Rate Equations - Introduction to Orders of Reactions (A-Level Chemistry)
Introduction to Orders of Reactions
Orders of Reaction
Changing the concentration of a reactant can change the rate of reaction.
In the rate equation as shown above, m and n are the orders of reaction with respect to the reactants, A and B.
Order of reaction of a reactant can be defined as the power to which the concentration of that reactant is raised in the rate equation.
It shows how the rate of reaction depends on each reactant concentration.
The order of reaction for each reactant can only be determined experimentally.
The order of the reaction is usually an integer, with the values 0,1 or 2.
To find the total order of a reaction, add up the powers, e.g. m + n.
The order of reaction with respect to the concentration to A is 1, first order.
The order of reaction with respect to the concentration of B is 2, second order.
The overall order of reaction is 3, third order.
Looking at the above rate equation we can see that the concentration of A is raised to the power of 0.
This means that the concentration of A has zero effect on the rate of reaction.
Therefore if the concentration of A changes, the rate of reaction will not be affected by this change.
Determining Order of Reaction
Orders of reactions can be determined through experimentation.
This is obtained by changing the concentrations of reactants and comparing the rate of reaction to that of a control experiment where the concentration of reactants have not been changed. The methods to determine the orders of reaction are discussed in detail in future tutorials.
Worked Example: Using the rate equation, 
, determine the order of reaction with respect to 
Determine the overall order of the reaction.
Answer:
Order of reaction with respect to [BrO3- ] = 1 (as the power = 1)
Order of reaction with respect to [Br- ] = 1 (as the power = 1)
Order of reaction with respect to [H+] = 2 (as the power = 2)
Overall order = 1+1+2 = 4
Rate equations in Chemistry describe how the rate of a chemical reaction changes as the concentration of reactants changes. They provide a mathematical expression of the relationship between the rate of a reaction and the concentration of reactants.
Orders of reactions in Chemistry describe the relationship between the rate of a reaction and the concentration of reactants. The order of a reaction can be zero, first, second, or third, depending on the reaction mechanism. The order of a reaction determines the rate equation of the reaction.
A zero order reaction is a reaction in which the rate of reaction is constant, regardless of the concentration of reactants. The rate equation for a zero order reaction is simply the rate constant multiplied by time.
A first order reaction is a reaction in which the rate of reaction is directly proportional to the concentration of reactants. The rate equation for a first order reaction is the rate constant multiplied by the concentration of reactants.
A second order reaction is a reaction in which the rate of reaction is proportional to the square of the concentration of reactants. The rate equation for a second order reaction is the rate constant multiplied by the concentration of reactants squared.
A third order reaction is a reaction in which the rate of reaction is proportional to the cube of the concentration of reactants. The rate equation for a third order reaction is the rate constant multiplied by the concentration of reactants cubed.
The order of a reaction can be determined by conducting experiments to measure the rate of reaction at different concentrations of reactants. From this data, the rate equation can be determined, and the order of the reaction can be calculated.
Understanding the order of reactions is important because it helps to predict the rate of reaction at different concentrations of reactants. This information can be used to optimize reaction conditions and design more efficient chemical processes. Additionally, a knowledge of the order of reactions is essential for understanding the mechanisms of chemical reactions and predicting their outcomes.
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