Chemistry Sec Note

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S3 Atoms, Elements and Compounds S3-CHEM-3.1

3.1 Atomic structure and the Periodic Table

A) Structure of an atom

1. The atoms of all elements are made up of three kinds of particles, hose properties are

tabulated belo!

Particle Char"e Mass

Proton Positi#e char"e $% 1) 1

&eutron &one 1

Electron &e"ati#e char"e $-1) 1/1840

'. The particles are arran"ed li(e this!

the protons

and neutrons

cluster to"ether in the centre, formin" the nucleus

the electrons occup ener" le#els outside the nucleus

*. Atomic number and mass number

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1. The atomic number of an element is the number of protons contained in one atom of

an element. Since the o#erall char"e of an atom is alas 0ero, the number of protons in

an atom is the same as the number of electrons.

'.The mass number of an element is the sum of the number of protons and the

number of neutrons contained in one atom of the element.

3. The atomic number and mass number of an element are usuall indicated ith the

chemical smbol in the folloin" manner.

Mass number

Smbol

Atomic number

C. sotopes

Each element has onl one atomic number. Hoe#er, the atoms of an element are notalas identical. sotopes are atoms ofthe same element ith the same number of

protons but different number of neutrons. n other ords, isotopes are forms ofan element

ith different mass numbers.

Mass number = the number of protons %number of neutrons


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S3 Atoms, Elements and Compounds 53-CHEM-3.2

D. Arrangement of electrons in shells

1.Electrons are arranged in energy leels around the nucleus of an atom. !he energy

leels are sometimes called shells. !he nearer the shell to the nucleus, the lo"er the

energy leel. !he first shell is the one nearest the nucleus.

2. Each energy leel can hold a certain num#er of electrons.

the first shell $lo"est energy leel% can hold up to 2 electrons

the second shell can hold up to & electrons

the third shell can hold up to &electrons

'e can "or( out ho" the electrons are arranged #y filling the lo"est energy shells first, i.e.

the first shell. 'hen the lo"est energy leel is full the ne)t electron goes into the second shell.

'hen the second shell is full "ith eight electrons then the electrons "ill fill the third shell. *or

e)ample, the atomic num#er of aluminium is 13. !he first shell is filled #y t"o electrons, the

second shell is filled #y eight electrons, and the remaining three electrons occupy the third

shell. !he atom is often "ritten as A+ $2,&,3%. !he $2,&,3% is the

electron arrangement orelectronic configuration of aluminium.

E)ample Arrangement of electrons in an aluminium atom

3. !he highest occupied energy leel is (no"n as the alence shell, and the

electrons in it are (no"n as alence electrons. *or e)ample, the fourth shell is the

alence shell of potassium, and each potassium atom has one alence electron.

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Atoms, Elements and Compounds S3-CHEM-3.2

The table below gives the electron configuration of the first 20 elements.

Atomic ElectronElement Symbol Electron shells

number configuration

st 2nd 3r! "th

shell shell shell shell

Hy!rogen H

Helium He 2 2 2

#ithium #i 3 2 2$

%eryllium %e " 2 2 2$2

%oron % & 2 3 2$3

Carbon C ' 2 " 2$"

(itrogen ( ) 2 & 2$&

*+ygen 0 , 2 ' 2$'

luorine 2 ) 2$)

(eon (e 0 2 , 2$,

So!ium (a 2 , 2$,$

Magnesium Mg 2 2 , 2 2$,$2

Aluminium A/ 3 2 , 3 2$,$3

Silicon Si " 2 , " 2$,$"

hos1horus & 2 , & 2$,$&

Sul1hur S ' 2 , ' 2$,$'

Chlorine C/ ) 2 , ) 2$,$)

Argon Ar , 2 , , 2$,$,

otassium 2 , , 2$,$,$

Calcium Ca 20 2 , , 2 2$,$,$2

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53 Atoms, Elements and Compounds S3-CHEM-3.3

3.2 Ions

1. The arrangement of electrons in three nole gases are gi!en elo"#

Helium atom 2 full outer shell

o$eon atom2,% full outer shell

Argon atom 2,%,% full outer shell

2. A full outer shell ma&es an atom stale. 'nl( the nole gas atoms ha!e full outer

shells. This e)plains "h( nole gases are chemicall( unreacti!e.

3. The atoms of all other elements ha!e incomplete outer shells. *( reacting "ith each

other, atoms of these elements can otain full outer shells and so ecome stale.

4. The atoms of some elements can otain full shells ( either losing or gaining

electrons.

(a) Metal atoms

losing one or

m . Eg:

can otain a full outer shell

more electrons to another

(

ato

Sodium atom +II p, II e Sodium ion +11 p, 1/ e

loses 1 e

Sodium atom has one !alence electron. It can otain a full outer shell ( losing this electron to

another atom. The result is a sodium ion.

This is no" the outer shell of a sodium ion. It is full and thus the ion is stale.

The charge on a sodium ion#

the charge on 11 protons 011the charge on 11 electrons -1/

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Total charge =+1

The sodium ion has a charge of +1, so it is a positive ion. Its symbol is Na",

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53 Atoms, Elements and Compounds 53-CHEM-3.4

(b) Atoms of non-metals can obtain full shells by gaining one o moe electons fom

anothe atom. !his pocess is called Electron transfer. Eg"

= = = = = = = = >

electongains one

A chloine atom has # $alence electons. %t can obtain a full shell by accepting & electon fom

anothe atom. !he esult is a chloride ion.

!his is no' the oute shell of a chloide ion. %t is full and thus the ion is stable.

!he chage on a chloide ion

the chage on potons =

the chage on & electons * -&

!otal chage * -&

!he chloide ion has a chage of -& , so it is a negati$e ion. %ts symbol is c.

&. Any atom becomes an ion 'hen it gains o loses electons. An ion is a chaged paticle

because it contains an une+ual numbe of potons and electons. !he ions fomed by some

of the elements ae gi$en belo'.

. eneally, metals and hydogen fom positi$e ions 'hile non-metals fom negati$e

ions. ome elements do not usually fom ions. !his is because thei atoms 'ould ha$e to

lose o gain se$eal electons, and this e+uies too much enegy.

3.3 %onic bonds

A. /omation of ionic bonds

%onic bonds ae fomed by the tansfe of electons bet'een metallic and non-metallic

elements. .

B. !he eaction bet'een sodium and chloine

&. 0hen a sodium atom and a chloine atom eact togethe, the sodium atom loses its

$alence electon to the chloine atom. %n the pocess, sodium and chloide ions ae fomed.

1oth the ions ae stable because they ha$e full oute shells.

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2. Once the ions are formed, they are held together by strong electrostatic attraction.

The electrostatic attraction between the ions is called an ionic bond, or sometimes an

electrovalent bond.

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53 Atoms, Elements and Compounds 53-CHEM-3.5

Example:The reaction between calcium and chlorine

A calcium atom has two outer electrons. It has to lose these to attain a stable, ull outer shell

o electrons. Chlorine !",#,$% needs to &ain an electron. 'ince the calcium atom needs to lose

two electrons and the chlorine atom needs to &ain onl( one electron, the calcium atom has to

combine with two chlorine atoms. The ormula o calcium chloride is thereore CaC)"* The

compound has no o+erall char&e.

a ",#,) CI",#,$ C)- ",#,# a ",#

3./ Molecules and co+alent bonds

Another t(pe o chemical bond is the co+alent bond which is ormed between non-metallic

elements. 0hen two non-metal atoms react to&ether, both o them need to &ain electrons to

reach ull shells. The( can onl( achie+e a ull outer shell o electrons b( sharin& electrons.A

co+alent bond is ormed when two atoms share a pair o electrons.

Example: Chlorine !two chlorine atoms%

Each chlorine atom needs a share a chlorine molecule

1rom let to ri&ht: add in one more electron to obtain a ull shell

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53 Atoms, Elements and Compounds S3-CHEM-3.6

3.5 Ionic and covalent compounds

A. Ionic compounds

Ionic compounds are formed when a metallic element comines with a nonmetallic element.

Electron transfers etween the atoms of these elements lead to the formation of ions. !hus,ionic solids are composed of ions with opposin" char"es held to"ether # stron" ionic

onds. A molecule is a small "roup of atoms which are held to"ether # covalent onds.

$acts

Ionic solids have hi"h meltin" and oilin" points. !he# are not volatile.!he# are usuall# solule

in water, ut are insolule in or"anic solvents %&e. solvents which have covalent onds'.

(hen solid, ionic solids do not conduct electricit#. !he# conduct electricit# when molten or in

solution.

!heor#

!he stron" force of attraction etween ions of opposite char"es ma)e it difficult to separate

the ions. A lot of ener"# must e asored to rin" aout a chan"e of state.

In the solid state, the ions cannot move. (hen the# are melted or dissolved, the ions are free

to move.

*. Covalent compounds

Atoms of non-metallic elements comine with each other # sharin" electrons. Man# covalent

compounds are composed of simple molecules+. !he onds which hold the atoms to"ether

in the molecule are stron", ut the attraction which eists etwen molecules are wea).

$acts

Molecular compounds are not ver# hard. !he# have low meltin" and oilin" points. Man# of

them are either "ases or volatile liuids at room temperature. !he# are usuall# insolule in

water ut are solule in or"anic solvents.

!he# do not cond uct electricit#

!heor#

!he molecules are held to"ether # onl# wea) forces, so not much ener"# is needed to

separate them.

!he molecules are unchar"ed.

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Atoms, Elements and Compounds 53-CHEM-3.6

Differences between ionic and covalent compounds

Ionic compounds Covalent compounds

Types of particle Ions with opposingSmall molecules

present charges

Strong ionic bonds Strong covalent bonds hold atoms

Bonding together in themolecule; separatebetween ions.

molecules are held by weak forces.

Properties: Non volatile.

Volatile.

Volatility Very high melting and

Low melting and boiling points.

boiling points.

Nonconductors when

Electrical solid.

!ood conductors when Nonconductors.

conductivity

molten or in solution.

Soluble in water but notSoluble in organic solvents" but not in

Solubility inwater.

organic solvents.

