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S-Block Elements

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S-Block Elements. ALKALI METALS. Chapter summary. Introduction Characteristic properties of the s-block elements . Variation in properties of the s-block elements of the First Group(Alkali Metals). Physical Properties. IA IIA. Li. Be. Mg. Na. Ca. K. Sr. Rb. Cs. Ba. Ra. Fr. - PowerPoint PPT Presentation
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S-Block Elements ALKALI METALS
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Page 1: S-Block Elements

S-Block Elements

ALKALI METALS

Page 2: S-Block Elements

Chapter summary

• Introduction• Characteristic properties of the s-block

elements .• Variation in properties of the s-block

elementsof the First Group(Alkali Metals). • Physical Properties

Page 3: S-Block Elements

Members of the s-Block Elements

Li Be

Na

K

Rb

Cs

Fr

Mg

Ca

Sr

Ra

Ba

IA IIA

IA Alkali metals

IIA Alkaline Earth metals

Page 4: S-Block Elements

Characteristic properties of s-block elements

• Metallic character• Low electronegativity• Basic oxides, hydroxides• Ionic bond with fixed oxidation states• Characteristic flame colours• Weak tendency to from complex

Page 5: S-Block Elements

Metallic character

• High tendency to lose e- to form positive ions

• Metallic character increases down both groups

Page 6: S-Block Elements

Electronegativity

• Low nuclear attraction for outer electrons

• Highly electropositive

• Small electronegativity

Group I Group II

Li 1.0 Be 1.5

Na 0.9 Mg 1.2

K 0.8 Ca 1.0

Rb 0.8 Sr 1.0

Cs 0.7 Ba 0.9

Fr 0.7 Ra 0.9

Page 7: S-Block Elements

Characteristic flame colours

Na+ Cl- (g) Na (g) + Cl (g)Na(g) Na* (g)[Ne]3s1 [Ne]3p1

Na*(g) Na(g) + h (589nm, yellow)

Page 8: S-Block Elements

Flame test

HCl(aq) sample

Li deep redNa yellowK lilacRb bluish redCs blue

Ca brick redSr blood redBa apple green

Page 9: S-Block Elements

Variation in properties of elements

• Atomic radii• Ionization enthalpies• Hydration enthalpies• Melting points• Reactions with oxygen, water, hydrogen and

chlorine

Page 10: S-Block Elements

Atomic radii (nm)

Li 0.152 Be 0.112

Na 0.186 Mg 0.160

K 0.231 Ca 0.197

Rb 0.244 Sr 0.215

Cs 0.262 Ba 0.217

Fr 0.270 Ra 0.220

Li

Fr

Be

Ra

Page 11: S-Block Elements

Ionization Enthapy

Group I 1st I.E. 2nd I.E.

Li 519 7300

Na 494 4560

K 418 3070

Rb 402 2370

Cs 376 2420

Group I 1st I.E. 2nd I.E. 3rd I.E.

Be 900 1760 14800

Mg 736 1450 7740

Ca 590 1150 4940

Sr 548 1060 4120

Ba 502 966 3390

Page 12: S-Block Elements

Ionization Enthalpy

Li Na

KRb

Cs

1st I.E.

300

400

500

600

500

1000

1500

2000

Be

CaBa

Be+

Ca+

Ba+

1st IE

2nd IE

Page 13: S-Block Elements

Ionization Enthalpy

Group I

1. Have generally low 1st I.E. as it is well shielded from the nucleus by inner shells.

2. Removal of a 2nd electron is much more difficult because it involves the removal of inner shell electron.

3. I.E. decreases as the group is descended. As atomic radius increases, the outer e is further

away from the well-shielded nucleus.

Page 14: S-Block Elements

Hydration Enthalpy

M+(g) + aqueous M+(aq) + heat

M+

-600

-300

Li+ Na+ K+ Rb+ Cs+

Page 15: S-Block Elements

Hydration Enthalpy

-600

-300

Li+ Na+ K+ Rb+ Cs+ Be2+ Mg2+ Ca2+ Sr2+ Ba2+

-2250

-2000

-1750

-1500

Page 16: S-Block Elements

Hydration Enthalpy

General trends:1. On going down both groups, hydration enthalpy decreases. (As the ions get larger, the charge density of the ions decreases, the electrostatic attraction between ions and water molecules gets smaller.)

2. Group 2 ions have hydration enthalpies higher than group 1. ( Group 2 cations are doubly charged and have smaller sizes)

Page 17: S-Block Elements

Ionization Energy

• Amount of energy required to remove an electron from the ground state of a gaseous atom or ion.– First ionization energy is that energy required to

remove first electron.– Second ionization energy is that energy required

to remove second electron, etc.

