Question

For a reaction $A+2 B \rightarrow$ Products, when concentration of $B$ alone is increased half life remains the same. If concentration of $A$ alone is doubled, rate remains the same. The unit of rate constant for the reaction is

• A. $S ^{-1}$
• B. $L\, mol ^{-1} \, S ^{-1}$
• C. mol $L^{-1} S^{-1}$
• D. $atm ^{-1}$

Answer 1

Answer: (B)

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 $Lmol ^{-1} s ^{-1}$.