Defining and Measuring Rates

The Rate of a chemical reaction can be thought of as how fast it goes. We just prefer to use the word 'rate' instead of 'speed' when dealing with chemical reactions.

Can you think of some chemical reactions which happen really fast?

Can you think of some which happen slowly?

Fast - any reaction which involves an explosion! Neutralisation reactions between acids and alkalis. Group 1 metals reacting with water....

Slow - chemical weathering of rocks. Rusting of iron/steel

How do we measure the rate of a reaction?

In a general chemical reaction.....

A + B --> C + D

A and B are the reactants and C and D are the products.

When we mix A and B together, they react together to produce C and D. So we could think of the rate of the reaction in one of two ways...

1. The rate at which A and B are used up
2. The rate at which C and D are made.

It tends to be easier to measure number 2, especially if one of the products in the chemical reaction is a gas.

Using a gas syringe

As the gas is produced, it pushes out the plunger in the gas syringe. If we start a stopwatch when the reaction starts, we can measure the volume of gas produced at different time intervals. These numbers can then be plotted on a line graph, a line of best fit drawn and then analysed!

We put time on the x axis because it is the independent variable here and volume of gas collected on the y axis, because it is the dependent variable. We can see that as time increase, the volume of gas also increases, but if we look closely we can analyse a little more.

The line starts at the origin (0,0) because when the time = 0, no gas has been collected! The graph starts off very steep (a steep gradient) because at the very start there are more reactants to react with each other. As they get used up over time, the 'rate' of the reaction slows down and so the gas gets produced at a slower rate. Eventually, the graph levels out because no more gas is produced (the rate of reaction / gradient is zero) because the reaction has stopped!

Instead of using a gas syringe to collect the gas, we can use an upturned measuring cylinder in a trough of water. This can be a little faffy to do, as you have to fully fill the measuring cylinder with water first! When the gas travels down the delivery tube, it then travels through the water and up into the measuring cylinder, displacing the water that is in there. You can then read the volume of gas easily using the scale on the measuring cylinder.