Higher and foundation tiers

# Rates of reaction

The rate of a chemical reaction tells us how fast it is going. Some reactions such as a firework exploding are very fast others such as the fermentation of sugar to make wine or the rotting of pears may take weeks while the rusting of iron is a chemical reaction that may take years. Other chemical reactions such as the digestion of food may only take a matter of hours to complete.

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 one of the reactants is used up or how quickly one of the products is produced. That is how quickly the solid calcium carbonate or the hydrochloric acid is used up. However one of the products is the 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.

One way to measure the rate of this reaction is to time how quickly the gas carbon dioxide is released. The apparatus shown below could be used to do this. You could use the gas syringe to measure how much carbon dioxide gas is given off every 30 seconds until the reaction stops.

Using the method shown above you could measure the rate of reaction by measuring the volume of carbon dioxide 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 is 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 was drawn and 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 carbon dioxide gas was released.
• In the next 30 seconds the volume of gas went from 40cm3 to 75cm3; so 35 cm3 of carbon dioxide was released.
• In the next 30 seconds the volume of carbon dioxide gas released went from 75cm3 to 100cm3; so 25 cm3 of gas was released.
• The curve levels off and is flat when 119cm3 of carbon dioxide gas has been released. The fact that the curve is flat tells us that no more products are being made and that the reaction has stopped.
• You can carry on and calculate the amount of gas that was released over the next 30s and continue it for all the results given. It is clear that the rate of production of carbon dioxide 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 rate but the gradient or slope of the line decreases as time passes. This is what is usually observed during a chemical reaction; the reactants will be getting used up as the reaction proceeds so there will be fewer reactant chemicals available to react and so less carbon dioxide gas will be produced. That is the rate of reaction is slowing down.

## 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 starts the carbon dioxide gas passes 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 would affect the results.

A reading of the mass on the balance could be taken every 30 seconds and the change in mass could be used to calculate the rate of the reaction. 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 proceeds there will be less reactants available to react so less carbon dioxide 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 the gas sulfur dioxide; so we could measure the rate of reaction as before by measuring how much gas is given off in certain time period 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. All of the reactants: sodium thiosulfate and hydrochloric acid are colourless solutions similarly water and sodium chloride solution on the products side of the above equation are also colourless. 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. This is usually done by timing how long it takes a large cross (X) drawn on a piece of paper to become obscured by the solid sulfur produced in the reaction. The procedure for this is outlined in the image below:

## Key Points

• The rate of a reaction can be measured by timing how long it takes one of the reactants to "disappear" or one of the products to "appear".
• The method chosen to measure the rate of the reaction will depend on what you intend to measure. For example you could:
• Use a gas syringe to measure how quickly a gas is produced.
• or
• Use a mass balance to record a fall in mass as a heavy gas is released
• or
• Time how long it takes for a solid precipitate to obscure a cross on a piece of paper
• or
• Use a pH meter connected to a data logger to measure changes in pH if one of the reactants or products is an acid or an alkali.