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ChemistryEdu Logo Alkali Metals | Chemical Characteristics#

In this article, we will discuss the chemical properties of alkali metals like reactivity towards air, reactivity towards water, reactivity towards dihydrogen, reactivity towards halogens, reducing nature and solutions in liquid ammonia.

Note

Alkali metals are highly reactive due to low ionization enthalpy and large size.

Reactivity towards air#

  • Alkali metals react with oxygen to form oxides. Due to formation of oxides, these tarnish in air.
  • Li forms monoxide(O2-), Na forms peroxide(O22-) and K, Rb and Cs form superoxide(O2-).
  • Superoxide is stable only in presence of large cations like K+, Rb+ and Cs+. This is due to the fact that larger cations are stabilized by larger anions(O2-) due to higher lattice energies.
\[4Li + O_2 → 2Li_2O\]
\[2Na + O_2 → Na_2O_2\]
\[M + O_2 → MO_2\]

Here, M = K, Rb, Cs

  • Lithium shows exceptional behaviour by reacting directly with nitrogen to form Li3N or lithium nitride.
\[6Li + N_2 → 2Li_3N\]

Reactivity towards water#

  • On reaction with water, alkali metals form hydroxide and dihydrogen.
\[2M + 2H_2O → 2M^+ + 2OH^- + H_2 ↑\]

Example

\[2Na + 2H_2O → 2Na^+ + 2OH^- + H_2 ↑\]
  • Although Li has the most negative E0 value, its reaction with water is less vigorous than that of sodium which has the least negative E0 value among the alkali metals. This is due to its small size and high hydration enthalpy.
  • Alkali metals also react with proton donors like alcohol, gaseous ammonia and alkynes.
\[2Na + 2R-OH → 2Na^+ + 2OR^- + H_2 ↑\]

Here, R = alkyl group

\[2Na + NH_3 → 2NaNH_2 + H_2 ↑\]

Reactivity towards dihydrogen#

  • Alkali metals react with dihydrogen at 400oC (lithium at 800oC) to form hydrides.
\[2M + H_2 → 2M^+H^-\]
  • Alkali metal hydrides are ionic solids with high melting points.

Reactivity towards halogens#

  • Alkali metals readily react with halogens to form ionic halides.
\[2M + X_2 → 2M^+X^-\]

Here, M = Li, Na, K, Rb, Cs and X = Cl, Br

Exception

Lithium halides are somewhat covalent due to its high polarising power. Due to its small size, Li+ has high tendency to distort electron cloud around the negative halide ion.

Reducing Nature#

  • Alkali metals are strong reducing agents due to their high tendency to lose electron. Reducing power increases from sodium (Na) to Cesium(Ca).
\[Reducing\ Power ∝ -ve\ E^0\ value\]
\[Reducing\ Power ∝ |Hydration\ energy|\]

Exception

Lithium is the most powerful reducing agent among alkali metals. It is due to high hydration energy of lithium.

The E0 value for Cl2/Cl- is 1.36, for I2/Cl- is 0.53, for Ag+/Ag is 0.79, for Na+/Na is -2.71 and for Li+/Li is -3.04. Arrange the following ionic species in decreasing order of reducing strength: I-, Ag, Cl-, Li, Na

\[Reducing\ Power ∝ -ve\ E^0\ value\]

Therefore, reducing power:

\[Li > Na > I^- > Ag > Cl^-\]

Solutions in Liquid Ammonia#

  • Alkali metals dissolve in liquid ammonia giving deep blue solutions which are conducting in nature.
\[M + (x + y)NH_3 → [M(NH_3)_x]^+ + [e (NH_3)_y]^-\]

Here, e is the ammoniated electron which gives blue colour to the solution and makes the solution paramagnetic.

  • When this solution is kept for some time, H2 gas is liberated slowly.
\[M^+_{(am)} + e^- + NH_3{_{(l)}} → MNH_2{_{(am)}} + {1 \over 2} H_{2_{(g)}}\]

Here, 'am' means solution in ammonia

  • In concentrated solution, the blue colour changes to bronze colour and the solution becomes diamagnetic.