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53 The Periodic Table 53-CHEM-4.1

The most important classification system in Chemistry is that arrangement of elements that we

call the periodic table. It was first constrcted by !mitri Mendelee"# a $ssian chemist in the

late 1%&'s. Mendelee"(ll table# howe"er# had many gaps in it becase only &3 elements were

)nown at that time. He predicted that elements wold be disco"ered to fill the gaps and he

frther predicted many of their properties. *ater# when sch elements were fond# ha"ing

almost e+actly the properties predicted for them# the great "ale of the periodic table was

clearly demonstrated. The periodic table enables s to correlate the properties of the

elements and to remember and predict their chemistry. There is a basic generali,ation

that lies behind the periodic table and this is )nown as the periodic law

The properties of the elements are periodic fnctions of their atomic nmbers.

The /oble 0ases

ppro+imately a do,en elements are gases at room temperatre and si+ of these# helim#

neon# argon# )rypton# +enon and radon are remar)ably similar. This similarity is illstrated as

follows

1 They are all colorless gases consisting of monatomic molecles.

2 There is a "ery narrow temperatre range o"er which anyone of them can e+ist as a

liid.

3 Their ioni,ation energies are "ery high. This means that their electron configrations

are "ery stable.

4 They are particlarly distincti"e in their almost total lac) of chemical reacti"ity

The noble gases illstrate the periodic law so clearly that they are the )ey to organi,ing

elements into the periodic table.

e shold also note that there is a small bt progressi"e trend in properties as we mo"e from

helim to radon. This is shown in the gradal rise of melting points and boiling points down the

grop.

He /e r r 6e $n

1tomic nmber 2 1 0 1 8 3 6 5 4 8 6

Melting point 758 1 . 1 at 2 5 8 4 1 1 6 1 6 1 2 0 22 6 atm

9oiling point 758 4 . 2 2 7 8 7 1 2 0 1 6 5 2 1 1

Ioni,ation energy2 3 7 8 2 0 8 7 1 5 2 6 1 3 5 7 1 1 7 6 1 0 4 4

7):;mole8


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53 The Periodic Table 53-CHEM-4.2

The Alkali Metals

The six elements with atomic numbers one greater than the six noble gases are ver similar

an! also illustrate the "erio!ic law. These are the alkali metals - lithium# so!ium# "otassium#

rubi!ium# caesium an! $rancium. Their similarities are illustrate! below%

& The are all so$t metals which corro!e easil.

2 Their li'ui! states exist over a wi!e tem"erature range an! have high boiling "oints an!

low melting "oints.

3 Their ioni(ation energies are ver low. This in!icates that the are strongl electro"ositive.

4 The are characteri(e! b their great reactivit to $orm ver stable com"oun!s in which

the alkali metal atom loses one electron an!# $orms a ) & ion. This can be seen in the $ollowing

reactions%

*a+ Each alkali metal reacts vigorousl an! ra"i!l with water to $orm h!rogen gas an! an

a'ueous solution o$ the alkali metal h!roxi!e

*b+ The react rea!il with chlorine *see "age &4, $or so!ium+# e.g. 2a*s+ ) C&2*g+ /

2aC0*s+

*c+ The burn brilliantl when heate! in air $orming the oxi!e M21 *an! usuall the

"eroxi!e M202 as well+ where M re"resents an alkali metal atom# e.g. 4a*s+ ) 12*g+ /

2a21*s+

*!+ The react ver vigorousl *almost ex"losivel+ with !ilute aci!s# e.g. 2a*s+ )

2H)*a'+ / 2a)*a'+ ) H2*g+

5 Their oxi!es an! h!roxi!es are basic an! !issolve in water to give alkaline solutions.

Thus the react with aci!s "ro!ucing salts "lus water onl.

Their chlori!es an! oxi!es are high melting "oint ionic soli!s.

0t shoul! be a""reciate! that the !ominant $eature o$ the chemistr o$ the alkali metals is their

marke! ten!enc to $orm the )& ion. 0n !oing so the achieve the stable noble gas electron

con$iguration. The alkali metals are ver reactive but com"oun!s containing the alkalimetal ions are com"arativel unreactive. ike the noble gases we notice that there is a

gra!ual small change in "ro"erties as we move $rom lithium to caesium. Thus the melting

"oints an! boiling "oints gra!uall !ecrease !own the grou".

Li a 4 b Cs

Atomic number 3 1 1 1 9 3 7 5 5

Melting "oint *4+ 4 53 3 7 1 3 3 6 3 1 2 3 0 2

6oiling "oint *4+ 1 59 9 1 1 6 2 1 0 3 0 9 52 9 6 3

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Ionization energy5 2 6 5 0 2 4 2 5 4 0 9 3 8 2

(kJ/mole)

Outer shell electrons 1 1 1 1 1

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S3 The Periodic Table S3-CHEM-4.3

The Halogens

The elements with atomic numbers one less than the noble gases form another family which

illustrates the periodic law. These are known as the halogens - fluorine chlorine bromine and

iodine. Their similarities are illustrated below!

" They are non-metals and consist of diatomic molecules.

# They are all coloured fluorine - a pale yellow gas$ chlorine - a yellow-green gas$

bromine a brown-red li%uid$ iodine - a &iolet solid.

3 Their melting points and boiling points are low and their li%uids e'ist only o&er a

narrow temperature range.

4Their ioni(ation energies are high$ in fact second only to those of the noble gases.

Hence they are strongly electronegati&e.

) They are &ery reacti&e. *n their reactions there is a marked tendency for the halogen

atom to gain one electron and form a stable -" halide ion. *n these reactions they act as

o'idi(ing agents of which fluorine is the strongest and iodine the weakest.

+a, They react &iolently with sodium e.g. #a+s, C*#+g, //0 #aC*+s,

+b, 1ll of the halogens e'cept iodine will o'idi(e iron+l*, compounds to iron+l**,

compounds e.g. for chlorine! e.g. H#S24+a%, #eS24+a%, C"#+g, ///0

e#+S24,3+a%, #HC*+a%,

+c, They combine with hydrogen to form gaseous co&alent hydrides which

dissol&e in water to form halogen acids e.g. H#+g,C* #+g,//0#HC*+g,.

+d, They react with most non-metals to form co&alent halidese.g.#+s,3C*#+g, //0

#C"3(1 )

+e, Chlorine and bromine o'idi(e sulphur dio'ide or sulphites to sulphates

(f) Displacement reactions The halogens displace each other from their salts

depending on their relati&e strengths as o'idi(ing agent

5 Halides ha&e distincti&e properties.

+a, Metal halides are ionic solids e.g. aC*. on-metal halides are co&alent

compounds e.g. C"3: Hydrogen halides are gaseous compounds that are &ery

soluble in water forming strongly acid solutions.

+b, 6hen sil&er nitrate solution is added to a%ueous solutions containing halide

ions the corresponding sil&er halides are precipitated +sil&er fluoride being

soluble is an e'ception, e.g. 1g+a%, Cl7+a%, //0 1gC*+s, or sil&er chloride

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The dominant feature of the chemistry of the halogens is that they are very reactive but

halides are comparatively unreactive and stable. There is a gradual trend in properties as

we move from fluorine to iodine, the oxidizing strength decreases and their melting points

and boiling points increase.

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53 The Periodic Table 53CHEM4.4

F CI Br I

Atomic number 9 1 7 3 5 5 3

Melting point (K) 5 5 1 7 2 2 6 6 3 8 7

Boiling point (K) 8 5 2 3 9 3 3 2 4 5 7

Ionization energy1 6 8 7 1 2 5 7 1 1 4 8 1 0 1 5

(!"mole)

#uter $%ell electron$ 7 7 7 7

&%e 'ir$t &ran$ition erie$

. &%e *ir$t tran$ition $erie$ element$ are all metal$. &%u$ t%ey are goo+ con+uctor$ o*

%eat an+ electricity. A$ e,pecte+ *or metal$- t%eir ionization energie$ are relatiely lo/.

0.&%eir +en$itie$ an+ melting point$ are %ig%. &%i$ i$ in contra$t to t%e alali metal$

(group ) /%ic% %ae relatiely lo/ +en$itie$ an+ melting point$.

3. Many o* t%eir compoun+$ are coloure+. By contra$t- t%e compoun+$ o* t%e alali metal$

are not coloure+.e.g. Copper compoun+$ are generally blue or green. Iron compoun+$ are

generally green- re+ or bro/n.

4.&%ey *orm ion$ or molecule$ /it% +i**erent o,i+ation $tate$. e.g. Copper *orm$ copper

(I) an+ copper (II) compoun+$. Iron *orm$ iron (II) an+ iron (III) compoun+$

5. &%ey *orm comple, ion$. e.g. Cu(1H3)l2 tetramminecopper(ll) ion or n(#H)l

zincate(ll) ion

.&%e tran$ition element$ an+ t%eir compoun+$ are o*ten u$e+ a$ cataly$t$. e.g. 'inely

+ii+e+ iron i$ u$e+ in t%e Haber proce$$ *or t%e manu*acture o* ammonia. 1icel i$ u$e+ in

t%e %y+rogenation o* un$aturate+ *at$.

c &i V Cr Mn 'e Co 1i Cu n

6elatie 7en$ity 3 . 0 4 . 5 1 6 . 1 7 . 1 9 7 . 4 3 7 . 8 6 8 . 9 0 8 . 9 0 8 . 9 6 7 . 1 4

$t Ionization6 3 4 6 5 9 6 5 1 65 5 7 1 8 7 6 0 7 6 0 7 3 9 7 4 8 9 1 1

Energy K!"mole

Melting 8oint. 1 5 4 0 1 6 7

1 9 0 0 1 8 7

1 2 5 0 1 5 4 0 1 4 9 0 1 4 5 0 10 8 3 4 1 90 0

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53 The Periodic Table 53-CHEM-4.S

Hydrogen

Hydrogen forms a family all by itself.

In its physical properties it resembles the halogens. It is a non-metal and consists of

diatomic molecles. Its melting point and boiling point are !ery lo" and its li#id e$ists onlyo!er a narro" temperatre range.