Page 18: S-Block Elements

Trends in First Ionization Energies

• As one goes down a column, less energy is required to remove the first electron.– For atoms in the same

group, Zeff is essentially the same, but the valence electrons are farther from the nucleus.

Page 19: S-Block Elements

Ionization Enthalpy of Alkali Metals

• ionization enthalpy decreases down the group from Li to cs because as we move down a group the number of valence electrons goes increasing separating the electrons away from the nucleus ,there is an increasing shielding of the nuclear charge by the inner shell electrons and thus the removal of electrons requires less energy as we move down.

Page 20: S-Block Elements

Physical Properties

• Silvery White• Soft & Light Metals• Due to large size , elements have low density• Low Melting & Boling Points indicates weak

bonding due to presence of only 1 valence electron .

Page 21: S-Block Elements

• In order of increasing atomic number the alkali metals are:

• Lithium•Sodium

•Potassium•Rubidium •Caesium•Francium

Increasing atomic number

Page 22: S-Block Elements

All alkali (group 1) metals react violently with water, forming Hydrogen gas and Hydroxides (pH above 7):

Alkali metals are:•Metals found in group 1 of the periodic table.•Soft when cut (compared to other metals).•Metals with low melting points and densities.•Powerful reducing agent and form univalent compounds.•Metals which tarnish in air.

Page 23: S-Block Elements

E.g.Li2ONa2O, Na2O2

K2O2, KO2

Rb2O2, RbO2

Cs2O2, CsO2

On cumbustion in excess of air, alkali metals form

1.Oxides- LiO2

2. Peroxides- Li2O2, NaO2

3. Superoxides- K2O2 ,Cs2O2 RbO2

Page 24: S-Block Elements

E.g.LiOHNaOHKOHRbOHCsOH

The oxides of the alkali metal are easily hydrolysed by addition of water.

E.g.:- LiO2 + H2O → LiOH

Page 25: S-Block Elements

The alkali metals combine directly with halogens under appropriate conditions forming halides of general formula MX.

These halides can also be prepared by the action of aqueous halogen acids (HX) on metals oxides, hydroxides or carbonate.

All these halides are colourless, high melting crystalline solids having high negative enthalpies of formation.

Page 26: S-Block Elements

M2O + 2HX → 2MX + H2O            MOH + HX → MX + H2O        M2CO3 + 2HX → 2MX + CO2 + H2O (M = Li, Na, K, Rb or Cs)

(X = F, Cl, Br or I)

Page 27: S-Block Elements

Since the alkali metals are highly electropositive, therefore their hydroxides are very strong bases and hence they form salts with all oxoacids.

( H2CO3, H3PO4, H2SO4, HNO3, HNO2 etc) .

They are generally soluble in water and stable towards heat.

Page 28: S-Block Elements

The carbonates (M2CO3)  of alkali metals are remarkably stable upto 1273 K, above which they first melt and then eventually decompose to form oxides.

Li2CO3 , however is considerably less stable and decomposes readily.

Δ          Li2CO3 → Li2O + CO2

This is presumably due to large size difference between Li+ and CO2-

3  which makes the crystal lattice unstable.

Page 29: S-Block Elements

SODIUM CHLORIDE

The most abundant source of sodium chloride is sea water. Crude sodium chloride, generally obtained by crystallisation of brine solution, contains sodium sulphate, calcium sulphate, calcium chloride and magnesium chloride as impurities. Calcium chloride,CaCl2, and magnesium chloride MgCl2 are impurities because they are deliquescent(absorb moisture from the atmosphere).

Page 30: S-Block Elements

To obtain pure sodium chloride–Crude salt is dissolved in minimum amount of water and filtered to remove insoluble impurities.The solution is then saturated with hydrogen chloride gas.Crystals of pure sodium separate out.Calcium and magnesium chloride, being more soluble than sodium chloride, remains in solution.

Page 31: S-Block Elements

USES OF NaCl −1) It is used as a common salt or

table salt for domestic purpose.

2) It is used for the preparation of Na2O2 , NaOH and Na2CO3.

Page 32: S-Block Elements

SODIUM HYDROXIDE(CAUSTIC SODA),

NaOHSodium hydroxide is generally prepared commercially by the electrolysis of sodium

chloride in Castner −Kellner cell.A brine soln. is electrolysed using a mercury cathode and a carbon anode. Sodium metal discharged at the cathode combines with

mercury to form sodium amalgam. Chlorine gas is evolved at the anode.

Page 33: S-Block Elements

NaCl → Na+ + Cl¯

AT ANODE: Cl ¯ ─ e¯ → Cl Cl + Cl → Cl2

AT CATHODE: Na+ + e ¯ → Na+Na + Hg → NaHg Amalgam

The amalgam is treated with water to give sodium hydroxide, mercury and hydrogen gas.

