Which of the following statements are correct?
A. The rate constant of a chemical reaction is nearly doubled with a 10°C rise in temperature.
B. A second-order reaction will obey first-order rate law if one of the reactant is taken in excess.
C. Inversion of cane sugar is a pseudo first order reaction.
D. In the concentration of reactant vs time graph of a zero-order reaction, the slope gives the value of the initial concentration of reactant.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A, B and C only
Three ideas from chemical kinetics are tested here:
Temperature: for most reactions the rate constant becomes nearly double for a 10° rise in temperature, because the fraction of molecules with energy above Ea rises sharply.
Pseudo first order reactions: if one reactant is present in large excess, its concentration hardly changes, so it is absorbed into the rate constant and the reaction behaves as first order.
Zero order graph: for a zero order reaction the integrated rate law is a straight line.
\( \displaystyle [R]=-kt+[R]_0 \)
A. The rate constant of a chemical reaction is nearly doubled with a 10°C rise in temperature. — Correct. This is the usual observation; the ratio k(t+10)/kt is called the temperature coefficient and is about 2.
B. A second-order reaction will obey first-order rate law if one of the reactant is taken in excess. — Correct. For Rate = k[A][B] with B in large excess, [B] stays almost constant, so Rate = k′[A] with k′ = k[B]: a pseudo first order reaction.
C. Inversion of cane sugar is a pseudo first order reaction. — Correct. In \( \ce{C12H22O11} \) + \( \ce{H2O} \) → \( \ce{C6H12O6} \) + \( \ce{C6H12O6} \) (in the presence of \( \ce{H^{+}} \)), water is in large excess, so Rate = k[C12H22O11].
D. In the concentration of reactant vs time graph of a zero-order reaction, the slope gives the value of the initial concentration of reactant. — Incorrect. Comparing [R] = −kt + [R]0 with y = mx + c, the slope is −k; the initial concentration [R]0 is the intercept.