In its chemical properties it largely resembles the al%ali metals. Ths it reacts "ith the

halogens to form a stable &' ion.

eg H2(g) & C'2(g) - *HC' (g)

+hen hydrogen chloride dissol!es in "ater, the strongly acid soltion prodced indicates that

H& ions are present.

H

-tomic nmber '

Melting point () '4

/oiling point () 20

Ioni0ation energy (%12mole) ' 3'

4ter shell electrons '

Constrction of the Periodic Table

' 5ertical colmns are called grops, indicated by roman nmerals.

a) The al%ali metals ma%e pgrop I.They form a stable & ' ion by loss of ' electron.

b) The al%aline earths ma%e p grop II. They form a stable &* ion by loss of *

electrons.

c) The halogens ma%e p grop 5II. They form a stable -' ion by gain of ' electron.

d) The noble gases are sometimes called grop .

* There is agradal change in properties as one mo!es do"n a grop, bt the

elements in any grop clearly belong to one family.Hori0ontal ro"s are called

periods. Mo!ement across a period from left to right also re!eals a trend of change in

properties bt this trend is mch more mar%ed than !ertical trends.

3 In its more complete form the table mst be di!ided bet"een grops II and III and

separated so that the transition metals can be placed in position in the centre.

4 Periodicity is also illstrated by the graph of ioni0ation energy against atomic nmber.

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53 Chemical Bonding and Structure S3-CHEM-S.1

Formation of Ions

A group of elements called noble gases hae er! high first ioni"ation energies #hich

indicates that their electrons are er! tightl! bound. Accordingl! these elements are er!

stable and do not enter into chemical combination #ith other elements.

$able %.1 Ioni"ation Energies of &oble 'ases

&oble gas Electron configuration 1st ioni"ation energ!

Helium 2 ( 3)1 *+,mole

&eon 2,8 () *+,mole

Argon 2,8,8 1 %(/ *+,mole

0r!pton 2,8, 18,8 1 3%( *+,mole

enon 2, 8, 18, 18, 8 1 1 *+,mole

2ther atoms or groups of atoms lose alence electrons to form positie ions or gain alence

electrons 4from other atoms5 to form negatie ions. In each case the ion obtains the electron

configuration of a noble gas atom and is therefore er! stable.

Example:

1 electron lost positie ion formed. sodium ion

( electrons gained negatie ion formed.o6ide ion

Ions are therefore atoms or groups of atoms #hich hae become electricall! charged through

the gain or loss of electrons. 7ositie ions are *no#n as cations and negatie ions as anions

4see page 199). Some common ions are gien belo#8

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S3 Chemical Bonding and Structure S3-CHEM-S.2

Table 5.2 Positive Ions and Negative Ions

+1ion +2 ion +3 ion

H+ hydrogenMg2+agnesi!

Ca 2+ "al"i!NH#+aoni!

$at+ bari! % 13+al!ini!

Na+ sodi!&n2+

Cr 3+"hroi!

'+ (otassi!)in"

*III,e2+

%g+ silveriron*II ,e 3+ iron*III

C!+ "o((er*IC! 2+"o((ertll .

Pb 2+ lead*II

-1ion -2 ion -3 ion

, l!oride

C1- "hloride .2- o/ide

$* broide S2- s!l(hide

, iodide S32- s!l(hite

0H- hydro/ide S0- s!l(hate P#3-(hos(hate*N3- nitrate

col- "arbonate

Mn#-anganate*II Cr20l- di"hroate*I

HC3-hydrogen Cr0- "hroate*I

"arbonate siol- sili"ate*I

Ioni" Co(o!nds and Covalent S!bstan"es

To a4or "lasses o "hei"al s!bstan"es are ioni" "o(o!nds and "ovalent s!bstan"es. Ioni"

"o(o!nds are "o(osed o ions not ole"!les. They oe their e/isten"e to the tenden"y o

"ertain atos to lose ele"trons and o others to gain the. 0n the other hand "ovalent

s!bstan"es "onsist o ole"!les ade o atos %tos in these ole"!les are held together by

the sharing o one or ore (airs o ele"trons in the ole"!les.

Ioni" Co(o!nds

The "on"e(ts o ions ioni" bonds and ioni" "o(o!nds "an be best !nderstood by

"onsidering6 the oration o a ty(i"al ioni" "o(o!nd - sodi! "hloride.

7hen sodi! and "hlorine rea"t one ele"tron is transerred ro a sodi! ato to a "hlorine

ato. %n ioni" bond is ored.

Notes: (a)The sodi! ato loses one ele"tron to be"oe a (ositive ion Na+ and

the "hlorine ato gains one ele"tron to be"oe a negative ion "r.

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S3 Chemical Bonding and Structure S3-CHEM-S.3

Molecular Substances (or Covalent Substances)

The simplest example to illustrate the concepts of the covalent bond and substances is the

hydroen molecule. ! hydroen molecule consists of t"o hydroen atoms in "hich the t"o

valence electrons are shared bet"een the t"o atoms# $iarammatically this may be

represented as e- follo"s#

%&.-H atom Hatom H 2 molecule

'i. . *epresentation of the Covalent +ond in the Hydroen Molecule

The shaded reion on each atom is ,no"n as an orbital. t is the reion in "hich the electron

"ill be found. hen the orbitals overlap/ the t"o electrons/ one from each atom/ are attracted

to the t"o nuclei at the same time thereby formin a sinle covalent bond.

(a) Electrons are not lost or ained by the atoms concerned.

(b) The chemical bond in the hydroen molecule forms because each of the t"o electrons

is attracted to t"o protons simultaneously. The bondin electrons are often said to be

shared by the t"o nuclei.

(c) Substances li,e hydroen/ in "hich the bondin bet"een the atoms in the molecules is

covalent/ are called molecular or covalent compounds or substances.

(d) +y sharin the t"o electrons the atoms in the hydroen molecule have achieved the

electronic confiurations of noble as atoms. (e)The covalent bond is electrostatic in nature.

Simpler "ays of representin the bondin in the hydroen molecule are#

H -H

The line represents a shared pair of electrons.

0alence electrons "hich are not involved in the covalent bond ma,e up pairs ,no"n as lone

pairs.

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53 Chemical Bonding and Structure 53-CHEM-S.4

Double and triple bonds

The atoms in many molecules are lined by more than one co!alent bond. "#emember that a

co!alent bond in!ol!es the sharing o$ % electrons only.& Such molecules may contain double

bonds "% co!alent bonds& or triple bonds "3 co!alent bonds&.

"a& Co!alent substances do not conduct electricity because there are no ions or $ree

electrons present.

"b& The boiling points and melting points o$ co!alent substances are much lo'er than

those o$ ionic solids. Ho'e!er 'e must not conclude that co!alent bonds are 'ea. (n $act)

they are !ery strong. The lo' boiling points and melting points result $rom the 'ea

attracti!e $orces bet'een molecules. These $orces are easily o!ercome by a small amount

o$ heat. Because many co!alent substances are li*uids at room temperature most sol!ents

in general use include 'ater and co!alent substances such as ben+ene) ethanol)

methanol) methylben+ene etc.

,ther Co!alent Substances

Sulphur and (odine are both co!alent substances

Table 5.3 Melting oints and Boiling oints o$ Sulphur and (odine

.ormula Melting Boiling

point oint

Sulphur S/ 1151l 444.61l

(odine b 1141l 1831l

0s e1pected $or co!alent substances) their melting points and boiling points are comparati!ely lo'.

The molecule 88 is present in both the rhombic and monoclinic crystalline $orms o$ solid sulphur

"see page 22& and in the li*uid 'hen it is $irst molten.

0t room temperature iodine "b& is a lustrous blue-blac crystalline solid. (t sublimes on heating

to $orm a !iolet !apour.

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53 Chemical Bonding and Structure 53 ..CHEM-S.S

Table 5.4 Differences between Ionic and Covalent Substances

Ionic compounds Covalent Substances

enerall! cr!stalline solids "e.#. salts$ enerall! li%uids or #ases

Hi#& meltin# points and boilin# points 'ow meltin# points and boilin# pointsl.e. l.e.

non-volatile volatile

Electrol!tes (on-electrol!tes

enerall! soluble in water but enerall! soluble in or#anic solvents

insoluble but

in or#anic solvents insoluble in water

)ondin# is &i#&l! directional. Hence

)ondin# is non-directional molecules &ave #eometrical s&apes*

wit&atoms fi+ed wit& respect to eac& ot&er.

T&eir reactions are fast T&eir reactions are often slow

Structures of Elements and Compounds

Elements and compounds can be divided into four structural classes.

"a$ Ionic compounds e.#. (aCI* M#,

"b$ Molecular substances e.#. Sa* H2, CH4"c$ Metals e.#. Cu* M#

"d$ iant molecular structures e.#. diamond* silica

Table 5.5 rincipal eatures of t&e our Classes of Elements and Compounds

iantIonic Covalent

molecular Metalscompounds substances

structures

Covalent

wit&inCovalent Metallic

)ond Ionic but not

between t&rou#&out bondin#

molecules

Meltin#

points andHi#& 'ow Hi#& Hi#&

boilin#

points

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Non-conductorsConduct when

Conductivit molten or in Non-conductor or

Conductorsemi-conductors

y aqueous s(an exception is

s

solutiongraphite)

Combination .Position in of

periodic elements from Right side Centre Left sidetable

both sides

Ionic solids and coalent substances hae already been discussed in this chapter.

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53 Chemical formulae and equations S3-CHEM-6.1

The central activity of the chemist is to understand and control chemical changes or chemical

reactions.

Characteristics of a Chemical Change

1 ne! su"stance al!ays forms.# $sually the change is difficult to reverse.

$sually much energy is associated !ith the change.

E%am&les'

(a) The rusting of iron.

(") The com"ustion of fuels to &roduce car"on dio%ide and !ater.

(c) The reaction of *inc metal !ith hydrochloric acid li"erating hydrogen gas.

E%am&les of changes !hich are not chemical changes'

(a) ny change of state+ e.g. the melting of ice.

