Chapter 11 Chemical Bonds: Formation of Compounds Periods in the Periodic System of elements are called shells; subshells (s, p, d, f…) contain orbitals (s:1, p: 3, d: 5, f: 7), and each orbital can be occupied by 2 electrons with opposite spins. Elements are classified in groups having similar chemical characteristics. Noble gases have completely filled p orbital with 8 e-. 1 st shell 2 nd shell d subshell 4s orbital is populated before 3d. Before we continue with filling 4p, 10 e- must go into 3d. We put them in a loop since we are stalled filling 4 th shell. s p d f his presentation has 8 groups.
Transcript
Chapter 11 Chemical Bonds: Formation of Compounds
Periods in the Periodic System of elements are called shells; subshells (s, p, d, f…) contain orbitals (s:1, p: 3, d: 5, f: 7), and each orbital can be occupied by 2 electrons with opposite spins. Elements are classified in groups having similar chemical characteristics. Noble gases have completely filled p orbital with 8 e-.
1st shell2nd shell
d subshell
4s orbital is populated before 3d. Before we continue with filling 4p, 10 e- must go into 3d. We put them in a loop since we are
stalled filling 4th shell.
s pd
f
This presentation has 8 groups.
Ionization Energy vs. Atomic Number
Ionization energy
… is the energy required to remove an e- from the atom.
It increases left to right across a period…
That is the behavior exactly opposite to that of atomic radii!
and increases bottom to top, up a group.
Atomic radii increase right to left across the period, and top to bottom down the group.
Atomic properties
Chemical BondingA molecule is a collection of atoms bound together. It is considered as an element if all atoms are of the same type (e.g. H2), or a compound if it is made of different atoms.
74 pm
1 pm = 10-12 m
The bond that is formed is called covalent bond. (Co-means partner, valent refers to valence electrons). It always releases energy.
A bond between two atoms of hydrogen will occur spontaneously if the atoms are within a close proximity, i.e. when attractive forces between the nucleus of one atom and the e- of the other overcome repulsive forces among their e-, thus pulling the atoms together.
Filled-shell e- (or core e-) are almost never involved in the bond as they are too close to their own nucleus.
Covalent bond can be presented as any of the following ways:
H : HH H. .
H – H
Remember that the line stands for a pair of e-!
Electrons are shared between two atoms.
In a covalent bond between two hydrogen atoms, each atom apparently has 2 electrons in its shell. That gives each H atom the electronic configuration of the closest noble gas, He.
2 He atoms repel each other and will never form a bond.
Molecules and the Octet Rule
Elements ‘want’ to have the electronic configuration identical to that of noble gas (8e-).When form a molecule, atoms achieve the octet (8e-) by sharing e- with other atoms.
How many electrons an element needs to satisfy the octet rule can be found by observing the Roman numerals in the periodic table.
All atoms in the molecule have the electronic configuration identical to that of the closest noble gas.
Hydrogen is an exception, as it only needs one more electron to fill its 1s orbital.
C: 8e-
H: 2e-
O: 8e-
H: 2e-Nonmetals usually gain electrons, metals loose them.
Core electrons
Valenceelectrons
Drawing Dot DiagramsStep 1: Find the total number of valence e- by adding up the group numbers of all atoms. For ions, adjust the dot count accordingly (subtract e- for cation, add for anion).
assume that the first non-hydrogen atom in the formula of the group is the central atom. Connect atoms with single bonds.
C – O
O – C – O
Step 4: Put in the remaining electrons, two at a time, as lone pairs. Start with the terminal atoms, and continue with the central atom if there are any electron pairs left.
: C – O :. .
. .
. .: O – C – O :. .
. .
. .
Step 5: Check that each atom has octet satisfied (doublet for H). If not, move electron pair(s) from the adjacent atom to form multiple bonds.
: C – O :. .
. .
: C Ξ O :
. .: O – C – O :. .
. .
. .
. .: O – C – O :. .
. .
. .
: O – C Ξ O :. .. .
