Chapter 16, Problem 60A

Interpretation Introduction

Interpretation:

The significance of the word initial in the method of the initial rates used to determine the rate law for a chemical reaction needs to be explained.

Concept introduction:

Consider the general reaction

$\begin{array}{l}\text{aA+bB}\to \text{Products}\\ \text{rate=}k{\left[\text{A}\right]}^{\text{m}}{\left[\text{B}\right]}^{\text{n}}\end{array}$

Here, $\left[\text{A}\right]$, $\left[\text{B}\right]$ concentration of reactant A and B, k represents rate and m, n are the orders of their corresponding reactant. They are determined by experimentally and are found by examining how altering the initial concentrations affects the initial reaction rate.

Answer to Problem 60A

The word initial in the method of initial rates indicates that it is the starting concentration used. The rates calculated based on the change from the initial concentrations during a period time.

Explanation of Solution

Consider the general reaction

$\begin{array}{l}\text{aA+bB}\to \text{Products}\\ \text{rate=}k{\left[\text{A}\right]}^{\text{m}}{\left[\text{B}\right]}^{\text{n}}\end{array}$

In order to determine the rate law of a reaction, the method of initial rate is used. This method is used for at least three trials with different amounts of the reactants A and B to determine the affect that each reactant has on the reaction rate.

In the first experiment, a common control concentration is used for each reactant to assess the initial reaction rate. In the second trial, reactant B has the same initial concentrations as the first trial, but the concentration of reactant A is doubled.

By comparing the initial rates measured by doubling the concentration of A from trials 1 to 2, the order of reactant A is determined.

This can be replicated to determine the impact of reactant B on the rate. In one trial, keep the concentration of reactants A and B constant, and in another trial hold reactant A constant but double the concentration of the B and examine the effect of doubling on initial rate. The order by which the change in the concentration of B altered the reaction rate is the order of reactant B.

Conclusion

The method of initial rate is used at least for three trials for the different amounts of the reactants.