(") ,inc o%ide turns from !hite to yello! on heating. (t returns to its original colour on

cooling.)

Chemical eactions

t is normal to call a chemical change a chemical reaction. chemical reaction can "e

re&resented "y using a chemical equation (see &age 51).

E%am&le'

The neutrali*ation reaction "et!een &otassium hydrogen car"onate and dilute hydrochloric acid

can "e re&resented "y

n a chemical reaction matter is neither created nor destroyed. This is /no!n as the la! of

conservation of mass. n &ractical terms this means that atoms are neither created nor

destroyed in a chemical reaction.

n many chemical reactions heat energy is released. These reactions are said to "e

e%othermic. n other chemical reactions heat energy is a"sor"ed. These reactions are

endothermic.

E%am&les'

Com"ustion of fuels is e%othermic.

0eutrali*ation of &otassium hydrogen car"onate !ith dilute hydrochloric acid is endothermic.

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S3 Chemical formulae and equations 53-CHEM-6.2

Equations and Formulae

An equation is a way of representing a cemical reaction. !t gi"es te formulae of te starting

materials #called reactants$ and of te products and it correctly portrays te ratios of te

compounds concerned.

E%ample&

'is equation means&

2 molecules of ydrogen react wit one molecule of o%ygen to form 2 molecules of water. (g)

means gaseous* (s) means solid #e.g. precipitate$* !t+q) means in aqueous solution* and (!)

means liquid.

An equation does not tell us&

, 'e conditions for te reaction.

2 'e speed #or rate$ of te reaction.

3 eter te reaction is endotermic or e%otermic.

'e mecanism #detailed steps$ in"ol"ed in te reaction.

/etermination of Formulae

0efore we write an equation it is essential to write te formulae of compounds correctly.

Formulae of !onic Compounds

1ince an ionic compound is electrically neutral we can page 3 to determine formulae.

E%ample& , 1il"er pospate#$

'e ions required are Ag4 and 4-3

For electrical neutrality we need 3Ag) and ,43-

.) te formula is 3Ag4 ,43

- or Ag34.

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53 Chemical formulae and equations 53-CHEM-6.3

Formulae of Molecular Compounds

We may make use of the concept of valency to determine the formulae of molecular

compounds.

efinition! "he valency of an element #or radical$ is the num%er of hydro&en atoms that 'illcom%ine 'ith or displace one atom of the element #or radical$.

(alency) in fact) can %e thou&ht of as the *com%inin& po'er+ of an element or radical. For

elements and radicals the valency has the same value as the ma&nitude of the char&e on the

ion #omittin& the si&n$.

E,ample!

,y&en has valency of #ion is /-$ and hydro&en has a valency of 0 #ion is H1$.

2se of valencies to find formulae of molecular compounds

E,ample 0! Methane. "his contains car%on and hydro&en) so 'e 'rite CH. Car%on has a

valency of and therefore it com%ines 'ith hydro&en atoms. We 'rite CH4 Hydro&en has a

valency of 0 so there is no further chan&e.

E,ample ! 4hosphorus#($ o,ide. 4hosphorus has a valency of 5) o,y&en . "herefore the

formula is 42/5.

E,ample 3! Car%on dio,ide. y a similar ar&ument) the formula is C2/4. We 'rite this in its

simplest ratio as C.

ote! "he term valency is avoided %y many chemists today %ecause it has different meanin&s

to different people and may therefore %e confusin&. 7tudents are often advised to learn the

formulae of molecular compounds. Ho'ever valency) as defined a%ove) can %e helpful.

7tudents 'ho remem%er that car%on has a valency of 'ill find this a &reat aid in the study of

or&anic chemistry.

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53 Chemical formulae and equations 53-CHEM-6.4

Writing Chemical Equations

Steps in writing an equation

! Wor" out the reactants and products.

# Write down the correct formulae of the reactants on the left side and the products on theright side.

3 $alance the equation %& selecting the correct coefficients. 'n a %alanced equation atoms

must %e conser(ed.

)ote Elements which do not e*ist as single atoms must %e represented correctl&. +hus ,# )# H#

C!#etc.

E*ample !

Write down the equation for the com%ustion of methane.

Step ! We ascertain that the reactants are methane and o*&gen and that the products are

water and car%on dio*ide.

Step # Write CH4 ,# /0 H#1 C,#Step 3 $alance to conser(e atoms CH42g #1#2g /0 #H#12g C,#2g

E*ample #

Write down the equation for the neutraliation of sulphuric acid with potassium h&dro*ide.

Step ! +he reactants are gi(en. +he products are potassium sulphate and water.Step # Write H#S14 ,H /0 #S14 H#1

Step 3 $alancing H#S142aq #,H2aq /0 #S142aq #H#12'

'onic Equations

'onic equations are to %e preferred where(er the& can %e used %ecause the& are simple and the&

indicate clearl& which ions react together.

E*ample

+he species H#S14 ,H and #S14 all ionie in water. ,nl& a small num%er of H#1 molecules

form ions. +herefore we can write

't can %e seen that man& of the ions appear on %oth sides of the equation. +hese ions do not

ta"e place in the reaction and are "nown as spectator ions. +he& can %e remo(ed lea(ing

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In an ionic equation, both the atoms and the charge are conserved. In the ionic equation

above it can be seen that there is a net charge of zero on both sides .Therefore charge is

conserved.

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S3 Chemical Reactions S3-CHEM-7.1

Types of Chemical Reactions

Combination or Synthesis

Examples:

(a !"rnin# ma#nesi"m ribbon: $M#(s % &$(# '' $M#)(s

(b *assin# chlorine o+er heate, iron ire: $e(s % 3C1$(#' $eC/3(s

0o"ble 0ecomposition or Metathesis

/n this type of reaction ions interchan#e or 2sap 4ormally a precipitate is forme, or else a #as

is #i+en off.

Examples:

(a 4aC1(a5 % 6#4&3(a5 '' 4a4&3(a5 % 6#C/(s

ritten ionically: Cl2(a5 % 6#%(a5 8 6#C/(s

(b CaC&3(s % $HC/(a5 ' CaCh(a5 % H$&(/ % C)$(#

lonically: C)l-(s % $H%(a5 ' H$C&3(a5 % H$&(/ % C)$(#

4e"trali9ation

4e"trali9ation is the name #i+en to a ,o"ble ,ecomposition reaction beteen an aci, an, a

base.

Example:

lonlcally:

HC1(a5 % 4a)H(a5 '' 4aC1(a5 % H2&(/

H%(a5% )H-(a5 '' H2&(/

Replacement

(a Some metals replace hy,ro#en in aci,s to form salts.

Example:

(b Metals replace metals loer on the acti+ity series from sol"tions of their salts.

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53 Chemical Reactions 53-CHEM-7.2

Oxidation and Reduction Reactions (til edox? for short)

Oxidation describes these reactions in hich ox!"en or the halo"ens are added to a

substance or h!dro"en is remo#ed from it.

Reduction describes those reactions in hich ox!"en or the halo"ens are remo#ed from

a substance or h!dro"en is added to it.

Oxidation and reduction ala!s occur to"ether.

Consider to exam$les%

&a) 2M"&s) ' (2&") * 2M"O&s) Ma"nesium burns in air.

Ma"nesium is oxidi+ed b! addition of ox!"en, and ox!"en is reduced. e sa! that ox!"en isthe oxidi+in" a"ent and ma"nesium the reducin" a"ent.

&b) n&s) ' 2HC/&a0) * nC12&a0) ' H2&") inc dissol#es in dilute h!drochloric acid.

inc is oxidi+ed b! addition of chlorine and h!dro"en is reduced. he chloride ions are neither

oxidi+ed nor reduced. inc is the reducin" a"ent and h!dro"en ions the oxidi+in" a"ent.

et us anal!+e these reactions further%

&a) he reaction beteen ma"nesium and ox!"en is made u$ of to half-reactions%

M" * M" 2'' 2e-

he o#erall e0uation ma! be obtained b! addin" the second half-reaction to tice the first

half-reaction. he electrons ill then cancel out. /t is clear that electrons are transferred from

ma"nesium to Ox!"en in the reaction.

&b) he reaction beteen +inc and dilute h!drochloric acid is also made u$ of to half

reactions%

n * n2'' 2e-

he o#erall e0uation ma! be obtained b! addin" the to half reactions to"ether. /t can be

seen that electrons are transferred from +inc to h!dro"en ions in the reaction.

Important: Oxidation-reduction reactions in#ol#e the transfer of electrons.

e are no in a $osition to formulate su$erior definitions of oxidation and reduction in terms

of electron transfer%

Oxidation is a chemical chan"e in hich electrons are lost b! an atom or a "rou$ of

atoms.

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~uction is a chemical change in which electrons are gained by an atom or a group of

atoms.

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Chemical Reactions S3 ..CHEM-7.2

Oxidation States or Oxidation Numbers

The concept of oxidation numbers is a device which enables us to keep track of electrons in

oxidation and reduction in terms of electron transfer: In assinin oxidation numbers!

"a# $%% free e%ements "e.. O2&H2# are i'en an oxidation number of (ero.

"b# $%% ions ha'e oxidation numbers e)ua% to their chares. e.

cr - oxidation number -*

+n2,- oxidation number ,2

"c# Except in peroxides& oxen in compounds has an oxidation number of-2

"d# Except in hdrides& hdroen in compounds has an oxidation number of ,*.

hen an e%ement is oxidi(ed its oxidation number increases. hen an e%ement is

reduced its oxidation number decreases.

Examp%es!

hen M atoms are chaned to M2,ions as in examp%e "a# abo'e then the ha'e been

oxidi(ed "oxidation state increases from / to ,2#.

hen 0e 2,ions are chaned to 0e 3,ions then the ha'e been oxidi(ed "oxidation state

increases from 2 to 3#.

hen cr ions are chaned to ch%orine as& then the ions ha'e been oxidi(ed "oxidation stateincreases from -* to /#.

The oxidation state "or number# is often indicated b a roman number in parentheses.

Examp%es!

Iron"%I# ch%oride. Iron has oxidation number ,2.

