Difference Between First and Second Order Reactions l First vs Second Order Reactions


The key difference between first and second order reactions is that the rate of first order reactions depends on the first power of the reactant concentration in the rate equation whereas the rate of second order reactions depends on the second power of the concentration term in the rate equation.

The order of a reaction is the sum of the powers to which the reactant concentrations are raised in the rate law equation. There are several forms of reactions according to this definition; zero order reactions (these reactions does not depend on the concentration of reactants), first order reactions and second order reactions.

CONTENTS

1. Overview and Key Difference
2. What are First Order Reactions
3. What are Second Order Reactions
4. Side by Side Comparison – First vs Second Order Reactions in Tabular Form
5. Summary

What are First Order Reactions?

First order reactions are chemical reactions of which the rate of reaction depends on the molar concentration of one of the reactants that involved in the reaction. Therefore, according to the above definition for the order of reaction, the sum of the powers to which the reactant concentrations are raised in the rate law equation will always be 1. There can be either a single reactant that takes part in these reactions. Then the concentration of that reactant determines the rate of the reaction. But sometimes, there are more than one reactants that take part in these reactions, then one of these reactants will determine the rate of the reaction.

Let us consider an example in order to understand this concept. In the decomposition reaction of N2O5, it forms NO2 and O2 gases as products. Since it has only one reactant, we can write the reaction and the rate equation as follows.

2N2O5(g)     →     4NO2(g)    +    O2(g)

Rate =  k[N2O5(g)]m

Here k is the rate constant for this reaction and m is the order of the reaction. Therefore, from experimental determinations, the value of m is 1. Thus, this is a first order reaction.

What are Second Order Reactions?

Second order reactions are chemical reactions of which the rate of reaction depends on the molar concentration of two of the reactants or the second power of one reactant that involved in the reaction. Therefore, according to the above definition for order of reaction, the sum of the powers to which the reactant concentrations are raised in the rate law equation will always be 2. If there are two reactants, the rate of reaction will depend on the first power of the concentration of each reactant.

Difference Between First and Second Order Reactions l First vs Second Order Reactions

Figure 01: A graph comparing the two types of order of reaction using their reaction time and the reactant concentration.

If we increase the concentration of a reactant by 2 times (if there are two reactants in the rate equation), then the rate of reaction increases by 4 times. For example, let us consider the following reaction.

2A        →        P

Here A is a reactant and P is the product. Then if this is a second order reaction, the rate equation for this reaction is as follows.

Rate = k[A]2

But for a reaction with two different reactants such as follows;

A   +     B        →        P

Rate = k[A]1[B]1

What is the Difference Between First and Second Order Reactions?

First order reactions are chemical reactions of which the rate of reaction depends on the molar concentration of one of the reactants that involved in the reaction. Therefore, if we increase the concentration of reactant by 2 times, the rate of reaction increases by 2 times. Second order reactions are chemical reactions of which the rate of reaction depends on the molar concentration of two of the reactants or the second power of one reactant that involved in the reaction. Hence, if we increase the concentration of reactant by 2 times, the rate of reaction increases by 4 times. The below infographic presents the difference between first and second order reactions in a tabular form.

Difference Between First and Second Order Reactions l First vs Second Order Reactions

Summary – First vs Second Order Reactions

There are three major types of reactions according to the order of the reaction; zero order, first order and second order reactions. The key difference between first and second order reactions is that the rate of a first order reaction depends on the first power of the reactant concentration in the rate equation whereas the rate of a second order reaction depends on the second power of the concentration term in the rate equation.

Reference:

1. Libretexts. “Methods of Determining Reaction Order.” Chemistry LibreTexts, Libretexts, 5 June 2017. Available here

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