s Block Elements Notes: – Alkali Metals

 S Block Elements – ALKALI METALS

S Block Elements having two groups alkali metals and alkaline earth metals. Alkali is a Arabic word which means ‘Aquali’ meaning the ashes of the plant from which compounds of sodium and potassium were first isolated.
Detail properties about S block element – Alkali metals are as follows.

Physical Properties

Physical state: Silvery white, soft and light
Electronic configuration: ns¹
Atomic and ionic radii, volume: Atomic and ionic radii increase from Li to Fr due to presence of extra shell of electrons. Volume increases from Li to Cs
Density: Densities are quite low and increases from Li to Cs. K is lighter than Na due to unusual increase in atomic size. Li, Na and K are lighter than water
Melting point and boiling points: Decrease in melting and boiling point from to Li to Cs due to weak intermetallic bonding.
Metallic character: Metallic character depends on the atomic size so metallic character Increases from Li to Cs.
Conductivity: Good conductor.
Oxidation state: +1 oxidation state
Ionization enthalpy: Ionization enthalpy decreases from Li to Cs due to decrease in atomic size.
Hydration of ions: Smaller the size of cation, greater degree of hydration Li⁺> Na⁺, K⁺ > Rb⁺ > Cs⁺
Hydration energy: Hydration energy of alkali metals decreases from Li⁺to Cs⁺
Flame colouration: Because of large size, less energy is required to promote the electron into excited state as the electrons return to ground state, they impart colour to the flame. Li Crimson, Na Yellow, K Pale violet, Rb Red violet, Sc Blue.
Reducing property: Strong reducing agent. Reducing character increases from top to bottom because size increase and metal can lose the electrons easily But Li is strongest reducing agent in solution Because of its high hydration energy.
Complex formation: Alkali metals have little tendency to form complexes. Since Lithium has a small size, it forms certain complexes. Alkali metals form stable complexes with polydentate ligand such as crown ether.
Action of air: Lithium form oxide(O^-2), Sodium forms peroxide(O₂^2-) while Potassium, Rubidium Cesium form superoxide (O₂^–) Stability of peroxides and superoxide increases from Li to Cs. It can be explained by stabilization of larger anion by larger cation through lattice energy. Peroxides and Superoxides are important oxidizing agent.
Nature of hydroxide and halide: Thermal stability of Group-I hydrides decreases down the group, hence reactivity increases from LiH to CsH. Melting and boiling point of halides follows order: Fluorides > Chlorides > Bromides > iodides. The ease of formation of alkali metal halides increases from Li to Cs

Watch MCQ of s block elements

Nature of oxide and hydroxide: Alkali metal oxides are basic in nature and their basic character increases gradually on moving down the group. The basic character of alkali metal hydroxide LiOH < NaOH < KOH < RbOH < CsOH
Nature of carbonates and bicarbonates: Alkali metal carbonates and bicarbonate stability increases down the group. Since electropositive character increases from Li to Cs All carbonates and bicarbonate are water soluble and their solubility increases from Li to Cs.

CHEMICAL PROPERTIES

Alkali metals are highly reactive due to low ionization energy. Reactivity decreases down the group

(i) Reaction with oxygen and air: –The alkali metals tarnish in air due to formation of Carbonates, oxides and hydroxides at their surface and hence kept in kerosene oil or paraffin wax. When burnt in oxygen lithium form Li₂O Sodium form peroxide Na₂O₂ and other alkali metals form super oxide MO₂ (M = K, Rb, Cs). Lithium when burnt in air it forms nitride by reacting with nitrogen along with Lithium oxide 6Li _(s) + N_2(g) ⟶ 2Li3N_(s)Other alkali metals do not react with Nitrogen. Lithium oxide is very stable due to small size of lithium and O^2- ions and have higher charge density. Sodium peroxide and K₂O₂ are stable because of ions are of comparable size. Increasing stability of peroxide and super oxide is due to stabilization of larger anions by larger cation through lattice energy. Superoxide ion (O^2- has a three-electron bond which makes it paramagnetic and coloured, whereas peroxides are diamagnetic and colour less. Both peroxides and oxide acts as a oxidizing agents. Alkali metal oxides are basic in nature and basic character increases down the groups.

ii) Reaction with water: Alkali metals reacts vigorously and readily with water to form hydroxides with liberation of hydrogen. The reactivity increases down the group due to increased electro positivity. K, Rb, Cs lower alkali metals in group reacts so vigorously that evolved hydrogen catches fire spontaneously. Because of their high reactivity they are kept in kerosene. Alkali metals reacts with compound containing acidic hydrogen atoms such as alcohol and acetaldehyde.

2M + 2 C₂H₅OH ⟶ H₂+2C₂H₅OH (metal ethoxide)

2M + HC ≡ CH →  H₂ + M – C ≡ C – M (alkali metal acetylide)
Alkali metal hydroxides are strong basic. Basic character increases from LiOH to CsOH.
LiOH < NaOH < KOH < RbOH < CsOH.
As metal ion size increases down the group distance between metal ion and OH group increases. Thus more basic hydroxides down the group also thermal stability of hydroxide increases down the group.

iii) Reaction with hydrogen: Hydrogen reacts with alkali metals to form hydride M⁺H^–. Reactivity increases down the group as electro positive character increases down the group. And thermal stability decreases and heat of formation decreases down the group. Hydrides liberate hydrogen at anode on electrolysis. Therefore, they are used as reducing agent.

