Higher and foundation tier

The rate of a chemical reaction tells us how fast it is going. Some reactions such as an explosion are very very fast, others such as iron rusting are much slower.

There are many ways to measure the rate of a reaction, you could measure:

• How long it takes for a colour or turbidity change to happen.
• How long it takes for a pH change to happen.
• How long it takes for a certain volume of gas to be given off.
• How long it takes for a mass change to happen.
To measure the rate of a reaction you need to be clear about what exactly it is you are measuring, e.g. Consider the reaction between calcium carbonate (chalk) and hydrochloric acid. An equation for the reaction is shown below:

We could measure the rate of this reaction by measuring how quickly the reactants are used up. That is how quickly the solid calcium carbonate or the hydrochloric acid is used up. However one of the products is a gas, carbon dioxide. It would be much easier to measure the rate of reaction by measuring how quickly this gas is given off. There are a number of ways of doing this, see below:

Using the method shown above you could measure the rate of reaction by measuring the volume of gas released every 30 seconds. Obviously the larger the volume of gas released the faster the reaction. A typical set of results for this experiment as shown in the table below:

 time/s volume of gas/cm3 0 30 60 90 120 150 180 210 240 0 40 75 100 111 116 119 119 119

A graph of these results is shown opposite. There are a few points you should note from the graph:

• In the first 30 seconds of the reaction 40 ml or 40 cm3 of gas was released.
• In the next 30 seconds the volume of gas went from 40cm3 to 75cm3, so 35 cm3 of gas was released.
• In the next 30 seconds the volume of gas went from 75cm3 to 100cm3, so 25 cm3 of gas was released.
• The curve levels off and is flat when 119cm3 of gas has been released. The fact that the curve is flat tells us that no more product is being made and that the reaction has stopped.
• You can carry on and calculate the amount of gas that was released over the next 30 s and continue it for all the results given. It is clear that the rate of production of gas is falling, that is the rate of the reaction is slowing down. This is obvious from the graph, the slope of the line will give an indication of the rate of reaction, the line to begin with has a steep gradient, indicating a fast reaction but the gradient of the line decreases as time passes. The reactants will be getting used up as the reaction proceeds, so there will be less chemicals available to react and so less gas will be produced.

## Method 2- Using a measuring cylinder to measure the volume of gas released

This method is very similar to the one above. You probably used one or other method in your science lessons. Below an inverted measuring cylinder is filled up with water. As the reaction happens the carbon dioxide gas passing down the delivery tube and rises up into the measuring cylinder, pushing the water out as it does. You can measure the volume of gas in the measuring cylinder every 30 seconds and calculate the rate of reaction from your results. This method works well for carbon dioxide gas, since its solubility in water is fairly low. Obviously this method would not suitable if the gas to be collected was very soluble in water.

## Method 3- Use a balance to measure mass loss

Since carbon dioxide gas is a heavy gas it is possible to measure the rate of this reaction by measuring the loss in mass of the reactants as the reaction takes place. In the apparatus below the carbon dioxide gas released by the reaction will escape through the pipe in the stopper. It is essential to have some kind of stopper as the effervescence (fizzing) from the bubbling can cause splashes which if they left the conical flask could affect the results. A reading of the mass on the balance could be taken every 30 seconds. A valid set of results should show that to begin with there will be a large drop in mass as large amounts of carbon dioxide is released. As the reaction takes place there will be less reactants available so less gas should be released, this means the mass will drop more slowly. This method would not be particularly suitable for lightweight gas such as hydrogen.

## Method 4- Colour change or change in turbidity (how clear a solution is)

Consider the reaction between sodium thiosulfate and hydrochloric acid:

One of the products of this reaction is a gas, sulfur dioxide, so we could measure the rate of reaction as before by measuring how much gas is given off in certain time using a gas syringe. However sulfur dioxide is a toxic gas and so it is probably not advisable to collect large amounts of it in a syringe. However all of the reactants, sodium thiosulfate and hydrochloric acid are colourless solutions as is water and sodium chloride solution on the products side of the above equation. This leaves sulfur, sulfur is a yellow solid that is insoluble in water. In this reaction as the solid sulfur is produced it causes the solution to gradually turn yellow. That is it changes colour. We could measure the rate of this reaction by timing how long it takes the solution to change colour or turbidity, study the image below for more information.