S-Block Elements

Li

Na

K

Rb

Cs

Fr - Radiaoactive

Be

Mg

Ca

Sr

Ba

Ra - Radioactive

S-Block

S-Block elements are those in which the last e- enters the outermost s-orbital

The first element of alkali & alkaline earth metals differs in many respects from the other group elements

S-BLOCK ELEMENTS

Group 1 (1s1) Group 2 (1s2)

Alkali Metals - named so as their hydroxides and oxides are alkaline in nature

Alkaline Earth metals - named so as the oxides and hydroxides are alkaline in nature & found in the earths crust

ALKALI METALS Properties Trend

Electronic Configuration [Noble Gas] ns1

Atomic & Ionic Radii As expected both increases down the group Size of M+ < size of M

Ionization enthalpy ** As expected it decrease down the group ** Values are considerably low in comparison to other elements of the periodic table as the metals are highly electropositive in nature

Hydration enthalpy Alkali metals

is the energy released when an ion gets hydrated in H2O. Generally smaller the ion more is the hydration enthalpy Decreases down the group as the size increases

Density Increases down the group (except in K, where Na>K)

Flame test Imparts characteristic colors to the flame

General features Silvery white, soft, light , low MP and BP (indicating weak metallic bonding)

Alkali Metals-Chemical properties

General points:

High reactivity of alkali metals is due to their large size and low ionization enthalpy

Due to the increase in size down the group, reactivity also increases

Oxides Hydroxides

A smaller cation can stabilize a smaller anion while a larger cation can stabilize a larger anion - So, the formation of Li2O, Na2O2, KO2

Reaction with H2O becomes increasingly violent down the group

Oxides & peroxides are colorless. superoxides are yellow/orange in color & paramagnetic

Alkali metal hydroxides are the strongest of all the bases & among them basic strength increases from LiOH ----> CsOH (strongest base)

Li

Na

K

Rb

Cs

+ O2

LiO2 (oxide)

Na2O2 (peroxide)

KO2

RbO2 superoxide

CsO2

N2 (air) Li3N

H2 + Hydroxides +H2O

H2O

1. Reactivity towards air (mainly O2) & H2O

2. Reactivity towards Halogens

M + X2 ------> MX Order of reactivity of M - X2

Li < Na < K < Rb < Cs F2 > Cl2 > Br2 > I2

Except LiX all MX are ionic in nature

Due to the high polarisation capacity of Li+ ion

Polarisation capacity of cation: The capacity of the cation to distort the e- cloud of anion. Smaller the cation, more is the polarisation

Polarisability of anion: The capacity of the anion to get distorted by the cation. Larger the anion, more is the polarisability

Higher the Polarisation & Polarisability, more would be the Covalent character

M2O/MOH/M2CO3 + HX (aq) ----> MX + H2O

All the MX have negative enthalpy of formation (-Hf)

Solubility: halides are water soluble except LiF & CsI which are sparingly soluble. LiF - Due to high lattice energy CsI Smaller hydration enthalpy Li halides being covalent in nature, soluble in organic solvents

MP & BP : MF > MCl > MBr > MI

3. With liq.NH3 & H2

[M(NH3)x]+ + [e(NH3)y]

-(x + y) NH3 (l)

on standing

MNH2 + H2

Deep blue

amides

M + solution in liq. NH3 which isparamagnetic & conducting

3M

H2

MH

Hydrides(Ionic solids)

3. With Oxo-Acids ---> Salts Oxo-acids: are those in which the acidic H+ is on a hydroxyl group

with an oxo (C=O, S=O) group attached to the same atom. Eg:

carbonic acid

CHO

O

OH

H2CO3 H2SO4sulphuric acid

S

O

O

HO OH

M + oxo-acids Oxo-salts ( carbonates - M2CO3

bicarbonates - MHCO3

sulphates - M2SO4 )

Soluble in H2O

Except Li2CO3 (which decomposes on heating to Li2O) and LiHCO3 (which does not exist as a solid at RT) all other carbonates and hydrogen carbonates are thermally stable and their stability increases down the group due to the increase in electropositive character

