A chemical reaction in which the rate of reaction is directly proportional to the first power of the concentration of the reacting substance is called:
- (a)second order reaction
- (b)first order reaction
- (c)zero order reaction
- (d)third order reaction
Answer
Why
Correct — B. A first order reaction is defined exactly as the stem states — the rate is proportional to the first power of the concentration.
rate = k[A]
double [A] and the rate doubles
Two things follow from that rate law. The rate constant k carries units of s⁻¹, and the half-life t½ = 0.693/k is independent of the starting concentration — which is why radioactive decay is the standard example.
Why the others are wrong
- (a)second order reaction — Second order means rate ∝ [A]², or first order in each of two reactants. Doubling the concentration would quadruple the rate, not double it.
- (c)zero order reaction — In a zero order reaction the rate is independent of concentration — rate = k, with k in mol L⁻¹ s⁻¹. That is the opposite of what the stem describes.
- (d)third order reaction — Third order means the exponents in the rate law sum to three, so rate ∝ [A]³ for a single reactant. The stem specifies the first power.
Concept
Order is an experimental quantity. It is the sum of the exponents in the rate law, and it is measured, not read off the balanced equation.
That distinction is the whole topic. Molecularity counts the species colliding in an elementary step, so it is a whole number and never zero. Order may be zero, fractional or even negative.
The integrated form for first order is ln([A]₀/[A]) = kt, which plots as a straight line of ln[A] against time.
A reaction can be first order without being simple. In pseudo first order reactions one reactant is in such excess that its concentration barely changes, so the observed rate depends on the other alone.
Key facts
- For a first order reaction, rate = k[A], and the rate constant has units of s⁻¹.
- The half-life of a first order reaction is 0.693/k and does not depend on the initial concentration.
- Order is determined experimentally and may be zero, fractional or negative, while molecularity is a whole number.
- Radioactive decay follows first order kinetics.
Study next
Common traps
- Reading the order off the stoichiometric coefficients of the balanced equation
- Confusing order with molecularity
Kinetics is asked either through the definition of an order or through a named example of one.
The zero order form appears at 10 Sep 2024, 09:00, General Awareness Q.22, keyed to the decomposition of ammonia on hot platinum, and again at 25 Sep 2024, 16:00, General Awareness Q.20.
Related PYQs
No directly related past PYQ was found.