Iron(lI/) chloride. Iron has oxidation number ,3.

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53 Chemical Reactions 53-CHEM-7.3

In radicals and complexes the oxidation number of the central atom is sometimes indicated at the

end, as follows:

Mananate!"II# ion. In this ion !Mn$%-# mananese has an oxidation number of &7.

'itric acid. In this compound !H'$3# which contains the nitrate ion, nitroen has an oxidationnumber of 5. Hence the I.(.).*.C. name - 'itric!"# acid. +e do not use oxidation numbers in

relation to carbon.

xidi-in *ents and .educin *ents

xidiin aents, b/ their presence, cause other substances to be oxidied0 reducin aents, b/

their presence, cause other substances to be reduced. 1ome common oxidiin aents include:

!a# x/en

!b# Chlorine

!c# Concentrated sulphuric acid!d# Concentrated nitric acid

!e# )otassium mananate!"II#

!f# )otassium dichromate!"I#

1ome common reducin aents include:

!a# H/droen

!b# Carbon

!c# Carbon monoxide

!d# Metals !especiall/ those that are most electropositi2e#

!e# 1ulphur dioxide

.e2ersible .eactions

* re2ersible reaction is one in which the reactants can bere-formed b/ reaction between the

products.

Example:

ed-hot iron reduces steam: 3e!s# &%H4$!# 556 e3$%!1#& %H4!#

*lso h/droen reduces heated iron !II# diiron !III# oxide: e3$%!1#&%H4!# 6 3e!s# &

%H4$!#

Hence this is a re2ersible reaction and we write: 3e!s# & %H4$!# 6 e3$%!1#& %H4!#

ther examples:

!a # '4!# & 3H4!# 556 4'H3!# Haber process

!b# Contact process

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53 Chemical Reactions S3-CHEM-7.3

Ionic equation

An ionic equation is written to express the actual happening during a chemical reaction

lea!ing out the non-participating ions.

An ionic equation is a "alanced equation in terms o#

$I% &um"er o# atoms'ions o# each element

$II% (he charges on the ions

)riting Ionic Equation

)rite an ionic equation according to #ollowing steps*

+% )rite a "alanced equation #or the reaction and include all state s,m"ols

% In aqueous state ionic compounds ionie to #orm #ree ions. Express the ionic compounds in

their ionic #orms

3% Identi#, / pectator ions0 and cross them out

1% Rewrite the a"o!e equation omitting all / pectator ions0 to #orm the ionic equation

2or example* )hen Magnesium reacts with h,drochloric acid to produce magnesium chloride

and h,drogen.

Step one* Mg$s% HCI$aq% / MgCI$aq% H$g%

Step two* Mg$s% H$aq% Cr$aq% / Mg$aq% Cr$aq% H$g%

Step three* Mg$s% H$aq% Cr$aq% / Mg$aq% Cr$aq% H$g%

Step #our* Mg$s% H$aq% / Mg$aq% H$g%

(he next example is the neutraliation process "etween sulphuric acid and sodium h,droxide

to produce water and sodium sulphate.

Step one* HS41$aq% &a5H$aq% / &aS41$aq% H4$I%

Step two* H$aq% SOl-(aq) &a$aq% 4H-$aq% / &a$aq% SOl-(aq) H4$I%

Step three* H$aq% S5l-$aq% &a$aq% 4H-$aq% / &a$aq% S5l-$aq% H4$I%

Step #our* 4H-$aq% H$aq% / H4$I% = > 5l$aq% H6$aq% / H4$I%

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S3 Chemical Reactions 53-CHEM-7.3

The last example is the reaction between nitric acid and calcium carbonate that results in the

production of calcium nirate, water and carbon dioxide.

Step one !H"#3$a%& ' CaC#3$a%& ( Ca$"#3&!$a%& ' H!#$)& ' C*!$+&

Step two !H'$a%& ' !"#3- ' CaC#3$a%& ( Ca!

'$a%& ' !"#3-$a%& 'H!#$)& ' C*!$+&

Step three !H'$a%& ' !"#3-$a%& ' CaC#3$a%& ( Ca!'$a%& ' !"#3-$a%& 'H!#$)& ' C*!$+&

Step four !H'$a%& ' CaC#3$a%& ( Ca!'$a%& ' H!#$)& ' C*!$+&

Extra notes

& Substances in the solid, li%uid and +aseous states do not ionise

!& The state of a compound can be determined b considerin+ that

i& cids react in the a%ueous state

ii& Solubilit of salt. soluble salt exists in an a%ueous state

A. ll nitrates are soluble

/. Most chorides are soluble except those of lead and sil0er

C. Most sulphates are soluble except those of lead, barium and calcium $Sli+htl

soluble&

1. Most carbonates are insoluble except those of sodium, potassium and ammonium.

3& The number of ions is determined b the numerals in front and subscripts on the ri+ht.

!HC) ( !H' ' !Cr and

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S3 Mole concepts and calculations S3-CHEM-S.1

Relative Atomic and Molecular Mass

Relative Atomic Mass (Symbol Ar)

Te relative atomic mass o! an element is te "ei#ted avera#e o! te relative isotopic

masses o! te element on a scale on "ic te mass o! atoms o! te isotope carbon -1$ ise%actly 1$.

Relative Molecular Mass (Symbol Mr)

Te relative molecular mass o! an element or compound is te sum o! te relative atomic

masses in one !ormula unit o! te element or compound&

'rom te de!inition it is clear tat te relative molecular mass can be obtained by addin#

to#eter te relative atomic masses in te !ormula unit. Tis can be seen in te e%amples

belo"&

'ind te percenta#e o! "ater in copper(l) sulpate crystals. Te !ormula is CuS*+,H$

5 x 18.0Pe rcentage of water = x100 =36.1

249.6

Te Mole

e!inition&Te mole is te amount o! a substance "ic contains as many particles astere are carbon atoms in .1$ /ilo#ram (i.e. 1$ #rams) o! carbon-1$. 0ote&

(a) Te number o! particles per mole is #iven by te Avo#adro constant (2.$3 % 1$3mol)4

t is very important to remember tat 1 mole is connected "it 2.$3 % 1$3particles. Tus te

amount o! "ater tat contains 2.$3 % 1$3molecules is 1 mole and "e can even say tat

2.$3 % 1$3ants are 1 mole o! ants.

(b) Te particles must be speci!ied. Tey may be atoms4 molecules4 ions4 radicals4 electrons4

etc.

(c) Te mole (symbol mol) is te unit by "ic "e measure te 5uantity +amount o!

substance6 (n a similar "ay te /ilo#ram is te unit by "ic "e measure te 5uantity

+mass6 )

Since te relative mass o! 1 carbon atom to 1 nitro#en atom is 1$& 1* (!rom teir relative

atomic masses) it !ollo"s tat te relative mass o! 2.$3 7 1$3carbon atoms to 2.$3 % 1$3

nitro#en atom is also 1$&1*. And since (by de!inition) 1 mole o! carbon atoms as a mass o!

1$ #4 it is clear tat 1 mole o! nitro#en atoms as a mass o! 1* #. n #eneral4

1 mole o! te atoms o! an element is #iven by te relative atomic mass o! te element in

#rams8.

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By a similar argument it can be easily shown that 1 mole of water molecules (6.023 x 1023

molecules) has a mass of 18 g (the relative molecular mass in grams) and in general

1 mole of the formula units of an element or com!ound is given by the relative

molecular mass in grams".

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53 Mole concepts and calculations S3-CHEM-S.2

Determination of Number of Moles

Since 1 mole of atoms is given by te relative atomic mass in grams and 1 mole of formula

units is given by te relative molecular mass in grams it follo!s tat"

Mass of elementsNumber of moles of atoms = .. andrelative atomic mass

7\T b .1' le .1' fi la Mass of element or compounds in gramsHum er oJ mo s oj ormu units = --~-------=----___;__--

Re lative molecular mass

#n cross-multiplying !e obtain te useful relationsips"

Mass of element in grams $ Number of moles % &elative atomic mass and

Mass of element or compound in grams = Number of moles % &elative molecularmass

Examples:

1' marble !eigs 10 g. Ho! many moles of suc marbles !ould e(ual te entire mass of te

eart !ic is about 6 % 1027 g) *'ssume 1 mole = 6 % 1023 particles+

Answer: Mass of 1mole ofmarbles = 10% 6% 1023g

2 Ho! many moles of atoms are tere in

*a+ ,- g aluminium *b+ /03 g iron)

A U N b .r 1 Masso! elem ent in grams

nswer: smg urn era) mates =--___;;_------=----Re lative atomic mass

Amountof Al = 9.0 = 033 moles *of atoms+27

0.83 Amountof Fe = - = 0.015 mo es55.8

3 Ho! many moles of o%ygen atoms are tere in one mole of *a+ nitric acid molecules *b+

sulpuric acid molecules)

Answer:

*a+ 1 molecule of nitric acid *HN3) contains 3 o%ygen atoms. % 1023 molecules of HN3

contains 3 % %1023 o%ygen atoms. 1 mole of HN3 contains 3 moles of o%ygen atoms.

*b+ Similarly 1 mole of sulpuric acid *H2S4) contains moles of o%ygen atoms.

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53 Mole concepts and calculations 53-CHEM-S.3

Application - Finding the Formula of a Compound.

Examples:

1 In an experiment it as found that 1!"3 g of a copper oxide ere reduced to 1#$% g of copper

metal. &etermine the formula of the compound.

Anser' It is clear that the oxide contains 1#$% g copper and 1"( g ox)gen.

Amount of copper * 1#.% = 02 mole +of atoms,635

Amount of ox)gen = 1.( = .1 mole +of atoms,16

Cu

Moles of atoms' "# "1

Simplest mole ratio' # 1

Simplest ratio of atoms' # 1

.? the empirical formula is C#.

/he empirical formula represents the simplest ratio of the num0ers of atoms present in the

compound.

# A certain liuid has the folloing approximate percentage composition 0) mass' car0on

1!"1 2 h)drogen #"352 chlorine 43"52. Calculate its true formula if the relatie molecular

mass is 45.