2NaHg + 2H2O→ 2NaOH + 2Hg + H2

Page 34: S-Block Elements

PROPERTIES-1. NaOH is a white, translucent solid and it melts at 591K.2. it is readily soluble in water to give alkaline solution.3. Crystals of NaOH are deliquescent. It reacts with the CO2 in the atmosphere to form Na2CO3.

USES- The manufacture of soap, paper and no. of chemicals.In petroleum refiningIn textile industry for mercerising cotton fabricsAs laboratory reagentFor preparation of pure oils and fats

Page 35: S-Block Elements

SODIUM HYDROGENCARBONATE(BAKIN

G SODA), NaHCO3

Sodium hydrogencarbonate is known as baking soda because it decomposes on heating to generate

bubbles of carbon dioxide. It is made by saturating a solution of sodium

carbonate with carbon dioxide. The white crystalline powder of sodium hydrogencarbonate , being less

soluble, gets separated out.Na2CO3 + H2O + CO2 → 2NaHCO3

NaHCO3 is a mild antiseptic for skin infections. It is used in fire extinguishers.

Page 36: S-Block Elements

BIOLOGICAL IMPORTANCE OF SODIUM AND

POTASSIUMA typical 70 kg man contains about 90g of Na and 170g of K

compared with only 5g of iron and 0.06g of copper.Potassium ions are present in higher concentration inside the cells than sodium ions and they are present outside the cell in

blood plasma.Because of large concentration gradient inside and outside the cells, the transport of sodium ion into the cells is favoured. To

pump out these ions again from the cell to maintain concentration gradient large driving force is carried out. The

energy for this process is provided by ATP molecules.Thus both sodium and potassium ions are essential for living

organisms.

Page 37: S-Block Elements

Anomalous properties of Lithium

Page 38: S-Block Elements

LithiumSymbol – LiAtomic no. - 3Atomic Weight – 6.94uElectronic Configuration - 1s22s1

Group no. – 1Period no. – 2Group name – Alkali MetalsBlock name – ‘s’Standard State(298 K)- SolidColor – Silvery-white/grey Classification - Metallic

Page 39: S-Block Elements

Anomalous Properties

• High melting & boiling point.• Much harder than other alkali metals.• Reacts with oxygen least readily to form normal

oxide(E.g. Li2O), whereas other alkali metals form peroxides and superoxides(E.g. MO2,M2O2).

4Li + O2 → 2Li2O• Unlike other alkali metals lithium reacts directly

with carbon to form an ionic carbide.

Page 40: S-Block Elements

• The carbonates, hydroxides and nitrates of lithium decompose on heating unlike those of other alkali metals which are somewhat stable towards heat.

4LiNO3 → 2Li2O + 4NO2 + O2

2LiCO3 → 2Li2O + CO2

2LiOH → Li2O + H2O

• LiOH is a weaker base than hydroxides of other alkali metals.

• Unlike elements of group 1, Lithium forms nitride with nitrogen.

3Li + N → Li3N

• Lithium halides are have more covalent nature than halides of other members of group 1.

Page 41: S-Block Elements

Li +

Li +

Li +

Li +

Due to appreciable covalent nature, the halides and alkyls of lithium are soluble in organic solvents.

Li + has very high hydration energy and charge/radius ratio, therefore it acts as an excellent reducing agent in solution.

Page 42: S-Block Elements

• Small size of atom results in relatively high cohesive properties associated with relatively strong inter-metallic bonding; large atoms usually form weak bonds.

Page 43: S-Block Elements

Diagonal Relationship

The properties of lithium are quite different from the properties of other alkali metals. On the other hand, it shows greater resemblance with magnesium, which is diagonally opposite element of it.

Similarly properties of Beryllium & Boron represent that of Aluminium & Silicon respectively.

The main reasons for the anomalous behavior of lithium are -:

B C

Page 44: S-Block Elements

The Reasons -: (i) The extremely small size of Lithium

& its ion. (ii) Greater polarizing power of

lithium ion ( Li+), due to its small size which result in the covalent character in

its compounds. (iii) Least electropositive character

and highest ionization energy as compared to other alkali metals.

(iv) Non availability of vacant d-orbitals in the valence shell.

Page 45: S-Block Elements

Some More ExamplesExamples For Diagonal Relationship

• Li and Mg form only normal oxides whereas Na forms peroxide and metals below Na, in addition, forms superoxide.

• Li is the only Group 1 element which forms nitride, (Li3N). Mg, as well as other Group 2 elements, also form nitride.

• Lithium carbonate, phosphate and fluoride are sparingly soluble in water. The corresponding Group 2 salts are insoluble. (Think lattice and solvation energies).

• Both Li and Mg form covalent organometallic compounds. LiMe and MgMe2 (of Grignard reagents) are both valuable synthetic reagents. The other Group 1 and Group 2 analogues are ionic and extremely reactive (and hence difficult to manipulate).

