Question

2(c) Consider each of the following substances: NaH, H2, H2S, NH4Cl.
(i) Describe the nature of the intermolecular forces holding the units or molecules together in the condensed (liquid or solid) state.
(ii) Explain briefly what happens when a sample of each of the substances is added to water.
(iii) Write the chemical equations of any reactions occurring or of any equilibria established. [14 marks]

Accepted Answer

(i) Intra-molecular forces (e.g. covalent/ionic bonds) hold atoms together within a molecule/compound, and are generally much stronger than inter-molecular forces, which hold separate molecules together in a liquid or solid and determine bulk properties such as melting and boiling points; they are electrostatic in nature. NaH is ionic (held by strong electrostatic ionic bonds); H2 and H2S are held together in the condensed state by weak van der Waals forces (H2S also has some dipole-dipole attraction); NH4Cl is ionic, held by strong electrostatic forces between NH4+ and Cl– ions.
(ii) • NaH: reacts vigorously with water; an alkaline solution (NaOH) is formed and hydrogen gas is liberated.
  • H2: no reaction occurs when hydrogen gas is passed into water.
  • H2S: dissolves to give a weakly acidic solution which turns blue litmus red.
  • NH4Cl: dissolves appreciably in water; the container feels cold to the touch (the dissolution is endothermic).
(iii) NaH + H2O → NaOH + H2
  NH4Cl + H2O ⇌ H3O+ + NH3 + Cl–  (or NH4Cl + H2O → NH4OH + HCl)

WAEC Chemistry 2019 Theory — Question 13

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