The number of incorrect statement/s from the following is ___________
A. The successive half lives of zero order reactions decreases with time.
B. A substance appearing as reactant in the chemical equation may not affect the rate of reaction
C. Order and molecularity of a chemical reaction can be a fractional number
D. The rate constant units of zero and second order reaction are mol L⁻¹ s⁻¹ and mol⁻¹ L s⁻¹ respectively

Solution

Chemistry ma’am · 3 steps

Check each statement against its formula, and keep order and molecularity apart: only order can be a fraction.

1. Zero order half-life. For zero order, t_(1/2) = [A]₀/2k, so the half-life is proportional to the starting concentration. Each new half-life starts from half the previous concentration, so it is half as long.
2. Order versus molecularity. Order is found by experiment and can be zero, a whole number or a fraction. Molecularity counts the particles colliding in one elementary step, so it is always a whole number: 1, 2 or 3.

   Statement	Reason	Verdict
   A	t_(1/2) ∝ [A]₀, and [A]₀ halves each time	correct
   B	A reactant whose order is zero does not change the rate	correct
   C	Order can be fractional, but molecularity cannot	incorrect
   D	k has units (mol L⁻¹)¹⁻ⁿ s⁻¹: n = 0 gives mol L⁻¹ s⁻¹, n = 2 gives mol⁻¹ L s⁻¹	correct

3. So. Only statement C is wrong, so the number of incorrect statements is 1.

The common slip: 2. Calling A incorrect because half-life is taken as constant gives 2. Half-life stays constant only for first order, and for zero order it falls as the concentration falls.

The idea to keep. Half-life goes as [A]₀¹⁻ⁿ: it shrinks for zero order, stays constant for first order and grows for second order. Order comes from experiment, while molecularity is always a whole number.

Answer from the NTA final answer key. Solution by PrepWiser.