Reducing property of Alkali Metals

Strong reducing agents

Reducing property is measured by Standard electrode potential Value

Smaller the value, greater will be the reducing character

for Li is less compared to other metals due to its high hydration enthalpy, so is the most powerful reducing agent

E

E

E

E is the overall change of

M (s) M(g) M+ (g) + e-

M+ (aq)

H2OSublimationenthalpy

Ionizationenthalpy

Hydrationenthalpy

Anomalous properties of Li

Anomalous properties are due to

1. Small size of Li atom & ion

2. High polarising power

3. Non-availability of d-orbitals

4. Diagonal relationship with Mg

Difference b/w Li & other Alkali Metals

1. Comparatively harder and having high MP & BP

2. Li is comparatively less reactive. But, coming to the reducing property it is the strongest reducing agent

3. It is the alkali metal on reaction with N2 (atm) forms LiN3 4. LiCl is the only alkali metal chloride which crystallizes in hydrate form i. e.

LiCl. 2H2O

5. Li unlike other alkali metals forms no ethynide on reaction with ethyne

6. Lithium nitrate decomposes in a different fashion

7. Salts of Li are covalent in nature

HC CHLi + LiC CLiXMNO3 MNO2 + O2

M = Na, K, Cs, Rb

Li2O + NO2 + O2

M = Li

Diagonal relationship b/w Li & Mg

Relationship is due to their comparable sizes i. e.

Similarities:

1. Both are harder and lighter than other elements in the group

2. Reaction towards H2O is slow in both the cases

3. Hydroxides of both the compounds decomposes on heating

4. With N2, they form nitrides (LiN3, MgN3)

5. Carbonates of Li & Mg decomposes on heating to give oxides + CO2 6. Solid hydrogen carbonates are not formed by Li & Mg

7. Solubility properties of their compounds are similar

8. Both LiCl and MgCl2 crystallizes in hydrate form ( LiCl.2H2O , MgCl2. 8H2O )

Li Mg

Atomic Radii 152pm 160pm

Ionic radii 76pm 72pm

Industrially important compounds of Na

1. Sodium Carbonate (Na2CO3.10H2O) Washing Soda:

Preparation: by Solvay process

Principle: Low solubility of NaHCO3 allows its precipitation from the solution. NaHCO3 thus obtained is heated to get Na2CO3

Process: ** pass CO2 to a concentrated solution of NaCl saturated with ammonia which gives NaHCO3 ** Heat NaHCO3 to get Na2CO3

[Note: Solvay process cant be extended to prepare K2CO3 bcoz KHCO3 is highly soluble]

2 NH3 + H2O + CO2 (NH4)2CO3H2O + CO2 2 NH4HCO3

NaCl

NaHCO3 (ppt) + NH4ClNa2CO3 + CO2 +H2OCa(OH)2

2 NH3 + CaCl2 + H2O

NH3-recovered

contd Properties:

It hydrolyses in H2O giving alkaline solution of NaOH

2 . Sodium Hydrogencarbonate (NaHCO3) Baking Soda:

Preparation: by saturating the solution of Na2CO3 with CO2. NaHCO3 being less soluble precipitates out

Na2CO3 + 2 H2O 2 NaOH + H2CO3

Na2CO3 .10H2O375K

Na2CO3.H2O + 9H2O>375K

Na2CO3 + H2O

Soda ashWashing soda

Na2CO3 + CO2 +H2O 2 NaHCO3

Na2CO3 + CO2 +H2ONaHCO3

Evolution of this gas leaves bubblesin baking items making it light

Industrially important compounds of Na

Properties 3. NaCl 4. NaOH (Caustic soda)

MP 808 o C (1081 K) 318 o C (591 K)

Preparation By the evaporation of sea water

Electrolysis of brine solution in Castner-Kellner cell

Procedure Impurities: CaCl2, MgCl2, CaSO4, Crude salt is dissolved in minimum amount of H2O & filtered to remove insoluble impurities Saturate the solution with HCl gas (common ion effect) Crystals of pure NaCl separates out Impurities being more soluble remain in solution