. .: O Ξ C – O :. .
or
Equivalent structures, or resonance forms.Practice with CO32-, SO3, etc.
The central atom must form multiple bonds, hence H can never be the central atom.
. . . .. . . .O = C = O
++
Step 3: Subtract 2e- for each bond from total #e- to get #e- in lone pairs.
10e – 2e = 8e-
16e – 4e = 12e-
orin lone pairs
in lone pairs
CO32-
CO O
O
4e + (3 x 6e)+ 2e = 24e-
24e – (3 x 2e) = 18e-
CO O
O
: :
: :. .
. .
. .
. .
. .
Octet rule not satisfied for C
CO O
O
: :
: :. .
. . . .
2-
SO3 is identical to this; try it yourself.
C2H4
Which C is the central atom?
BOTH !
CC
HH
H H
2x4e+4x1e=12e- total
12e – (5x2e) = 2e- in lone pairs
CC
HH
H H
:
Octet rule not satisfied for C
CC
HH
H H
CC
ClCl
H H
CC
ClH
Cl H
CC
HCl
Cl H
C2H2Cl2
: : : :
:
::
:: :
: :
. . . .
. .
. . . .
. .
2x4e + 2x1e + 2x7e = 24e-
. .Two more resonance structures for CO3
2- ion are possible.
C cannot pull electrons from Cl
in lone pairs
totaltotal
H has no e- to give in for double bonds
HW chapter 11: 1, 3, 13, 29, 37
Electronegativity (EN) is numerical rating of an atoms ability to attract to itself the shared electrons in a covalent bond. Generally, electronegativity of metals is low, and that of nonmetals is high.The least electronegative atom (except H!) is the central atom in dot structures.
Polar covalent bond is a covalent bond in which e- are shared unequally (large EN).
A partial negative charge (-) occurs on the more EN atom. A partial positive charge (+) occurs on the less EN atom.
Ionic and covalent are two extremes at the ends of a continuum bonding types.
EN = 0, covalent; EN = 1.0, polar covalent (23% ionic); EN = 1.9, polar covalent (60% ionic); EN > 1.9, ionic.
The bond between metals and non-metals is usually ionic. Metals give away their e- and become positively charged (cations). Nonmetals accept them and become anions. The ionic bond is formed as a result of attraction between oppositely charged ions. The compound is called ionic compound, and the three-dimensional ordered network of the ions is called ionic lattice.
Electrons are rarely shared equally between atoms.
The difference in EN defines the bond.
Electronegativity and the Polar Covalent Bond
The Shape of the Molecules
Valence Shell Electron Pair Repulsion (VSEPR) theory is the model mostly used to predict molecular shape.
Electron pairs on the central atom repel one another.
H |H – C – H | H
The two dimensional dot structure of methane, CH4. gives the angles between electron pairs of 90o. But the dot structure angles are arbitrary. Molecules are three dimensional, and the electron pairs would be further away if the third dimension is considered. In fact, the shape of methane molecule is tetrahedral; the bond angles between electron pairs is 109.5o.
Four electron pairs around an atom assume tetrahedral arrangement.
When there are not enough electrons for single bonds the molecule forms multiple bonds and the structure differs. VSEPR theory treats each multiple bond as a single electron group, because it occupies roughly the same region of space. The number of electron groups around an atom is called the atom’s steric number (SN).
: O : | |H – C – H
H H| |C Ξ C
: O : | | C
H H
H – C Ξ C – H
formaldehyde acetylene
Dot structures True geometry
Dot structures of formaldehyde and acetylene are arbitrarily shown with angles of 90o.
Their true geometry has bond angles of 120o and 180o, respectively.
VSEPR arrangements of electron groups around an atom having no lone pair electrons
If the central atom has a lone pair of electrons, that electron pair is included in the molecular shape.
. .H – N – H | H
The steric number on nitrogen is 4 (3 bonding pairs and a lone pair). The e- pairs on the N assume tetrahedral arrangement.
The dot diagram of ammonia presents the atom in a plane.
Electrons in the lone pair occupy more space than the bonding pairs. They squeeze H-N-H bond angle to approx. 107o.
