Chemistry A level revision resource: Calculating reaction rates

How do we monitor rates of reaction?

Firstly, it’s important to understand what a rate of reaction is. When a reaction occurs, molecules are colliding together with enough energy for reactants to be broken down or changed into a new species known as a product (often there is more than one product). So, the rate of reaction is effectively the speed the product is formed and the speed with which the reactant is used up. Since reactions require the molecules to overcome a particular energy barrier to collide successfully, the rate of reaction often indicates whether the conditions are adequate for this to happen. For example, a slow rate of reaction might indicate that few of the collisions are happening with the right amount of force to break the reactants chemical bonds, so the product isn’t made as quickly. If this is known, then manufacturers can research the best way of increasing the number of successful molecule collisions to increase the yield. 

So, it’s important to be able to measure rates of reaction but how do we do it? It would be very difficult to monitor a specific chemical being produced or used as reactions are often a confusing mixture, but quite often we can observe obvious side effects that are easy to measure. For example, an exothermic reaction might produce heat and we can monitor the temperature change over time. Other reactions such as adding hydrochloric acid to a sample of magnesium produces hydrogen gas. This produces effervescence (the posh word for bubbles!). Bubbles can be easily counted and comparing the number of bubbles produced over a set time when you vary another aspect of the reaction such as temperature or acid concentration allows us to see how the rate of reaction varies.

Another common A Level experiment you might encounter is the iodine clock. This time the reaction is monitored by recording how long it takes to see the solution change colour and is explained in the video below. It’s important to make sure everything about the experiments is the same apart from the variable you’re changing, in this case the concentration of thiosulphate, to prevent false results. It can help to work in pairs, one person presses the timer, and another starts the reaction, so you are able to start the clock at the same point for each experiment.  

Why do we need to know about rates of reaction?

Rate of reaction equations

This is the bit that makes everyone nervous; the equations! They can look complicated and sometimes like a completely different language but they are actually very useful. They allow us to work out which reactants are responsible for the rate of reaction from very simple experimental measurements like those mentioned above.

During your exam or in the classroom you may be given a table of data which shows you how the rate of reaction changes when you vary the concentration of each of the reactants. From this information we can create a rate equation.

Rate equations take the form:

k is the rate constant. This is the value that tells us how fast or slow a reaction is. Since the rate of reaction can be affected by a range of variables such as temperature or reactant concentration, the rate constant will also vary. Anything in a square bracket ([ ]) just means we are referring to the concentration of the reactant in the brackets, in this case the concentration of A and B. The last two letters, m and n, are given as powers of the concentration. The numbers which replace m and n indicate how the rate is dependent on the individual reactant. This is known as the order of reaction for that species.

Rate = k [A]^m [B]ⁿ

If a reaction rate does not change when you vary the concentration of A then we know the rate is not dependent on A. In this case we can write m as zero and say that the order with respect to A is zero. Anything to the power zero is equal to one and so we can remove it as we are simply multiplying the rest of the rate equation by one. That’s the easy one!

For first order reactions m is equal to one. As A is to the power 1 we can just write [A]. In practice these means that as you increase the concentration of A, the rate of reaction will increase by the same proportion, for example if you double the amount of A, the rate would double as well. The same principle applies to second order reaction but this time m is equal to 2 so if you double A you have two times the original amount of A to the power of 2 and 22 is equal to 4. So if you double A, the rate of reaction would increase by four times.

Great, so that’s A done, what about B? Well handily it’s exactly the same which makes everyone’s life easier! You will never be asked for an order of more than 2 and if there are more than 2 reactants don’t worry, just add them on like you did with A and B.

Laboratory Confessions

In the Laboratory Confessions podcast researchers talk about their laboratory experiences in the context of A Level practical assessments. In this episode we look at appropriate apparatus to record measurements and the measurement of rates of reaction by continuous monitoring and initial rate methods.

Rates of reaction in industry

Being able to interpret data on the rates of reaction is essential in many areas of manufacturing and research. One of the most important industry applications is in the Haber process which you may have studied at GCSE or A Level. This involves the conversion of nitrogen and hydrogen gas into ammonia, and ammonia has a massive range of applications from cleaning agents to weapons. By monitoring the rates, Haber discovered that the speed of the reaction depended largely on the fact that the triple bond in nitrogen is really hard to break. He and his assistant were able to develop catalysts which allowed this process to occur at much faster rate. This process is 100 years old but is still used today!