Electrolyte: Saturated solution of NaCl Anode (oxidation): Carbon electrode Cathode (reduction): Hg-electrode

NaOH solution at the surface reacts with atmospheric CO2 giving Na2CO3

Anode

Cathode

2 Cl- Cl2 + 2e-

2Na+ + 2e-2Hg

2Na-Hg

2H2O

2 NaOH + H2 + 2Hg

Cell 2NaCl +2Hg 2Na -Hg + Cl2

Uses of alkali metals & its compounds

Alkali Metals Na2CO3 NaHCO3 NaCl NaOH

Li -to make alloys In thermonuclear reactions Liq Na is used as a coolant in nuclear reactors Cs in photoelectic cells

etc

H2O softening Laundering Used for qualitative & quantitative analysis

Mild antiseptic In fire extinguisher

For domestic purpose For the preparation of Na2O2, NaOH, Na2CO3 etc

in the manufacture of soap, paper etc In the purification of bauxite ore In petroleum refining Preparation of pure fats & oils

etc

Biological importance of Na+ & K+

A 70 kg man contains about 90g of Na+ & 170 g of K+

Belonging to the same group both Na & K are chemically similar, but they perform different biological functions

Na+ K+

Occurrence

Abundant in blood plasma & in the interstitial fluid which surrounds the cell

Abundant in the cell fluid

Blood Plasma : Na+ = 143mmol/L & K+ = 5mmol/L Within red blood cell: Na+ = 10mmol/L & K+ = 105mmol/L This ionic gradient within & out of the cell is necessary for the functioning of the body. To maintain this a Sodium-Potassium Pump always operates across the cell membrane by consuming the energy.

Function

Transmission of nerve signals Regulate the flow of H2O across cell membrane Transport of different sugars & amino acids into cells

Transmission of nerve signals To activate a variety of enzymes in the cell fluids Promote the oxidation of glucose into ATP

ALKALINE EARTH METALS

Properties Trend

Electronic Configuration

[Noble Gas] ns2

Atomic & Ionic Radii

As expected both increases down the group Size of M+ < size of M

Ionization enthalpy

** As expected it decrease down the group ** First IE of alkaline earth metals are higher than those of the Gp 1 Metals due to their comparatively smaller size

Hydration enthalpy

Decreases down the group as the size increases Values are larger in comparison to that of alkali metal ions. As a result compounds of the elements are extensively hydrated

Flame test Ca, Sr & Ba imparts characteristic colors to the flame As it is difficult to knock of e- from Be & Mg, they do not impart any color Helpful in qualitative analysis

General features

Strongly electropositive in character & the behavior increases down the group

Alkaline Earth Metals-Chemical properties

Group 2 elements show the same trends in properties as were observed with Group 1 elements

But, Be stands apart from the rest of the group, & differs much more from them than Li does from the rest of Group 1

Reason: Relative increase in size from Be2+ to Mg2+ is 4 times greater than the increase between Li+ & Na+

Elements are highly reactive metals , but are less reactive in comparison to alkali metals

Compounds of Group 2 are less ionic compared with Group 1 compounds

Compounds of Be & Mg have more covalent property

As the electro positivity increases down the group, reactivity also increases

Alkaline Earth Metals-Chemical properties

1. Reactivity towards Air & H2O:

With Air With H2O

M + (O2 + N2) ------> MO + M3N2 M + H2O ------> M(OH)2

Be is relatively unreactive in the massive form, but the powder is much more reactive

Be does not react with H2O, whereas Mg demands boiling H2O or steam conditions Other members react even with cold H2O

Mg being more electropositive, burns with intense light & evolves a lot of heat which is used in thermite reactions & flash photography

Oxides Hydroxides

MO + H2O -------> M(OH)2 Be(OH)2 is amphoteric in nature whereas other hydroxides are basic

