Changing the concentration of substances taking part in a reaction usually changes the rate of the reaction. A rate equation shows this effect mathematically. Orders of reaction are a part of the rate equation. This page introduces and explains the various terms you will need to know about. | ||

Note: If you aren't sure about why changing concentration affects rates of reaction you might like to follow this link and come back to this page afterwards - either via the rates of reaction menu or by using the BACK button on your browser. | ||

There are several simple ways of measuring a reaction rate. For example, if a gas was being given off during a reaction, you could take some measurements and work out the volume being given off per second at any particular time during the reaction. A rate of 2 cm | ||

Note: Read cm^{3} s^{-1} as "cubic centimetres per second". | ||

However, for this more formal and mathematical look at rates of reaction, the rate is usually measured by looking at how fast the For example, suppose you had a reaction between two substances For this reaction you could measure the rate of the reaction by finding out how fast the concentration of, say, You might, for example, find that at the beginning of the reaction, its concentration was falling at a rate of 0.0040 mol dm | ||

Note: Read mol dm^{-3} s^{-1} as "moles per cubic decimetre (or litre) per second". | ||

This means that every second the concentration of
For the purposes of rate equations and orders of reaction, the rate of a reaction is measured in terms of how fast the concentration of one of the reactants is falling. Its units are mol dm
I'm not going to define what order of reaction means straight away - I'm going to sneak up on it! Orders of reaction are always found by doing experiments. You can't deduce anything about the order of a reaction just by looking at the equation for the reaction. So let's suppose that you have done some experiments to find out what happens to the rate of a reaction as the concentration of one of the reactants,
That means that if you double the concentration of You can express this using symbols as: Writing a formula in square brackets is a standard way of showing a concentration measured in moles per cubic decimetre (litre). You can also write this by getting rid of the proportionality sign and introducing a constant, k.
This means that if you doubled the concentration of
By doing experiments involving a reaction between This is called the The concentrations of A and B.For UK A' level purposes, the orders of reaction you are likely to meet will be 0, 1 or 2. But other values are possible including fractional ones like 1.53, for example. If the order of reaction with respect to The A and B (a = 1 and b = 1), the overall order is 2. We call this an overall second order reaction.
Each of these examples involves a reaction between
In this case, the order of reaction with respect to both | ||

Note: Where the order is 1 with respect to one of the reactants, the "1" isn't written into the equation. [A] means [A]^{1}. | ||

This reaction is zero order with respect to
This reaction is first order with respect to
It doesn't matter how many reactants there are. The concentration of each reactant will occur in the rate equation, raised to some power. Those powers are the individual orders of reaction. The overall order of the reaction is found by adding them all up.
Surprisingly, the rate constant isn't actually a true constant! It varies, for example, if you change the temperature of the reaction, add a catalyst, or change the catalyst. The rate constant is constant for a given reaction only if all you are changing is the concentration of the reactants. You will find more about the effect of temperature and catalysts on the rate constant on another page. | ||

Note: If you want to follow up this further look at rate constants you might like to follow this link. Alternatively, you could visit it later via the rates of reaction menu. | ||

You will almost certainly have to be able to calculate orders of reaction and rate constants from given data or from your own experiments. There are all sorts of ways of doing these sums, and it is important that you practice the methods that your syllabus wants. Check your syllabus and past exam papers to see what sort of examples you need to be able to work out. | ||

Note: For UK A'level students, if you haven't got copies of your syllabus and past papers follow this link to find out how to get hold of them. | ||

Many text books make these sums look really difficult. In fact for A' level purposes, the calculations are usually fairly trivial. You will find them explained in detail in my chemistry calculations book. | ||

Note: There are several reasons why there are very few calculations on this site. It is much easier to learn to do sums from a carefully organised book than from a website; I would be in breach of my contract with my publishers if I included material similar to what is in the book; and I need to sell a few books to generate some income!
If you are interested in my chemistry calculations book you might like to follow this link. | ||

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© Jim Clark 2002 (modified October 2013) |