C H CI

1 6 of compound contain' 1!"1 6 #"35 6 43"5 6

141 = 1175235 =235 835 =2352Moles of each element in 1 g' 12

1 355-

Simplest mole ratio' 1 # #

Simplest ratio of atoms' 1 # #

.? the empirical formula is CH#C1#.

Compound CH#C1#would have relative molecular mass of 45.

/he relatie molecular mass gien is 45.

.? Molecular formula is CH2C/2

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Mole concepts and calculations 53-CHEM-S.3

Gases and the Mole

AvogadrofD Law states that equal volumes ofgases, measured at the same temperature

and pressure, contain equal numbers ofmolecules. This law, based on observation, wasproposed in 1811 by Amadeo Avogadro, an Italian scientist. Now 1mole of any gas contains

6023 x 1023 molecules. It follows from AvogadrofD law that 1 mole of all gases at the same

temperature and pressure occupies the same volume.

Experiment shos that!

"ne mole o# an$ %as occupies 22-& drrr' at 0 1l and (60 mm o# mercur$ pressure )i.e* at

s.t.p+. ),t room temperature and pressure 1 mole o# %as occupies approximatel$ 2& drn+

Eamples!

1 hat is the /olume at s.t.p. o# 0& mole o# ox$%en and o# 0& mole o# chlorine

Answer!

1 mole o# an$ %as at s.t.p. occupies 22.& drrr'*

hus 0& mole o# ox$%en and chlorine occup$ the same /olume. his /olume is 0& x 22.& drrr'

= .6 drrr'*

2 Ho man$ moles o# ar%on are there in 1 drrr' hat mass does this ar%on ha/e )4elati/eatomic mass = 3. ,ssume s.t.p. conditions.+

Answer!

uantit$ o# ar%on = 1 70.0&&622.4

8sin%! Mass in %rams = 9um:er o# moles x 4elati/e molecular mass

Mass = 0.0&&6 x 399 71.78 %

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53 Mole concepts and calculations 53 ..CHEM-S.4

Determination of Number of Moles

Since 1 mole of atoms is given b t!e relative atomic mass in grams and 1 mole of formula

units is given b t!e relative molecular mass in grams it follo"s t!at#

Mass of elementsNumber of moles of atoms $ .. and

relative atomic mass

7 \.T b 1 -1 ' le 1 -1 'fi la . Mass of element or compounds ingli.'amsHum er oj mo sOJ ormu units=--___;_--------=------=---Relative molecular mass

%n cross-multipling "e obtain t!e useful relations!ips#

Mass of element in grams = Number of moles & 'elative atomic mass and

Mass of element or compound in grams = Number of moles & 'elative molecular

mass

Examples:

1( marble "eig!s 10g. Ho" man moles of suc! marbles "ould e)ual t!e entire mass of t!e

eart! "!ic! is about * & 10+, g (ssurnet mole = * & 10+3 particles/

Answer: Mass of 1mole of marbles $ 10& 6 & 10

+3

g

+ Ho" man moles of atoms are t!ere in

a/ 0- g aluminium2 b/ 3 g iron

A 53 N b .I' 1 Masso! elementin grams

nswer: smg um er oj motes $---$---------$----Re lative atomic mass

Amount of Al = 9.0 = 033 moles of atoms/27

0.83 IAmountojFe=-=.!" moes

55.8

3 Ho" man moles of o&gen atoms are t!ere in one mole of a/ nitric acid molecules2 b/

sulp!uric acid molecules

Answer:

a/ 1molecule of nitric acid HN3/ contains 3 o&gen atoms. * & 10+3 molecules of HN3

contains 3 & * & 10+3 o&gen atoms. 1mole of HN3 contains 3 moles of o&gen atoms.

b/ Similarl 1 mole of sulp!uric acid H2S4) contains 4 moles of o&gen atoms.

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S3 Mole concepts and calculations S3-CHEM-8.S

Understanding Chemical Equations

Consider the equation: N2(g) 3H2(g) !!" 2NH3(g)

#rom this equation $e see that:

% molecule o& N2reacts $ith 3 molecules o& H2to 'ield 2 molecules o& NH3. hus * %+23

molecules o& N2react $ith 3 * * %+23molecules o& H2to 'ield 2 * * %+23molecules o& NH3.

(,ounding o&& the ogadro constant.)

% mole o& N2 reacts $ith 3 moles o& H2to 'ield 2 moles o& NH3

#urther more &rom the equation:

% molecule o& N2reacts $ith 3 molecules o& H2to 'ield 2 molecules o& NH3/ thus n molecules o&N2 react $ith 3n molecules o& H2 to 'ield 2n molecules o& NH3

% olume o& N2 reacts $ith 3 olumes o& H2 to 'ield 2 olumes o& NH3.

0t &ollo$s that $e can inter1ret an equation in three $a's.

% he coe&&icients gie the numer o& molecules reacting.

2 he coe&&icients gie the numer o& moles reacting.

3 #or reactions inoling gases/ the coe&&icients gie the numer o& olumes reacting.

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S3 Mole concepts and calculations S3-CHEM-8.6

Stoichiometric Calculations General Approach

Suppose two substances, A and B, combine accordin to a !nown reaction. "e wish to !now

how much B will react with #or, be produced $rom% a measured &uantit' o$ A. (he solution

in)ol)es three steps*

Step + Epress the amount o$ A in moles.

Step Con)ert the moles o$A to moles o$ B usin the balanced e&uation.

Step 3 Con)ert the amount o$ B #in moles% into whate)er &uantit' is re&uired.

E,amples*

+ H'droen was passed o)er heated copper#l% oide, and the oide con)erted to copper

metal. (he reaction can be represented b' the e&uation* Cu/ 0 H 11112 C 0 H4

"hen 5.64 o$ the oide was completel' chaned, +.6 o$ water were produced. se these

$iures to show that the relati)e atomic mass o$ copper is 6. #A,#H% +.4, A,#7% 1 +6%.

Answer:

Step + Epress the amount o$ water in moles. +.6 H0is moles H0. 126 U6

Step rom the e&uation we see that - moles H0result $rom -. - 9moles o$ Cu/.18 18

Step 3 Con)ert the amount o$ Cu/ #moles% into mass #rams%.

:et relati)e atomic mass o$ Cu be .

;ow Mass in rams = ;umber o$ moles


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S3 Mole concepts and calculations 5l-CHEM-S.7

Answer:

Step 1 Express the amount of zinc in moles. 130 g Zn is 130

moles Zn ! moles Zn"5

Step ! #rom the e$uation% ! moles of zinc pro&uce ! moles of H2

an& ! moles of ZnC12.

Step 3

'i( Mass of h)&rogen *um+er of moles x ,elatie molecular mass !x!g g

'ii( *o/ 1 mole of h)&rogen occupies !! &m% ! moles of h)&rogen occup) 2 &rn%

'iii( Mass of ZnC2 *um+er of moles x ,elatie molecular mass ! x 13" g !7! g

Note: 4he /or&s Step 16 Step !6 Step 36 a+oe are gien for gui&ance an& /oul& not

+e inclu&e& in an examination ans/er.

3 otassium manganate'89 &ecomposes /hen gentl) heate& accor&ing to the e$uation:

;hen 1"5 < of a certain sample of cr)stals of potassium manganate'89 /ere heate& until no

more gas /as eole&% the olume of ox)gen collecte& un&er s.t.p con&itions /as 11! ern.

Calculate the percentage purit) of the cr)stals of potassium manganate'89.

=%'H9 1.0, Ar(O) 1".0% Ar(K) 32 is 11! mole >2 0.005 mole >2!!00

Step ! 0005 mole >! is pro&uce& +) ! x 0005 mole ?Mn0% l.e% +) 001 mole ?Mn0

Step 3

9

Mass of ?Mn0 *um+er of moles x ,elatie molecular mass 0.01 x 152 < 1.52

:. % purit) of sample is 152x 100


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53 Mole concepts and calculations 53-CHEM-S.S

Concentration of Solutions

The composition of a solution is specified by its concentration. Both the solute and the solvent

have concentration, but it is usual to consider only that of the solute.

Concentration is the amount of substance per unit volume. t is !enerally measured in"

either Moles per cubic decimetre #rnol$drrr%& or 'rams per cubic decimetre #!$dm3 )

Note: Concentration is sometimes measured in moles per litre #mol$(& since ( litre ) ( drrr% ) (

*** ern+ and ( ml = ( ern+.

Examples:

f ( mole of HC is dissolved in ( drn+ the concentration is ( mol$drn+ or 3.5 !$dm3. f moles

of a/H are dissolved in *.5 dm3 the concentration is 0 mol$drn+ or (* !$dm3.

Problems:

( Calculate the concentration in !$dm3 and mol$drrr% in the follo1in!"

#a& 2 of aC dissolved in ** crrr% of solution.

#b& 0 2 /H dissolved in 0 litres of solution.

4r#a& = 3.*, 4r#C& = 35.5 A,(O) = (.*,A,(H) = (.*,A,(K) ) 32.(

4ns1er"

#a& f 2 aC is dissolved in ** ern+, then 5 2 aC is dissolved in ( *** em+ #6e. ( drrr%&,

3* 2 aC is 30 mole = *%5( mole23 +355

the concentration is 3* !$dm3 or *%5( mol$drrr%

#b& f 0 2 /H is dissolved in 0 litres, then 0 2 /H is dissolved in ( litre.

/H is mole = *.(( mole39.1+16+1

". the concentration is !$dm3 or *.(( mol$drn+.

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53 Identification of Ions and Gases 53-CHEM-9.1

Table: Tests for Identifying Ions

Ion Tests

K+ Te fla!e test is s"fficient confir!ation at tis le#el

$a+ K+ - transient #iolet% $a& - orange-yello'.

1 (la!e test - bric)-redCa *+ 2 it $H3,a% no /reci/itate for!s. it $a0H,a% a 'ite /reci/itate for!s

'ic is insol"ble in ecess.

C" *+ it $H3,a%a ligt bl"e /reci/itate for!s. It dissol#es in ecess to for! a dee/ bl"esol"tion.

