(a) Give one reason why a collision between... | WAEC SSCE - Chemistry - 1990 (Essay)

Question 1 Report

(a) Give one reason why a collision between reactants may not produce new species.

(b) Explain, illustrating with appropriate equation(s), why an aqueous solution of aluminium chloride is acidic.

Answer

(a) Why a collision between reactants may not produce new species

For a collision to be effective (fruitful) the colliding particles must have energy equal to or greater than the activation energy and must collide with the correct orientation. A collision fails to form products if the particles collide with too little energy (below the activation energy) or with an unfavourable orientation, so they simply bounce apart unchanged.

(b) Why aqueous aluminium chloride is acidic

The aluminium ion, \(Al^{3+}\), is very small and highly charged, so it strongly attracts and polarises the water molecules bonded to it. This causes hydrolysis (cationic hydrolysis): a bonded water molecule releases a hydrogen ion, so the solution contains an excess of \(H^+\) ions and is acidic.

Using the hydrated ion:

\[[Al(H_2O)_6]^{3+} \rightleftharpoons [Al(H_2O)_5(OH)]^{2+} + H^+\]

or, written simply:

\[AlCl_3 + 3H_2O \rightleftharpoons Al(OH)_3 + 3HCl\]

The liberated \(H^+\) (hydrochloric acid) makes the solution acidic.

Read lesson note on Kinetic Theory Of Matter And Gas Laws (JAMB)

Read lesson note on Acids, Bases And Salts (Ghana Only) (WAEC)

Answer Details

(a) Why a collision between reactants may not produce new species

(b) Why aqueous aluminium chloride is acidic

Using the hydrated ion:

\[[Al(H_2O)_6]^{3+} \rightleftharpoons [Al(H_2O)_5(OH)]^{2+} + H^+\]