When it comes to writing rate laws there is no one-size-fits-all answer. Every situation is unique and will require a different approach. However there are some general tips that can help you write an effective rate law.
Before you start writing it is important to have a clear understanding of the reaction you are studying. This includes knowing the reactants products reaction mechanism and any other relevant information. Once you have this information you can begin to write your rate law.
There are two main types of rate law: differential and integrated. Differential rate laws are usually easier to write but they can be less accurate. Integrated rate laws are more accurate but can be more difficult to write.
Differential rate laws express the rate of reaction in terms of the concentrations of the reactants. For example if you were studying the reaction A + B → C a differential rate law might look like this:
rate = k[A][B]
This equation says that the rate of the reaction is proportional to the concentrations of A and B. The proportionality constant k is called the rate constant.
Integrated rate laws express the rate of reaction in terms of the time that has elapsed since the reaction started. For example using the same reaction as before an integrated rate law might look like this:
[A]t = [A]0 – kt
This equation says that the concentration of A decreases linearly over time. The rate constant k determines the slope of the line.
To write a rate law you need to know the order of the reaction. The order of a reaction is the sum of the powers to which the concentrations of the reactants are raised. In the example above the order of the reaction is 1 since there is only a single concentration term for each reactant.
Once you know the order of the reaction you can write the rate law. For a first-order reaction the rate law is simply:
rate = k[A]
For a second-order reaction the rate law is:
rate = k[A][B]
And so on. The rate constant k can be determined experimentally.
It is important to note that the rate law only applies to a particular set of conditions. If the conditions are changed the rate law may no longer be valid. For example if the temperature is increased the rate constant will usually increase as well.
Now that you know the basics of how to write a rate law you can start working on your own rate law equations. Remember to carefully consider the reaction you are studying and to experimentally determine the value of the rate constant. With a little practice you should be able to write accurate and effective rate laws.
How do you write a rate law?
The rate law for a chemical reaction is a mathematical expression that relates the rate of the reaction to the concentrations of the reactants.
What is the rate law expression for a first-order reaction?
The rate law expression for a first-order reaction is -rA= k[A].
What is the rate law expression for a second-order reaction?
The rate law expression for a second-order reaction is -rA= k[A]^2.
What is the rate law expression for a zero-order reaction?
The rate law expression for a zero-order reaction is -rA= k.
What is the rate law expression for a reaction that is half first order and half second order with respect to reactant A?
The rate law expression for a reaction that is half first order and half second order with respect to reactant A is -rA= k[A]^(1/2).
How do you determine the overall order of a reaction?
The overall order of a reaction is determined by the sum of the exponents in the rate law expression.
How do you determine the rate constant for a reaction?
The rate constant for a reaction can be determined from the rate law expression by plugging in the known values for the rate and the concentrations of the reactants.
How do you determine the rate of a reaction?
The rate of a reaction can be determined from the rate law expression by plugging in the known values for the rate constant and the concentrations of the reactants.
What is the rate law expression for a reaction that is first order with respect to reactant A and second order with respect to reactant B?
The rate law expression for a reaction that is first order with respect to reactant A and second order with respect to reactant B is -rA= k[A][B].
What is the rate law expression for a reaction that is second order with respect to reactant A and first order with respect to reactant B?
The rate law expression for a reaction that is second order with respect to reactant A and first order with respect to reactant B is -rA= k[A]^2[B].
What is the rate law expression for a reaction that is second order with respect to the sum of reactants A and B?
The rate law expression for a reaction that is second order with respect to the sum of reactants A and B is -rA= k[A][B].
What is the rate law expression for a reaction that is second order with respect to the product of reactants A and B?
The rate law expression for a reaction that is second order with respect to the product of reactants A and B is -rA= k[A]^2[B]^2.
What is the rate law expression for a reaction that is third order with respect to reactant A?
The rate law expression for a reaction that is third order with respect to reactant A is -rA= k[A]^3.
What is the rate law expression for a reaction that is first order with respect to reactant A and third order with respect to reactant B?
The rate law expression for a reaction that is first order with respect to reactant A and third order with respect to reactant B is -rA= k[A][B]^2.
What is the rate law expression for a reaction that is first order with respect to reactant A second order with respect to reactant B and third order with respect to reactant C?
The rate law expression for a reaction that is first order with respect to reactant A second order with respect to reactant B and third order with respect to reactant C is -rA= k[A][B][C].
