Reaction kinetics is the study of rates of reaction, from understanding the factors that affect the basic rate of a reaction to using experimental data to derive rate equations that allow us to pull apart the mechanism for a multi-step reaction.
‘Rates of reaction’ is treated in a very simplistic way at GCSE (it’s not all about the collisions). At A level we develop our understanding from first principles, building in a mathematical framework as we go. You need to be confident in using a range of equations to draw graphs and be able to interpret them in a variety of ways. Unfortunately, we can’t get away from the maths in this topic …
You need to have an understanding of natural logarithms so if you are not studying Maths A level I strongly suggest you work your way through powers and indices and logarithms and natural logarithms before you start, just to revise what you need to know. Alternatively, I can highly recommend the Mathsisfun website for all things maths related.
Finally, it is vitally important that you are able to explain how to set up and run various kinetics experiments, how you would use the results to draw appropriate graphs and interpret them to find the order of reaction with respect to individual reactants, the rate equation and the rate constant for a reaction. The practical and the theory go hand-in-hand, especially where exam questions are concerned!
What does the rate of a reaction depend on?
Just because a reaction is feasible, it doesn’t mean that it will happen at a rate by which we are able to observe reactants changing into products.
Its really all about the collisions!
How does a heterogeneous catalyst work?
Catalysts speed up reactions by lowering the activation energy and are the key component in catalytic converters …
What is homogeneous catalysis?
- What’s the difference between an homogeneous and a heterogeneous catalyst?
- How do we draw an enthalpy profile for catalysed reaction?
Find out with examples from radical reactions in the stratosphere and transition metal chemistry.
Understanding the rate law and orders of reaction
- How do I write the rate law for a reaction?
- What is the rate constant?
- How do I know the order of a reaction?
Experimental methods for following the rate of a reaction
A detailed explanation of quenching and measuring gas volumes (with exam tips on describing methods) …
Determining orders of reaction and a rate equation from experimental data
In order to write a rate equation (rate law) for a reaction we need to know the orders of reaction with respect to each reactant i.e. how changing the concentration of each reactant in turn affects the initial rate of the reaction.
The actual details of the experiment are not important, we simply need the results!
Using colorimetry to follow the rate of a reaction
We can use a colorimeter to follow the rate of a reaction if one of the reactants or products is coloured.
This is an example of continuous monitoring as measurements are taken throughout the course of the reaction.
What is a clock reaction?
A clock reaction can be used to measure the rate of reaction in certain circumstances. In a typical reaction the initial rate (at t=0) doesn’t actually change over a short period of time whilst there are still plenty of reactants present in the reaction mixture.
Half life and first order reactions
All first order reactions have a constant half life, t½, which means that the time taken for the concentration of the reactant to fall to half its initial value is constant and it is independent of the initial concentration.
We can determine the half life for a first order reaction from a graph of concentration against time.
Differential and integrated rate equations for first order reactions
Since the rate of the reaction is constantly changing (just look at the gradient of the curve above), it would be useful to have an expression that describes how the concentration of A changes with respect to time.
We can’t determine this directly since we can’t measure the rate of the reaction directly – hence the need for differential and integrated rate equations …
Using the Arrhenius equation to find the activation energy for a reaction
- Where does the Arrhenius equation come from?
- How can I use it to find the activation enthalpy for a reaction?
Reaction mechanisms, intermediates and the rate determining step
- How do I propose a reaction mechanism for a reaction given the rate equation?
What are autocatalytic reactions?
There are some reactions that can catalyse themselves …