LiH_(s)+ H2O ⟶ LiOH_(aq)+ H2(g)

NaH + CH3OH  ⟶ CH3ONa + H2(g).

iv) Reaction with halogens: – The alkali metals combine readily with halogens(X₂) forming halides.

2M + X₂ ⟶ 2MX.
The ease of formation of halides increases down the group Li < Na < K < Rb < Cs Reactivity of halogen towards particular alkali metal follows the order F₂> Cl₂ > Br₂ > I₂.
Except halides of Li all are ionic and are soluble in water. K, Rb, C forms simple and mixed Polyhalides because of large size e.g CsI₃, KI₃, CsI₂Cl, RbIBr₂, RbClI₄ .Polyhalides of Cs are thermally more stable. Melting point and boiling point of particular alkali metal follow the order Fluorides > Chlorides > Bromides > Iodides. Lithium halides LiBr and LiI are covalent compound. LiF is soluble in non-polar solvents like kerosene.

v) Solubility in liquid ammonia: – Alkali metals dissolves and form solution in liquid ammonia. When alkali metals are dissolved in liquid ammonia, there is a considerable expansion in total volume hence such solutions are called expanded metals.

M ⟶ M⁺ + electron

M⁺ + xNH₃ ⟶[M(NH3)_x]⁺ (Ammoniated metal ion)
Electron + yNH₃ ⟶ [e(NH3)_y]^– ( Ammoniated electron ).

vi) Reaction with oxoacids: – Alkali metal hydroxide being basic in nature react with oxoacid (such as H₂CO₃), H₃PO₄HNO₃, H₂SO₄ etc.) to form different slats such as metal carbonates, bicarbonates, sulphates, nitrates, etc. Alkali metal carbonates and bicarbonates are highly stable towards heat and their stability increases down the group, since electropositive character increases from Li to Sc. However, Li₂CO₃ is less stable and readily decomposes to form oxide.

Li₂CO₃ ⟶ Li₂O + CO₂.
Alkali metal bicarbonates on heating decompose to give respective carbonates

2MHCO₃ ⟶M₂CO₃ + CO₂ + H₂O All carbonates and bicarbonates are water soluble. Their solubility increases down the group since their lattice energy decreases more rapidly than their hydration energy in the group.

vii) Alkali metal nitrates: – Alkali metal nitrates (MNO₃) decompose on strong heating to corresponding nitrite and O₂ except LiNO₃which decomposes to its oxides.

2NaNO₃ ⟶ 2NaNO₂ + O₂

But 4LiNO₃ ⟶ 2Li₂O + 4NO₂ + O₂

ANOMALOUS BEHAVIOUR OF LITHIUM: –

Lithium, the first member of alkali metals differs in many properties from the other alkali-metals due to the following reasons:
i) Li has smallest atomic and ionic size in the group
ii) Li+ has highest polarizing power in its group which makes its compounds covalent
iii) Li has highest ionization energy, high heat of hydration, highest electro-negativity or minimum electropositive character in its group.
iv) Li does not have d-orbitals also.

Difference between lithium and other alkali metals: –

i) Lithium is harder and higher than other alkali metals due to strong metallic bonding.
ii) Its m.pt. And b.pt are higher than the rest of alkali metals
iii) Li on burning in air or oxygen forms monoxide while other alkali metals form higher oxides like peroxides and Superoxides.
iv) Li forms nitride with nitrogen whereas other alkali metals do not 6Li + N₂ ⟶ 2Li₃N
v) Some lithium salts like LiF, Li2CO3 and Li3PO4 are sparingly soluble in water whereas corresponding slats of other alkali metals are freely soluble.
vi) Li form imide (LiNH) with ammonia while other alkali metals form amides (MNH₂)
vii) LiHCO₃does not exist as solid but it occurs in solution. Other alkali metals bicarbonates are known in solid state.
viii) Unlike other alkali metals Li does not form alum.

Similarities between and magnesium or diagonal relationship between lithium and magnesium: –

Similarities between and magnesium or diagonal relationship between lithium and magnesium
i) Both Li and Mg form monoxides Li2O and MgO on heating with air or oxygen.
ii) Both Li and Mg form ionic nitrides when heated in nitrogen
6Li + N₂  ⟶2Li₃N
3Mg + N₂ ⟶ Mg₃N₂
iii) Hydroxides, carbonates and nitrates of both Li and Mg decomposes on heating to yield respective oxide
2LiOH ⟶ Li₂O + H2O
Mg(OH)₂ ⟶ MgO + H₂O
Li2CO₃ ⟶ Li₂O + CO₂
MgCO₃ ⟶ MgO + CO₂
4LiNO₃ ⟶ 2Li₂+ 4NO₂ + O₂
2Mg(NO₃)₂ ⟶ 2MgO + 4NO₂ + O₂
iv) Fluorides, carbonates, oxalates and phosphates of both metals are sparingly soluble in water.
v) Both LiCl and MgCl2 are deliquescent salts.