Except BeO all MO have rock-salt structure Solubility & the basic strength increases down the group

BeO is amphoteric, whereas other MO are basic in nature

Solubility:

Solubility of any salt is influenced by lattice enthalpy as well as hydration enthalpy

Lattice enthalpy (LE): Energy required to separate the ions from one mole of the solid to infinite distance in the gaseous state

Hydration enthalpy (HE): Energy releases due to the hydration of one mole ions of the salt

If LE > HE, then salt does not dissolve

If LE < HE, then salt dissolves

Coming to Group 2, As we move down the group both LE & HE will decrease

But the decrease in LE is more compared to HE, so the solubility increases

2. Reactivity towards Halogens:

M + X2 -------> MX2 (At elevated temperature)

Halides

Tendency to form halide hydrates decreases down the group MgCl2.8H2O, CaCl2.6H2O, SrCl2.6H2O, BaCl2.2H2O

All the MX2 are ionic in nature except BeX2 which are covalent & soluble in organic solvents

BeCl2

Solid state Vapour phase High T 1200 K

BeO + 2NH3 + 4HF (NH4)2[BeF4] 2BeF2 + 2NH4F

C + Cl2

BeCl2 + CO

600-800 K

Cl

Be

Cl

BeCl Cl Cl Be Cl

Alkaline Earth Metals-Chemical properties

Towards Hydrogen All the elements form Hydrides upon heating with H2 except BeH2 as it is less stable However impure BeH2 can be prepared by 2 BeCl2 + LiAlH4 ------> 2BeH2 + LiCl + AlCl3

Towards Acids React with acid liberating H2 M + 2 HCl -----> MCl2 + H2

Reducing property Strong reducing agents indicated by their negative E- values But, reducing power is less compared to alkali metals Reducing property is found to increase down the group

Solutions in liq. NH3 Dissolves in liq NH3 giving deep blue black solution of ammoniated ions

[M(NH3)x]2+ + 2[e(NH3)y]

-

on standing

M(NH2)2 + H2

amides

evapor[M(NH3)6]2+

ammoniates

slow decomp

M(NH2)2 + 4NH3 + H2

Salts of Oxo-Acids

Carbonates Decompose on heating to give Oxide & CO2 Thermal stability increases as we move down the group Carbonates are insoluble in H2O & precipitates upon addition of Na2CO3/(NH4)2CO3 (Removal of hardness of H2O) Solubility in H2O decreases as we move down the group --- As the size of the anion is much bigger, the increase in the size of cation down the group will not affect the Lattice enthalpy to a considerable extent. Whereas hydration enthalpy decreases down the group & the solubility decreases

Sulphates Are quite stable to heat upon heating to a very high T may decompose to oxide, SO2 & O2 BeSO4 & MgSO4 are readily soluble in H2O Solubility decreases down the group

Nitrates MCO3 + dil.HNO3 -----> M(NO3)2 + H2O + CO2 2M(NO3)2 decomposes on heating ------> 2MO + 4NO2 +O2 Mg(NO3)2.6H2O crystallizes in the hydrate form whereas Ba(NO3)2 is existing in anhydrous form

Anomalous properties of Be

Be stands apart from the rest of the group, & differs much more from them than Li does from the rest of Group 1

Difference in properties with the group members

Diagonal relationship b/w Be & Al

Compounds of Be are largely covalent & get easily hydrolyzed

Like Al, Be is not readily attacked by acids because of the presence of an oxide film on the surface of the metal

As the valence shell of Be contains only 4 orbitals (one 2s and three 2p), highest coordination number exhibited is 4

Be(OH)2 dissolves in excess of alkali to give a beryllate ion, [Be(OH)4]

2 just as Al(OH)3 aluminate ion, [Al(OH)4]

The oxide and hydroxide of beryllium, unlike the hydroxides of other elements in the group, are amphoteric in nature

The chlorides of both Be & Al have Cl bridged structure in vapour phase. Both the chlorides are soluble in organic solvents and are strong Lewis acids. They are used as Friedel Craft catalysts.

