(a) Reactions of which order will show the rate to be independent of the concentration of the reactant? Give one example of this order.

• Step 1 (Definition & Governing Law): [0.5 Mark]

  Reactions for which the rate of reaction is independent of the concentration of the reactant are known as Zero Order Reactions. The rate law for such a reaction is expressed as:

  $$\text{Rate} = k[R]^0 = k$$
• Step 2 (Example): [0.5 Mark]

  A classic example is the thermal decomposition of ammonia on a hot platinum surface:

  $$2NH_3(g) \xrightarrow{Pt} N_2(g) + 3H_2(g)$$

  Here, the rate is independent of the concentration of $NH_3$ because the metal surface is fully covered by reactant molecules.

(b) State the condition under which a bimolecular reaction may be kinetically a first order reaction.

• Step 1 (Governing Principle): [0.5 Mark]

  A bimolecular reaction follows first-order kinetics when one of the reactants is present in large excess. This is known as a pseudo-first order reaction.

• Step 2 (Mathematical Representation): [0.5 Mark]

  Consider the hydrolysis of an ester (e.g., ethyl acetate) in the presence of a large excess of water:

  $$CH_3COOC_2H_5 + H_2O \rightarrow CH_3COOH + C_2H_5OH$$

  The rate law is theoretically $\text{Rate} = k'[CH_3COOC_2H_5][H_2O]$. Since $[H_2O]$ is in large excess, its concentration remains effectively constant. Thus, the rate equation simplifies to:

  $$\text{Rate} = k[CH_3COOC_2H_5] \quad \text{where } k = k'[H_2O]$$