i. A catalyst provides an alternative pathway associated with lower activation energy.

Potential energy barriers for catalyzed and uncatalyzed reactions

ii. The potential energy diagram compares the potential energy barriers for the catalyzed and uncatalyzed reactions. The barrier for the uncatalyzed reaction (E_a) is larger than that for the same reaction in the presence of a catalyst, E_a.

iii. Consider the decomposition of H₂O₂ in aqueous solution catalyzed by I^- ions:

2H2O2(aq) → [I-]2H2O(l) + O2(g)

At room temperature, the rate of reaction is slower in the absence of a catalyst with its activation energy being 76 kJ mol^-1. In the presence of a catalyst, iodide ion (I^-), the reaction is faster since the activation energy decreases to 57 kJ mol^-1.