The molecule of NH3 has a pyramidal shape.
Rule of thumb: each lone pair of e- on a period-2 atom compresses the remaining bond angles around that atom by ~2o.
We describe the shape of the molecule, not that of its electrons. Lone pair(s) of electrons are therefore ignored.
O = C = O
Using VSEPR
1. Draw a dot diagram.2. Count the number of e- pairs around the central atom, including lone pairs (i.e. the steric number, SN). A multiple bond counts as a single e- group.3. Find the best arrangement of the electrons using SN. 4. Pretending the lone e- pairs are invisible, describe the resulting shape of the molecule.
Practice on H2O and O3.
H|O
H
O| |O
O
H-O-Hbond angle ~105o.
O-O-O bond angle 120o. SN=4 SN=3
:: :
bent
bent
Chapter 13
Liquids have intermediate properties between solids and gases. Liquids are almost incompressible, have definite volume and assume the shape of the container.
Water and Properties of Liquids
Evaporation or vaporization is the escape of molecules from liquid into gaseous state. During evaporation, liquid that stays behind is cooler. The opposite process is condensation.
Sublimation is the escape of molecules directly from solid into gas, bypassing liquid state.Vapor pressure is the pressure exerted by a gas at equilibrium with its liquid, so that: evaporation
liquid gas condensationVapor pressure depends only on
temperature, not on the amount of liquid.
Open container completely evaporates.Closed container reaches equilibrium between liquid and gas.
Densities of liquids are usually lower than that of their solids. Water is an exception.
Vapor Pressure Measurement
Manometer attached to the flask shows equal pressure in both legs.
a. The system is evacuated.
20 oC
a.
20 oC
b.
b. Water is added.Liquid evaporates.Manometer shows increase in pressure.
20 oC
c.
c. Equilibrium established.Manometer shows constant pressure difference, 17.5 torr.
d. Temperature raised to 30 oC.Equilibrium reestablished.Manometer shows constant pressure difference of 31.8 torr.
d.
30 oC
1 atm = 760 torr
Vapor pressure of ethyl ether is the highest at any temp.
Vapor pressure:Ether > Alc. > Water.
Rate of evaporation:Ether > Alc. > Water.proportional to vapor pressure.
Substances that readily evaporate are volatile.
Vapor pressure of ethyl ether at 20 oC: 442.2 torr
Vapor pressure of water at 20 oC: 17.5 torrVapor pressure of mercury at 20 oC: 0.0012 torr
Volatile
Moderately volatile
Nonvolatile
Volatility
Vapor pressureand temperature
Vapor pressure of any gas at the boiling point is equal to the atmospheric pressure.
TBP
TBP
TBP
Ether < Alc. < WaterBoiling point:
1 atmosphere pressure
TBP alcohol
78.4oC
Each point on the curve represents a vapor-liquid equilibrium at a particular temperature and pressure.
TBP water
100.0oC
TBP ethyl ether
34.6oC
Boiling point at standard pressure (1 atm, or 760 torr).
At 500 torr, ethyl ether boils at ~22 oC, alcohol at ~68 oC, and water at 89 oC.
Normal Boiling Point
Freezing or Melting Point
The temperature at which the solid and liquid are in equilibrium.
meltingsolid liquid
freezing
evaporationliquid gas
condensation
Changes of State
Majority of substances change phases upon heating: solid liquid gas.CO2 is an exception (dry ice sublimes).
Heating curve for a pure substance
Boiling Point Curves
A – B: solid state B – C: meltingC – D: liquid state D – E: evaporationE – F: vapor state
Temperature is constant during melting and boiling – all heat used to break solid (at boiling point) or liquid forces.
Heat of Fusion and Heat of Vaporization
Energy (heat) needed to change 1 g of a solid at its melting point into liquid is heat of fusion.
Energy (heat) needed to change 1 g of a liquid at its boiling point into vapor is heat of vaporization.
