Free Order of Reaction MCQs with Answers

1. The order of a reaction with respect to a particular reactant

• A. is always equal to its coefficient in the balanced equation
• B. is the power to which its concentration is raised in the experimentally determined rate equation
• C. can never be zero
• D. is always a whole number

Explanation: Order is an experimental quantity and must be measured, because the balanced equation describes the overall stoichiometry while the rate depends only on the slow, rate determining step. This is why orders can be zero, fractional or even negative, none of which a coefficient can be. Only for a genuinely single step reaction do order and coefficient happen to agree.

2. For a reaction with the rate equation rate = k[A][B]^2, the overall order of the reaction is

• A. 1
• B. 2
• C. 3
• D. 4

Explanation: Overall order is the sum of the individual orders, so 1 for A plus 2 for B gives 3. The reaction is first order in A and second order in B, and doubling the concentration of B alone would therefore quadruple the rate while doubling A alone would only double it. Adding rather than multiplying the exponents is the step candidates most often get wrong.

3. In a zero order reaction, the rate

• A. doubles when the concentration doubles
• B. is independent of the concentration of the reactant
• C. falls to zero immediately
• D. is proportional to the square of the concentration

Explanation: A zero order reaction proceeds at a constant rate until the reactant is nearly exhausted, because something other than concentration limits it, typically a saturated catalyst surface or a fixed light intensity. The graph of concentration against time is therefore a straight line with a constant slope. Enzyme catalysed reactions become zero order once every active site is occupied.

4. For a reaction A gives products, doubling the concentration of A leaves the rate unchanged. The reaction is

• A. first order in A
• B. second order in A
• C. third order in A
• D. zero order in A

Explanation: If concentration has no effect on rate then the exponent of that term must be zero, since anything raised to the power zero equals one. Had the rate doubled the reaction would be first order, and had it quadrupled it would be second order, which is the reasoning used to find orders experimentally. Zero order behaviour usually signals a saturated catalyst or enzyme.