(e *+ it $H3,a or $a0H,a % a dirty green /reci/itate for!s.

(e 3+ 1 it $H3,a or $a0H,a% a reddis-bro'n /reci/itate for!s.2 it a!!oni"! tiocyanate sol"tion a dee/ red colo"ration is /rod"ced.

1 it $H3,a or $a0H,a% a 'ite /reci/itate for!s2 te latter /reci/itate onlyb *+ dissol#es in ecess.

2 it dil"te HCI% a 'ite /reci/itate for!s 'ic is sol"ble in ot sol"tion.

4n*+ it $H3,a or $a0H,a% a 'ite /reci/itate for!s% bot /reci/itates dissol#ing in

ecess.

1 it $a0H, a% a 'ite /reci/itate for!s 'ic is sol"ble in ecess. it13+ $H3,a% a 'ite gelatino"s /reci/itate for!s 'ic is sligtly sol"ble in ecess.

2 it dil"te HCI% no /reci/itate for!s. ,Tis distina"ises 13+fro! b *+.

0n 'ar!ing 'it $a0H,a a!!onia is gi#en off. ,$H3is detected by its caracteristic$H6+ s!ell% by its t"rning red lit!"s /a/er bl"e and by te for!ation of dense 'ite f"!es 'itydrogen cloride.

col- it dil"te acids carbon dioide is gi#en off. ,C7* is detected by its t"rning li!e'ater!il)y.

it dil"te acids carbon dioide is gi#en off. HC73is disting"ised fro!col- by reactionHC73 'it Mg876 sol"tion. Hydrogencarbonates% gi#e no /reci/itate in te cold. /reci/itate

for!s on boiling. Carbonates gi#e a /reci/itate in te cold.

sol- ields 87* 'it dil"te HCI acid ,87* t"rns /otassi"! dicro!ate,:I /a/er green.

80;- it


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1 The solid on heating produces 02(g) and often N02(g) also. (02 ignites a glowing

splint; N02 is a reddish-brown acidic gas.)

2 The solid on warming with concentrated H2S0 !ields N02 and HN03.

" Solutions gi#e the $brown ring% test. The solution is mi&ed with 'eS0(a) and

the test-tube held at % *oncentrated H2S0 is slowl! and carefull! poured

down the inside. + brown rinc forms at the ,unction of the two liuid la!ers.

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Identification of Ions and Gases S3-CHEM-9.2

Tests for Oxidizing Agents in Solution

Oxidizing agents !ill li"erate iodine fro# solutions of $otassiu# iodide. T%e iodine turns

t%e solution "ro!n. On s%a&ing !it% CC4, t%e iodine for#s a 'iolet solution !%ic% settles

"elo! an a(ueous la)er. Iodine #a) also "e identified ") t%e "lue colour it for#s !it%

starc% solution.

2 Oxidizing agents !ill con'ert iron*II+ salts to iron*III+ salts. T%e solution containing a

sus$ected oxidizing agent s%ould "e added to a fres%l) $re$ared solution of ferrous sul$%ate

acidified !it% dilute sul$%uric acid. T%e $ale green colour ,e2 ions *a(+ !ill "e c%anged to t%e

)ello!is%-red or )ello!is%-"ro!n colour of ,e 3 ions in solution ") an oxidizing agent.

Tests for educing Agents in Solution

educing agents !ill decolourize acidified solutions of $otassiu# #anganate*/II+ and

t%e) !ill c%ange acidified solutions of $otassiu# dic%ro#ate*/I+ fro# orange to green.

2 educing agents !ill decolourize a(ueous solutions of "ro#ine. In t%is case0 free "ro#ine

is reduced to "ro#ide ions *1r+0

Ta"le Tests for identif)ing gases

Gas Colour and S#ell Test

itrogen *2+ Colourless4Odourless o si#$le test. 1urn Mg in it to for# Mg32. 5%en

!ater is added to Mg32 a##onia is e'ol'ed.

Ox)gen *62+ Colourless4Odourless Ignites a glo!ing s$lint

H)drogen *H2+ Colourless4Odourless Ignites !it% a s(uea&) $o$

Heliu# *He+ Colourless4Odourless on-reacti'e4 o test

A##onia *H3+ Colourless47ungent Turns #oist red lit#us $a$er "lue. 7roduces dense

s#ell !%ite clouds of H8C !it% %)drogen c%loride gas.

Car"on dioxideColourless4Odourless

Turns li#e!ater #il&). *Excess CO2clears t%e

*CO2+ #il&iness. +

H)drogen c%loride Colourless47ungent ,u#es in #oist air0 t%e fu#es turn "luelit#us $a$er

*HC+ s#ell red. 7roduces dense !%ite clouds of H8C !it%

a##onia.

Greenis%-)ello!47oisono

us Turns #oist "lue lit#us $a$er red and t%enC%lorine *C2+

47ungent s#ell "leac%es it

edd is%-"ro!n47oisonou

itrogen s

dioxide*62+ 47ungent s#ell Turns #oist "lue lit#us $a$er red

Sul$%ur dioxide Colourless4 C%o&ing Turns orange 2Cr26: $a$er green*S62+ s#ell

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Water vapour (H20) Colourless/Odourless Condenses on a cold surface. Turns white

anhydrous CUS04 lue.

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53 Atmosphere and Environment S3-CHEM-13.1

Oxygen

Oxygen is the most abundant element in the earthlll crust. It is found free in air (as diatomic

! molecules" and combined in #ater and oxides.

$hysical $ro%erties of Oxygen

It is a colourless and odourless gas& and is only slightly soluble in #ater. Oxygen is slightly

more dense than air. It su%%orts combustion and life and is %roduced in %hotosynthesis

Chemical $ro%erties of Oxygen

1 Most metals and non-metals burn in air or oxygen to form oxides. Substances that burn in

air burn more readily in oxygen.

(a" Metals form basic oxides& e.g. !Mg(s" ' !(g" -- !MgO(s "

!Ca(s" ' !(g" - !CaO(s" bric)-red flame

brilliant #hite flame

(b" *on-metals form acidic oxides& e.g. S(s" ' !(g" ++, S!(g" large blue flame

C(s" ' !(g" - C!(g" glo#s red hot

! uels such as hydrocarbons (e.g. methane" and alcohol (ethanol" burn to form #ater and

carbon dioxide. Incom%lete combustion yields carbon monoxide rather than carbon dioxide.

Methane CH/(g" ' !!(g" -- C!(g" ' !H!(g"

Ethanol. CH3CH!H(I" ' 3!(g" ++, !C!(g" ' 3H!(g"

3 0mmonia burns at a et in oxygen. /*H3(g" ' 3!(g" -, !*!(g" '2H!(g"If a mixture of ammonia and oxygen is %assed oer heated %latinum a different reaction

occurs

/*H3(g" '4!(g" /*O(g" '2H!(g"5his latter reaction is im%ortant in the %re%aration of nitric acid. 6oth reactions illustrate

ammonia acting as a reducing agent.

7ses of Oxygen

1 In steel ma)ing.

! In oxy-acetylene #elding and cutting.

3 0s a roc)et fuel.

/ 0s an aid to breathing for %atients #ith res%iratory difficulties.

4 In dee% sea diing.

2 or the aeration of se#age in treatment %lant.

8 In %a%er %roduction& for the bleaching of %a%er %ul%.

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53 Speed of Chemical Reactions S3CHEM10.1

It is clear that chemical reactions proceed at very different speeds. The rustin of

iron! the e"plodin of a mi"ture of methane and o"yen! and the action of an acid on a metal

have reatly varyin reaction speeds.

#actors that affect the speed of reaction include$

1 The type of %ond involved.

& The concentration of the reactants.

3 The temperature of the reaction.

' The Surface area of the reactants.

( The presence of catalysts.

) The presence of liht

1 The Type of *ond Involved

Most reactions %et+een ions are very fast.

Example:

,recipitation reactions are fast! e..

Ca 2+( a- /C o l- ( a- CaC032s

*y contrast most reactions %et+een molecular compounds are slo+ due to the time taen in

%reain and main covalent %onds.

Example:

Esterification reactions in +hich alcohols react +ith car%o"ylic acids to form esters are slo+

reactions$

C2H50H / H44CCH3 C2H5000CH3 / H20

Ethanol Ethanoic acid Ethyl ethanoate

The a%ove reaction +hich re-uires the presence of sulphuric acid taes several hours +ith

heatin

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53 Speed of Chemical Reactions 53-CHEM-10.1

2 The Concentration of the Reactants

The speed of a chemical reaction is usually increased y increasin! the concentrations of the

reactants. This e"plains #hy the !reatest speed is oser$ed as soon as the reactants aremi"ed% i.e. #hen they are oth at their hi!hest concentrations. &s the reaction proceeds the

concentrations decrease and the speed of reaction also decreases.

E"planation'

Reaction can only ta(e place #hen molecules collide. )hen the concentration is increased

more collisions per unit time #ill ta(e place and #e predict an increased reaction speed.

Example 1:

The reaction et#een ma!nesium rion and e"cess dilute hydrochloric acid'

2HC1*a+, + M!*s, M!C12*a+, + H2*!,

/f the concentration of the acid is increased from 1 mo l /dm to 2 mol /dm it is found that the

speed of reaction is increased four times. This can e determined y measurin! the rate of

e$olution of hydro!en !as. )hen the !raph of $olume of hydro!en !as produced is plotted

a!ainst time% the speed of reaction can e determined from the !radient.

3 Temperature of Reaction

The speed of a chemical reaction is increased y raisin! the temperature of the reaction.

E"planation'

/ncrease in temperature increases the (inetic ener!y of the reactin! particles. The rate

increases for t#o reasons' firstly there #ill e more collisions per second of the reactin!

particles secondly more ener!y is in$ol$ed in the collisions et#een particles and therefore a

!reater proportion of collisions #ill lead to reaction.or most reactions an increase of 10 doules the reaction speed.

Example 1:

/n an ordinary saucepan food oiled in #ater cannot e coo(ed at temperatures hi!her than

the oilin! point of #ater. Ho#e$er in a pressure coo(er the pressure uild-up ele$ates the

oilin! point of #ater enalin! the food to e coo(ed at hi!her temperatures. This !reatly

speeds up coo(in!% thus sa$in! fuel and time.