Example 1: How many joules is needed to change 20.0 g of ice at 0 oC to steam at 100. oC? Qtot = Qfusion + Qheating + Qvaporization
Qfusion = (20.0 g) x (335 J/g)
Qheating = (20.0 g) x (4.184 J/goC) x (100. oC)Qvaporization = (20.0 g) x (2260 J/g)
Qfusion = (mass) (spec.heat of fusion) Qvaporization = (mass) (spec.heat of vaporization)
Qheating = (mass) (spec.heat) (temp.change)
Qtot = 60.3 kJ}Hydrogen Bond produces unusually high melting & boiling point
Qheating = (mass) (spec.heat) (temp.change)We learned before that amount of heat depends on mass and temp. change.
Constant temperature!
H bonding exists between H directly bonded to one of the three most electronegative elements (Fluorine, Oxygen, and Nitrogen), and F, O or N of another molecule.
Hydrogen Bonding (cont.)
H – C – O – C - H| . . |
| . . |
H H
H H
H – O :. .
|H
. . .
H
H – O :|
. .H bond No H bond
H bonded
to O
No H bonded to F, O, or N
Surface Tension and Capillary ActionA droplet of liquid falling forms a sphere due to attractions to other liquid molecules – surface tension.Spontaneous rise of liquid in a narrow tube – capillary action.
Cohesive forces within mercury liquid (left) are stronger than adhesive forces between Hg and walls of the container. Opposite is true for H2O.
H bonds are intermolecular forces.
Ethyl ether
HydratesSome ionic solutions retain water upon evaporation. It becomes the part of the crystalline compound – water of crystallization.
The formula is written as: ionic compound, dot , # water molecules…CuSO4
. 5 H2O and name them by adding # (Latin) hydrate.
Copper(II) sulfate pentahydrate. Hydrates are true compounds and the water is an integral part of it.
Formula mass CuSO4 . 5 H2O: 63.55+32.07+64.00+5x18.02 = 249.7
Percent composition of water is (5x18.02 / 249.7) x 100 = 36.08%
dry CuSO4 – whiteHydrate = blueWater indicator
Water can be removed by intense heat:The reaction is reversed when water is added.
Water, a Unique Liquid
Water covers ~75% of Earth. 97% of water is in the oceans. Only 3% is fresh water, of which 2/3 is locked up in ice polar caps.
. .
. .
OH H
+
-
Water is very stable molecule, can stand temperatures up to 2000 oC. It does not conduct electricity when pure, but decomposes into H2 and O2 in solutions of ions.
Water can be formed by Combustion, Neutralization,Metabolic reaction
CuSO4 . 5 H2O(s) CuSO4(s) + 5 H2O(g)
Solid form (ice) has lower density than liquid water.
Water reactions with metals:Cold water reacts with Na, K, Ca: Na + H2O H2 + NaOH
Steam reacts with Zn, Al and Fe: Fe + H2O(g) --> H2 + Fe3O4 Remind yourself of the activity series: the above six metals are the most active. Another three metals are more active than H: Pb, Sn, and Ni and react with acids only; Cu, Ag, Hg and Au are below H in the series and do not react with acids or H2O.
Water also reacts with certain nonmetals. Most reactive: 2 F2 + 2 H2O(l) --> 4 HF(aq) + O2
Less reactive: Cl2(g) + H2O(l) HCl(aq) + HOCl(aq)
Least reactive: C(s) + H2O(g) CO(g) + H2(g)
Water reacts with metal and non-metal oxides:Basic anhydride: CaO + H2O(l) Ca(OH)2(aq)
Acid anhydride: CO2(g) + H2O(l) H2CO3(l)
Reactions of water
Anhydride means: without water.
To test whether a metal or nonmetal is an anhydride, try to remove H2O until all hydrogen is removed.
CaOH
OH
CaO + H2O
H2SO4 SO3 + H2OWater Purification
Screening, flocculation and sedimentation, sand filtration, aeration, disinfection.Hard water contains Mg2+ and Ca2+ ionsAdditional water purification is done by distillation, Ca2+, Mg2+ precipitation, ion exchange and demineralization.
