Question

Consider the reaction, 2A + B $\to$ Products. When the concentration of B alone was doubled, the half-life did not change. When the concentration of A alone was doubled, the rate increased by two times. The unit of rate constant for this reaction is:

• A. L mol${}^{-1}$ s${}^{-1}$
• B. unitless
• C. mol L${}^{-1}$s${}^{-1}$
• D. s${}^{-1}$

Answer 1

Answer: (A)

L mol${}^{-1}$ s${}^{-1}$

For first-order reaction, half-life $t_{1/2}=\frac{0.693}{k}$
Since half-life does not depend on the concentration of reactants.

According to the given data, the order of reaction with respect to B is one, hence, does not change half-life.

Similarly, an order of reaction with respect to A is also one since the rate increase twice on doubling the concentration.

Hence, total order $=1+1=2$

Second order reaction and its unit is $Lmo{l}^{-1}{s}^{-1}$.