Chemguide: Support for CIE A level Chemistry

26.1: Simple rate equations, orders of reaction and rate constants

Learning outcome 26.1.5

This statement is about the relationship between orders of reaction and organic reaction mechanisms for multi-step reactions, and is in several bits.

Before you go on, you should find and read the statements in your copy of the syllabus.

Start by reading the page orders of reaction and mechanisms.

You don't specifically need to know about molecularity. Just read that bit of the page quickly so that you can refer back to it in case you come across another source that uses the term.

In the section titled "More about reaction mechanisms and orders of reaction", be sure that you understand how to do problems where the slow step is the first step.

The five bits of this statement are slightly confusing when you read them in one go - at first sight there seems to be a lot of similarity between them.

The very best way to work out what you need for this is actually to do lots of questions. There is a question on the specimen paper 4 for the 2022 syllabus (Q3), and lots of examples from previous paper 4s. Look at the questions together with the mark schemes and, preferably, the Examiner's Reports as well.

Look at the most recent ones that you can find. If you go back too far, the style of these questions seems to change.

A few points . . .

if you have read the bit about reactions where the slow step isn't the first one, you will know that I have given you a simple (but flawed!) way of looking at this. That seems to be the way CIE expect students to use in the limited number of questions that I could find.

You can identify an intermediate in a set of equations because it wasn't a part of the original mixture. It is produced and then goes on to produce other things.

You can recognise a catalyst because it will be involved somewhere in the mechanism, but is formed again by the end.

For a simple example of a catalyst during a mechanism, read the first example under the heading "Homogeneous Catalysis" about 2/3 the way down the page types of catalysis. You will meet this reaction again in statement 26.2.3

A few questions have given you a mechanism involving two or three steps and asked you to write the overall equation for the reaction.

You could do this by carefully tracing through everything that happens, but there is a much easier way. Write down everything on the left-hand sides of all the equations. Then write down everything on the right-hand sides. Then cross out everything that occurs on both sides.

In one of their papers CIE gave this sequence of equations although they didn't ask you for the overall equation - I am just using it as a typical example of how to work out an overall equation.

2NO     N₂O₂

N₂O₂ + H₂    N₂O + H₂O

N₂O + H₂   N₂ + H₂O

If you write down everything on the left-hand side, you will get

2NO + N₂O₂ + H₂ + N₂O + H₂

If you write down everything on the right-hand side, you will get

N₂O₂ + N₂O + H₂O + N₂ + H₂O

The N₂O₂ and the N₂O appear on both sides and so you can cancel them out, leaving

2NO + 2H₂   N₂O + 2H₂O

This doesn't involve any thought whatsoever!

In this example, both N₂O₂ and N₂O are intermediates.

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