Alkali Metals | Compounds of alkali metals#

Oxides of alkali metals#

Lithium(Li) forms mainly monoxide and some peroxide.
Sodium(Na) forms mainly peroxide and some superoxide.
Potassium(K), Rubidium(Rb) and Cesium(Cs) mainly form superoxide because large anions (O₂^-) are stabilized by large cations.

Stability:

\[CsO_2\ > RbO_2\ >KO_2\]

\[Li_2O + H_2O → 2Li^+ + 2OH^-\]

\[Na_2O_2 + 2H_2O → 2Na^+ + 2OH^- + H_2O_2\]

\[2MO_2 + 2H_2O → M^+ + 2OH^- + H_2O_2 + O_2\]

Here, M = K, Rb or Cs

Oxides and peroxides are colourless while superoxides are yellow or orange.
Superoxides are paramagnetic because O₂^- is paramagnetic according to Molecular Orbital Theory.

\[2M + 2H_2O → 2M^+ + OH^- + H_2↑\]

Here, M = Li, Na, K, Rb or Cs

\[Li_2O + H_2O → 2Li^+ + 2OH^-\]

\[Na_2O_2 + 2H_2O → 2Na^+ + 2OH^- + H_2O_2\]

\[2MO_2 + 2H_2O → M^+ + 2OH^- + H_2O_2 + O_2\]

Here, M = K, Rb or Cs

The hydroxides which are obtained by the reaction of oxides with H₂O are all white crystalline solids.
The alkali metal hydroxides are strongest of all bases (Example: NaOH, KOH).
These dissolve freely in water with evolution of much heat on account of intense hydration or high hydration enthalpy.

Alkali metal halides are prepared by heating with dihydrogen at 400^oC [for lithium, it is 800^oC].

\[2M + H_2 → 2M^+ H^-_{[ionic]} + Heat\]

Here, M = Li, Na, K, Rb or Cs

It is an exothermic reaction, ie, enthalpy of formation (Δ_fH = -ve).
Δ_fH becomes less negative down the group in case of fluorides. This is because the covalent character decreases as one moves down the group. This is because of the increase in electropositive character of the alkali metal from Li to Cs. Thus the tendency to form ionic bonds increases as one moves from Li to Cs. Hence, the consequent enthalpy of formation also increases down the group because an equal amount of energy will be released when the ionic bond breaks (called the Lattice enthalpy for ionic solids or Bond dissociation enthalpy). Stability order of these compounds is - LiF > NaF> KF > RbF > CsF
In case of chloride, bromide and iodide Δ_fH becomes more negative down the group because of increase in stability of halides formed down the group.
For a given alkali metal, Δ_fH becomes less negative from fluoride to iodide: MF > MCl > MBr > MI, ie, MF has most negative Δ_fH [M = Li/Na/K/Rb/Cs ]. This is due to increasing polarizability of halide ion from F^- to I^-. Thus, fluorides are the most stable while iodides are the least stable.

Alkali metal halides have very high melting and boiling points. Order of melting and boiling points:

\[MF > MCl > MBr > MI\]

Alkali metal halides are colourless crystalline solids like NaCl.
All alkali metal halides are soluble in water.
LiF is less soluble in water due to its high lattice energy. For solubility in water, hydration energy (|H.E|) should be greater than lattice energy (|L.E|).
CsI is less soluble in water due to smaller hydration energy of its two ions, Cs⁺ and I^-.
Halides are Li (except LiF) are soluble in ethanol, acetone and ethyl acetate. LiCl is also soluble in pyridine.

Examples

\[2Na + H_2SO_4 → 2Na^+ + SO_4^{2-} + H_2 ↑\]

Salts of oxoacids of alkali metals are soluble in water and thermally stable.
Their carbonates and bicarbonates are stable towards heat.
Stability of carbonbate and bicarbonate increases from Li to Cs because of increase in electropositive characteristic and increase in size of cation.
Lithium carbonate is not so stable to heat. Lithium being very small in size polarizes a large CO₃^2- ion leading to the formation of more stable Li₂O and CO₂.

\[Li_2CO_3 → Li_2O + CO_2\]