Page 46: S-Block Elements

IONIZATION ENTHALPY.

THE REACTIVITY OF THESE METALS INCREASES DOWN THE GROUP.

THE ALKALI METALS ARE HIGHLY REACTIVE. CAUSINGCONTRIBUTING FACTORS ARE LARGE SIZE AND LOW

Page 47: S-Block Elements
Page 48: S-Block Elements

1. ALKALI METALS TARNISH IN DRY AIR DUE TO FORMATION OF THEIR OXIDES.

2. THEY BURN IN OXYGEN VIGOURSLY.

3. THEY REACT WITH MOISTURE FORMING HYDROXIDES.

Page 49: S-Block Elements

EX-4LI+O2 2LI2O

2NA+O2 NA2 O2

M+O2 MO2

OXIDATION STATE- +1

Page 50: S-Block Elements

LITHUIM IS AN EXEPTIONREACTING DIRECTLY WITH NITROGEN OF AIR TO FORM THE NITRIDE.

DUE TO THEIR HIGH REACTIVITY TO AIR AND WATER THEY ARE KEPT IN KEROSENE OIL.

Page 51: S-Block Elements

2. REACTIVITY TOWARDS WATER-2M+2H2O 2M+2OH+H2

THE ALKALI METALS REACT WITH H2O TO FORM HYDROXIDE AND H2.REACTION WITH H2O IS EXPLOSIVE FOR ALKALI METALS.

Page 52: S-Block Elements

3. REACTIVITY TOWARDS HALOGENS- THE ALKALI METALS READILY REACT VIGOURSLY WITH HALOGENS TO FORM IONIC HALIDES,M+X-.LITHUIM HALIDES ARE CONVALENT DUE TO POLARISATION.

Page 53: S-Block Elements

5. REDUCING NATURELITHUIM-STRONGESTSODIUM-LEAST POWERFUL REDUCING AGENT.

A) M - M (SUBLIMATION ENTHALPY)B) M=+M+E- (IONISATION ENTHALPY)C)+M+H2O=M+ (HYDRATION ENTHALPY)

Page 54: S-Block Elements

THE ALKALI METALS DISSOLVE IN LIQUID AMMONIA GIVING DEEP BLUE SOLUTIONS - CONDUCTING IN NATURE.THE PRODUCTS OF THIS REACTION ARE -

Page 55: S-Block Elements

1.HYDROGEN.2. AMIDE ION.

THIS REACTION TAKES PLACE AT SLIGHTLY ELEVATED TEMPERATURES OR IN THE PRESENCE OF CATALYST.WHEN LIQUID AMMONIA IS EXPOSED TO LIGHT OF

Page 56: S-Block Elements

SPECTRA REGION OF UV LIGHT THERE IS NO OBSERVABLE CHANGE.

WHEN METTALIC SOLUTION ARE SO EXPOSED,REACTION OCCURS.THIS REACTION IN PRESENCE OF UV IS COMPLETE PHOTOCHEMICAL REACTION. REACTION-

Page 57: S-Block Elements

+M + E- + NH3 MNH2 +1/2H2

THE BLUE COLUR IS DUE TO THE AMMONIATED ELECTRON WHICH ABSORBS ENERGY IN VISIBLE REGION OF LIGHT IMPARTING BLUE COLOUR TO SOLUTION. THE SOLUTION IS PARAMAGNETIC.

Page 58: S-Block Elements

IN CONCENTRATED SOLUTION THE BLUE COLOUR CHANGES TO BRONZE COLOUR AND BECOME DIMAGNETIC.

Page 59: S-Block Elements

1.LITHUIM IS USED TO MAKE ALLOYS.EX-WITH LEAD TO MAKE WHITE METAL,BEARINGS FOR MOTORS ENGINES,WITH ALUMINIUM TO MAKE AIRCRAFTS PARTS AND WITH MAGNESIUM TO MAKE ARMOUR PLATES.

LITHUIM IS ALSO USED TO MAKE ELECTROCHEMICAL CELLS.

Page 60: S-Block Elements

2. SODIUM IS USED TO MAKE A Na or Pb ALLOY NEEDED TO MAKE PbMe4. used as anti-knock additives to petrol.

Liquid sodium metal is used as a coolant in fast breeder nuclear REACTORS. 3. POTASSIUM HAS A VITAL ROLE IN BIOLOGICAL SYSTEMS.

Page 61: S-Block Elements

POTASSIUM CHLORIDE IS USED AS A FERTILISER.

POTASSIUM HYDROXIDE IS USED AS AN EXCELLENT ABSORBENT OF CARBON DIOXIDE.

4.CAESIUM IS USED IN DEVISING PHOTOELECTRIC CELLS..


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