Be & Al ions have strong tendency to form complexes, BeF4

2, AlF6 3.

Industrially important compounds of Ca

CaO Quick lime Ca(OH)2 Slaked lime

Prepn

CaO + H2O -----> Ca(OH)2

Addition of limited amount of H2O breaks the quick lime lump & the process is called as Slaking of lime

Lime water : Aqueous solution of Ca(OH)2 Milk of lime : Suspension of slaked lime in water

On exposure to atmosphere, absorbs Moisture & CO2

On passing CO2 through lime water

Being a basic oxide it combines with acidic oxides at high temperature

Milk of lime + Cl2 ------> Hypochlorite

CaCO3 CaO + CO2lime stone

CaO H2OCO2 Ca(OH)2CaCO3

CaO CaSiO3P4O102Ca3(PO4)2

SiO2

Ca(OH)2 (aq)CO2

excessCO2

CaCO3 + H2O

Ca(HCO3)2

2Ca(OH)22Cl2

Ca(OCl)2 + CaCl2 + 2H2O

Industrially important compounds of Ca

CaCO3 Limestone CaSO4.1/2H2O Plaster of Paris

Prepn: 1. By passing limited amount of CO2 through slaked lime 2. CaCl2 + Na2CO3 ------> CaCO3 + 2NaCl

Hemihydrate of CaSO4

Reacts with dilute acids liberating CO2 CaCO3 + 2HCl ------> CaCl2 + CO2 + H2O CaCO3 + H2SO4 -------> CaSO4 + CO= +H2O

On mixing with an adequate quantity of H2O it forms a plastic mass that gets hard in 5-15 min

2(CaSO4. 2H2O) 2(CaSO4). 1/2H2O + 3H2Ogypsum

(dead burnt plaster)

393 K

above393K

CaSO4 anhy

Cement Also called as Portland cement (P.Cement) due to its resembles with the

natural limestone extracted in Isle of Portland

Average Composition of P.Cement

CaO 50-60% MgO 2-3% Ca2SiO4 26%

SiO2 20-25% Fe2O3 1-2% Ca3SiO5 51%

Al2O3 5-10% SO3 1-2% Ca3Al2O6 11%

For a good quality cement 1. Silica:Alumina in between 2.5 to 4 2. CaO: (SiO2+Al2O3+Fe2O3) close to 2

Manufacture of cement

Setting of Cement When mixed with H2O cement becomes a hard mass due to the hydration & rearrangement of molecules Purpose of addition of gypsum is to slow down the process of setting of cement

clay & limecementclinker

2-3% by weightof gypsum

cement

Uses of Group 2 elements & its compounds

Group 2 Elements

Cu-Be alloys are used in the manufacture of high strength springs Be metal is used in making windows of X-ray tubes Mg-Al alloys being light is used in air crafts Milk of magnesia {suspension of Mg(OH)2 in H2O} is used as antacid Ca-is used as reducing agent for the extraction of Metals Radium salts are used in Radiotherapy

CaO Quick lime

In the manufacture of cement, dyes and sodium carbonate Cheapest form of alkali In the purification of Sugar

Ca(OH)2 Slaked lime

Preparation of Mortar A building material Used in white wash due to disinfectant nature In glass making industry, for the preparation of bleaching powder etc

CaCO3- Lime stone

Used as a building material in the form of marble As flux in the extraction of metals As an antacid, mild abrasive in tooth paste, a constituent of Chewing gum, filler in cosmetics in the manufacture of high quality paper

CaSO4. 1/2H2O Plaster of Paris

for making statues Used as plasters

Biological importance of Ca2+ & Mg2+

A n adult body contains about 25g of Mg & 1200g of Ca

Ca2+ Mg2+

Functions: neuromuscular functions Interneuronal transmission Cell membrane integrity Blood coagulation

All enzymes that utilize ATP in phosphate transfer require Mg as the Cofactor

Ca concentration in plasma is 100 mg/L The main pigment for the absorption of light in plants is chlorophyll which contains Mg