The mathematical expression that relates the rate of a reaction to the concentrations of the reactants is known as the rate expression, rate equation or rate law.
For the reaction \(A + B \rightarrow \text{Products}\), the rate equation is
\[\text{Rate} \propto [A][B]\]
\[\text{Rate} = k[A][B]\]
k is known as the rate constant or specific reaction rate. The units of the rate constant are \((\text{mol})^{1-n}\,(\text{L})^{n-1}\,\text{s}^{-1}\), where n is the order of the reaction.
The rate law for any reaction cannot be predicted by merely looking at the balanced chemical equation, i.e., theoretically, but must be determined experimentally.
Several chemical reactions take place in a sequence of steps, and the overall rate of the reaction is governed by the slowest step, called the rate determining step.
Identify the false statement from the following:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → If the unit of concentration is changed from mol L−1 to mmol L−1, the value of the rate constant of a first order reaction increases 1000 times
The passage states that the units of the rate constant are (mol)1−n (L)n−1 s−1, where n is the order of the reaction. For a first order reaction, n = 1:
\( \displaystyle (\mathrm{mol})^{1-1}\,(\mathrm{L})^{1-1}\,\mathrm{s^{-1}}=\mathrm{s^{-1}} \)
The unit has no concentration term, so changing the unit of concentration leaves the value of k unchanged.
Option (1) If the unit of concentration is changed from mol L−1 to mmol L−1, the value of the rate constant of a first order reaction increases 1000 times — False statement. A first order k is in s−1, so its value does not depend on the concentration unit.
Option (2) The rate equation is the expression that gives the relation between the rate of a reaction and the concentrations of the reactants — True statement. The passage defines the rate equation (rate law) in these words.
Option (3) The rate law of a reaction cannot be predicted merely by looking at its balanced chemical equation — True statement. The rate law must be determined experimentally, as the passage states.
Option (4) The rate of a reaction depends on experimental conditions such as the concentration of the reactants — True statement. In the rate law, Rate = k[A][B], the rate changes when the concentrations of the reactants change.