Choose the correct statements for a zero order reaction
(A) The rate of the reaction is independent of the initial concentration of the reactant
(B) The rate of the reaction increases linearly as the initial concentration of the reactant is increased
(C) The plot of concentration against time is a straight line with a positive slope
(D) The unit of rate constant is \( mol\, L^{-1} s^{-1} \)
Choose the correct answer from the options given below.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) and (D) only
In a zero order reaction the rate does not depend on the concentration of the reactant, because the concentration term carries the power zero. Integrating the rate law gives a straight-line relation between concentration and time:
\( \displaystyle \text{Rate} = -\frac{d[\mathrm{R}]}{dt} = k[\mathrm{R}]^0 = k \)
\( \displaystyle [\mathrm{R}] = -kt + [\mathrm{R}]_0 \)
(A) The rate of the reaction is independent of the initial concentration of the reactant — Correct. Since \( [\mathrm{R}]^0 = 1 \), the rate equals k whatever the concentration. Decomposition of \( \ce{NH3} \) on a hot platinum surface at high pressure is an example: the surface is fully covered, so adding more gas does not speed it up.
(B) The rate of the reaction increases linearly as the initial concentration of the reactant is increased — Incorrect. A rate that rises in proportion to the concentration is the behaviour of a first order reaction (rate = k[R]); a zero order rate stays constant.
(C) The plot of concentration against time is a straight line with a positive slope — Incorrect. Comparing \( [\mathrm{R}] = -kt + [\mathrm{R}]_0 \) with y = mx + c, the line has slope −k (negative) and intercept \( [\mathrm{R}]_0 \); the concentration falls with time.
(D) The unit of rate constant is \( \mathrm{mol\,L^{-1}\,s^{-1}} \) — Correct. Here k equals the rate, so it has the unit of rate:
\( \displaystyle k = \frac{\text{rate}}{[\mathrm{R}]^0} = \frac{\mathrm{mol\,L^{-1}\,s^{-1}}}{1} = \mathrm{mol\,L^{-1}\,s^{-1}} \)
NCERT (2026-27 edition):
Zero Order Reactions – “Zero order reaction means that the rate of the reaction is proportional to zero power of the concentration of reactants. …”
\( [\mathrm{R}] = -kt + [\mathrm{R}]_0 \)
“Comparing (3.6) with equation of a straight line, y = mx + c, if we plot [R] against t, we get a straight line (Fig. 3.3) with slope = –k and intercept equal to [R]0.”