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4 Surface Area of Reaction

The speed of reaction invovin! a soid reactant is increased "# increasin! the surface area of

the soid. The surface area of a soid is increased "# "rea$in! it into s%aer pieces or "# usin!

it in po&der for%. 'nti%ate %i(in! of i)uids and !ases aso increases the speed of reaction.

E(panation*

Reaction ta$es pace &hen the surface of the soid reactant is hit "# %ovin! partices of the

other reactant. 'f the surface+is ar!er it &i "e hit "# %ore %ovin! reactant partices per unit

ti%e and the rate of reaction &i "e faster.

Example 1:

Reaction of %a!nesiu% ri""on &ith diute h#drochoric acid*

2HC',a) M!,s // M!C12,a) H2,!

'f a conditions are $ept constant incudin! the en!th of %a!nesiu% ri""on it is found that

reaction is so&er if the ri""on is cut into t&o and the pieces !ued face to face ,thus havin!

the surface area.

Example 2:

Reaction of caciu% car"onate &ith diute h#drochoric acid*

CaC03

,s 2HC',a) // CaC12,a) C2,! H20,

.'f ar!e %ar"e chips ,havin! a reative# s%a surface area are used the reaction rate is

%uch so&er than &ith po&dered i%estone ,havin! a reative# ar!e surface area.

'f suphuric acid is used instead of h#drochoric acid the reaction &i initia# "e rapid "ut &i

soon so& do&n as the %ar"e chips "eco%e coated &ith insou"e caciu% suphate thus

preventin! further reaction.

E(a%pe 3* E(posions

An e(posion is a che%ica reaction that ta$es pace at a ver# rapid rate producin! a ar!e

a%ount of ener!#. E(posions are i$e# to occur &hen reactants are inti%ate# %i(ed e.!.

,a There is dan!er of e(posive co%"ustion in four %is &hen fine po&der is %i(ed &ith air.

," There is dan!er of e(posions in coa %ines &hen coa dust and %ethane %i( &ith air.

hen %ethane ,or 1 M ire da%p is present to the e(tent of 5 percent in air the %i(ture is

e(posive.

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5 The Presence of Catalysts

Many reactions proceed quite slowly when the reactants are mied alone! "ut they can "e

made to ta#e place much more rapidly "y the presence of other su"stances! called catalysts.

$ catalyst is a su"stance which speeds up a chemical reaction without itself "ein% used up

and it remains chemically unchan%ed at the end of the reaction.

Eplanation&

$ catalyst pro'ides an alternati'e easier pathway for the reaction.

Example 1:

Hydro%en peroide decomposes 'ery slowly in aqueous solution&

The rate of reaction can "e determined "y measurin% the rate of e'olution of oy%en %as. The

dia%ram "elow illustrate the effect of addin% small samples (0.1 %) of a num"er of su"stances

to a quantity of hydro%en peroide. *t can "e seen that a num"er of su"stances catalyse the

reaction and that some are "etter than others. Man%anese (*+) oide is a particularly %ood

catalyst for the a"o'e reaction. ,hen it is added to hydro%en peroide! oy%en is produced

with 'i%orous effer'escence.

$ su"stance which slos down a chemical reaction is called a ne%ati'e catalyst or an

inhi"itor.

Example 2:

Enymes are su"stances which catalyse chemical reactions in li'in% or%anisms. Each enyme

is quite specific in its reaction! and wor#s "est at an optimum temperature. ali'a contains the

enyme amylase which is an effecti'e catalyst in the chan%e of starch to su%ar. This chan%e!

#nown as hydrolysis! can "e represented "y the equation&

(CsH1005) n / nH20 nCH120

starch %lucose

Note: Starch is a polymer of glucose

Examples of catalysts used in industry

1 Platinum aids the oidation of ammonia in the manufacture of nitric acid.

2 +anadium(+) oide is used in the contact process of manufacturin% sulphuric acid.

3 4inely di'ided iron (promoted with alumina and potassium oide) is used in the Ha"er

process for the manufacture of ammonia.

6ic#el is used in the hydro%enation of unsaturated fats (e.%. in ma#in% mar%arine).

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5 Silica-aluminum or p latinum catalysts are used in the ' cracking? and reforming of crude

petroleum.

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S3 Speed of Chemical Reactions 53-CHEM-10.4

6 The Presence of i!ht

T"o e#amples "ill ma$e this clear%

E#ample 1%

&f a mi#t're of h(dro!en and chlorine is e#posed to )ri!ht s'nli!ht it e#plodes

H*+!, C&*+!, / *HC1 +!,

o reaction ta$es place in the dar$. &n diff'se li!ht the reaction proceeds slo"l(.

E#ample *%

The s')stit'tion reaction of methane "ith chlorine ta$es place smoothl( in diff'se li!ht ande#plosiel( in s'nli!ht. o reaction occ'rs in dar$ness. The first s')stit'tion step is%

CH4+!, C12+!,2 CH3C&+!, HC1 +!,

Reersi)le Reactions

&n this chapter "e hae not considered ho" catal(sts and chan!es in factors s'ch as

press're concentration and temperat're affect reersi)le reactions. This is an important topic)'t )e(ond the scope of this )oo$. S'ffice it to sa( that the factors a)oe infl'ence )oth the

position of e'ili)ri'm and the speed at "hich e'ili)ri'm is attained in s'ch reactions.

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S3 Acids, Bases and Salts 53-CHEM-11.1

Acids

Most acids are produced when non-metallic oxides, such as, sulphur dioxide or sulphur!"#

oxide, dissol$e in water. Howe$er not all acids are solutions o% non-metallic oxides in water,

e.&. h'drochloric acid is made (' dissol$in& the &as h'dro&en chloride in water.

)eneral *roperties o% Acids

1 +he' are &enerall' colourless.

+he' ha$e a sour taste.

3. +he' chan&e the colour o% indicators, e.&. acids turn litmus %rom (lue to red and

phenolphthalein %rom pin to colourless.

+heir a/ueous solutions conduct electricit', i.e. the' are electrol'tes.

5. +he' react with metals that are more electropositi$e than h'dro&en on the acti$it' series

to &i$e h'dro&en &as plus a salt, e.&. M&s# 0 H2Sa/# 22 M&Sa/# 0 H2

4 +he' react with car(onates and h'dro&en car(onates to 'ield a salt plus car(on dioxide,

e.&.

HC1a/# 0 CaC3s# 22 CaC1a/# 0 H1# 0 C

6.+he' neutrali7e (ases to %orm a salt and water onl', e.&.

HC1 a/# 0 8aHa/#9 8aC"a/# 0 H"#

Explanation %or the properties o% acids

All acids contain the element h'dro&en and their a/ueous solutions conduct electricit'.

+here%ore we deduce that all acids release h'dro&en ions in a/ueous solution and that this is

responsi(le %or their acidic properties.

:e%inition; An acid is asu(stance that releases h'dro&en ions in a/ueous solution.

Examples of the release of hydrogen ions in aqueous solution:

i# H'dro&en chloride; HC"a/# 222 H


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S3 Acids, Bases and Salts 53-CHEM-11.2

Bases and Alkalis

The most common bases are metallic oxides. Most metallic oxides are not solble in !ater.

Bases !hich are solble are kno!n as alkalis. Belo! is a list o" common alkalis#

$a% &a'H# sodim h(droxide or castic soda

$b% )'H# *otassim h(droxide or castic *otash

$c% Ca$'H%2# calcim h(droxide or slaked lime

$d% &H3# ammonia

(e) &a2C+3# sodim carbonate or !ashin soda

eneral ro*erties o" Alkalis

1 The( are enerall( colorless.

2 The( taste bitter.

3The( are sli**er( to toch.

4The( chane the color o" indicators, e.. alkalis trn litms "rom red to ble and

*henol*hthalein "rom colorless to *ink.

5.Their a/eos soltions condct electricit(, t.e, the( are electrol(tes.

0 The( netralie acids to "orm a salt *ls !ater onl(, e..

2&a'H$ a/% H2S+$a/% 44 &a2S+$ a/% 2H2+$6%This reaction is also tre "or insolble bases, e.. C'$s% H,S+$a/% 44 CS+$a/% H2+$6%

Ex*lanation "or the *ro*erties o" alkalis

H(droxide ions $'H-% are "ond in alkaline soltions and i7e them their distincti7e *ro*erties.

Sodim h(droxide releases h(droxide ions in soltion as "ollo!s#

&a'H$s% 44 &a8$a/% 'l$a/%

9e shold note that ammonia and sodim carbonate "orm h(droxide ions as belo!#

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Definition: A base isa substance which releases hydroxide ions in an aqueous solution

or a substance which reacts with an acid to form a salt plus water only. Many bases are

also insoluble in water.

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S3 Acids, Bases and Salts S3CHEM11.3

The pH Scale

The degree of acidity or alkalinity of an aqeos soltion is !easred on a scale called the pH

scale.

The reference point on this scale is the pH of pre "ater "hich has the #ale of $. Sch asoltion is said to %e netral. pH #ales lo"er than $ are acidic& the lo"er the pH the stronger

the acidity. pH #ales greater than $ are alkaline& the higher the pH, the stronger the alkalinity.

Measre!ent of 'H

(1)

13 1*

3 * 5 + 7 8 9 1( 11 1)

< = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

= >

ncreasing acidity

alkalinity

-etral ncreasing

The si!plest "ay to !easre pH is to !ake se of indicators. ni#ersal ndicator "hich is a

!i/tre of se#eral indicators, is particlarly sefl since it changes color gradally fro! red

throgh orange and green to #iolet o#er a "ide pH range. 0or #ery accrate !easre!ent of

pH, pH !eters !ay %e sed.

Strong and eak Acids

A strong acid is one "hich ioni2es co!pletely in "ater.

E/a!ples& hydrochloric acid, nitric acid. A dilte soltion 31 M4 of hydrochloric acid has a

pH of O .

A "eak acid is one "

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