SOME IMPORTANT COMPOUNDS OF ALKALI METALS

1. SODIUM CHLORIDE, NaCl (Common salt)

NaCl obtained from sea water may have impetrates like CaSO₄, Na₂SO₄, CaCl₂, MgCl₂ etc. MgCl₂ and CaCl₂are deliquescent in nature (absorbs moisture from air) hence impure common salt gets wet in rainy reason. Pure NaCl can be prepared by passing HCl gas into saturated solution of commercial salt. Pure salt gets precipitated due to common ion effect.
NaCl is used as table salt NaCl is used in preparation of number of compounds such as Na₂CO₃, NaOH, Na₂O₂ etc

2. SODIUM HYDROXIDE, NaOH (CAUSTIC SODA)

Sodium hydroxide is known as caustic soda, since it breaks down the protein of skin to a pasty mass.

PREPARATION

1.Causticization process (Gossage process): – This process involves heating of sodium carbonate with milk of lime.
Na₂CO₃+ Ca(OH)₂ ⇌ CaCO₃↓ + 2NaOH.
2. Electrolysis of NaCl:- Electrolysis of saturated aqueous solution of NaCl gives NaOH , Cl2 and H₂
𝑁𝑎𝐶𝑙(𝑎𝑞) → 𝑁𝑎⁺ + 𝐶𝑙⁻
At anode: 2Cl^–à Cl₂ + 2e^–
At Cathode: 2H₂O + 2e- ⇌ H₂ + 2OH^–
Na⁺ + OH^– à NaOH.
Cl₂ gas, one of the byproducts reacts with NaOH to form other byproduct.
2NaOH + Cl₂ à NaCl + NaOCl + H₂O.

3. Porous diaphragm process (Nelson cell process) In this process a perforated cathode made up of steel lined up with asbestos is used. In this process Cl2 formed at anode is taken out so that extent of impurities in NaOH is quite low.

4.Castner-kellner cell (Mercury cathode process): – This process involves the electrolysis of conc. Brine solution in such a way so that reaction between NaOH and Cl₂ does not takes place. In this process three compartments are made in electrolytic cell and mercury used as cathode moves freely from one compartment to another. Graphite rods are used as anode.

Properties:- NaOH is deliquescent, white crystalline solid which absorbs moisture and carbon dioxide from atmosphere to form aq.NaOH layer around pellet first and finally white powder of Na₂CO₃.
2NaOH + CO₂à Na₂CO₃ + H₂O NaOH dissolves readily in water to yield higher alkaline solution which is corrosive, soapy in touch and bitter in test.

Uses: NaOH is widely used in soap industry, paper industry, textile industry (for mercerization of cotton). It is used in the manufacture of dyes and drugs NaOH is used for absorbing acids and gases, in petroleum refining and as a regent in laboratories.

Uses:- Sodium carbonate is used in laundries as washing soda It is also used to remove hardness of water Na₂CO₃ is used to manufacture glass, caustic soda etc It is used in petroleum refining and in textile industry.

4. SODIUM BICARBONATE, NaHCO3 (BAKING SODA)

Preparation: Sodium bicarbonate or sodium hydrogen carbonate is obtained as intermediate compound in Solvay process. It can also be prepared by passing CO₂through solution of sodium carbonate Na₂CO₃ + CO₂+ H₂O ⟶ 2NaHCO₃
Properties: – NaHCO₃ on heating decomposes to produce bubbles of CO2 which make the cakes and pastries fluffy.
2NaHCO₃ ⟶ Na₂CO₃ + H₂O + CO₂ 
It is amphiprotic i.e.; it can act as H⁺ donor as well as H⁺acceptor.
HCO₃^–+ H⁺ ⇌ H₂CO_3,  HCO₃^– ⇌ H⁺ + CO3^2-
USES:- NaHCO₃ is used in the preparation of baking powder.
[Baking powder = NaHCO₃ (30%) + Ca(H₂PO₄)₂ (10%) + Starch ( 40% ) + NaAl(SO₄)₂]
It is used in fire extinguisher: NaHCO₃ + HCl à NaCl + CO₂ + H₂O Such kind of fire extinguisher are known as soda-fire extinguisher. It is used as antacid and mild antiseptic.
5. MICROCOSMIC SALT, Na(NH)₄HPO₄.4H₂O:- Microcosmic salt exists in colourless crystalline solid form. It is prepared by dissolving NH₄Cl and Na₂HPO₄ in 1:1, molar ratio in hot water.
NH₄Cl + N2HPO₄ à Na(NH₄)HPO₄ + NaCl.
USES:- It is used for performing ‘bead test’ (like borax) for detecting colour ions in qualitative analysis. On heating microcosmic salt form NaPO3 which form coloured beads of orthophosphates with oxides of transition metal and cloudy bead with SiO₂