Rate of reaction

We can predict the effect of changes in factors, such as surface area, temperature, concentration and the use of a catalyst, on the rate of reaction using what we know from collision theory. However, without scientific evidence we cannot prove that these predictions are correct.

We can conduct various experiments to investigate the effects of these changes on the rate of reaction. The methods used are varied and the choice of method often depends upon the factor being changed, the states of the reactants used, the products formed and the time available for the investigation. There may be more than one possible method that can be used for each investigation.

One way in which the rate of reaction can be measured is by measuring the change in mass of the reaction mixture. This method is usually used when a gas is given off. One such example of a reaction is the reaction which occurs when calcium carbonate is added to hydrochloric acid. Calcium carbonate reacts with hydrochloric acid to produce carbon dioxide gas, water and calcium chloride solution, as shown by the equation:

The hydrochloric acid is placed in a conical flask. The flask is placed on a mass balance and the initial mass recorded. The calcium carbonate is added to the acid in the flask and the stopwatch is started immediately. A piece of cotton wool is inserted into the neck of the flask to prevent any acid spraying out. The cotton wool also allows the carbon dioxide gas to escape from the flask, thus preventing any gas build up in the flask, which increases the pressure and could cause an explosion. The mass is recorded every 30 seconds until the bubbling has stopped, indicating that the reaction has stopped. The mass of the reaction mixture should decrease due to the loss of carbon dioxide gas from the reaction vessel.

The amount of carbon dioxide gas lost every 30 seconds can be worked out by subtracting the mass recorded from the initial mass of the reaction mixture. A graph can then be plotted with amount of gas produced against time. The rate of the reaction can be calculated by drawing a tangent to the line of best fit over the first 30 seconds.

Another way in which the rate of reaction could be measured for this same reaction is by collecting the carbon dioxide gas produced in a gas syringe, or inverted measuring cylinder, and recording the volume collected every 30 seconds.

A graph of the volume of carbon dioxide gas against time could then be plotted and the rate of reaction calculated by drawing a tangent to the line of best fit over the first 30 seconds.

You should be able to describe experiments used to investigate the effects of changes in surface area of a solid, concentration of a solution, temperature, and the use of a catalyst on the rate of a reaction.

When investigating the effect of changing one factor, it is important that other factors are kept the same so that only the factor being investigated influences the rate of reaction. For example, if you are investigating the effect of changes to the surface area of a solid on the rate of reaction between the solid and an aqueous solution, the concentration and volume of solution used must be kept constant for each experiment.

Examples of these experiments will now be described using the reaction between calcium carbonate and hydrochloric acid and the catalytic decomposition of hydrogen peroxide solution, but you must be aware that there may be other methods that can be used and other experiments that you may conduct or observe.

Changes in surface area of a solid

The surface area of a solid can be changed by using different sized pieces. The total mass of the solid used each time must remain the same.  Calcium carbonate is a solid which can be used as large or small pieces of marble chips or as a powder.

In the reaction between calcium carbonate and hydrochloric acid we can collect the carbon dioxide gas produced in a gas syringe as previously discussed. The method is outlined below:

1. Add a known volume of hydrochloric acid into the flask. This volume must be kept the same for each experiment completed to ensure it is a fair test and that only the effect of changing the surface area is investigated.
2. Set up a gas syringe ready to be connected to the bung and delivery tube which will be placed into the neck of the reaction flask containing the acid.
3. Add a known mass of large marble chips to the acid and immediately replace the bung to ensure the minimal amount of carbon dioxide is lost to the surroundings. The flask and gas syringe should now be connected.
4. As soon as the chips are added, start the timer.
5. Record the volume of carbon dioxide gas collected in the gas syringe at regular intervals until the reaction has stopped.
6. Repeat the experiment with the same volume and concentration of acid and the same mass of marble chips. This time use the smaller marble chips instead.
7. Repeat the experiment again replacing the smaller marble chips with the same mass of powdered calcium carbonate.

The powdered calcium carbonate has the largest surface area and the large marble chips have the smallest surface area. Therefore, we would expect the rate of reaction to be greatest when the powdered calcium carbonate is used. The graph produced would show that the volume of gas produced overall from the reaction would be the same for all three forms of calcium carbonate but the gradient of the line of best fit drawn would be steeper initially for the powdered calcium carbonate as it produces a large amount of carbon dioxide very quickly and the reaction stops sooner.

Changes in the concentration of a solution

The calcium carbonate and hydrochloric acid experiment can be used to investigate the effects of changes in concentration of a solution on the rate of reaction.

The method used would be the same as described previously but this time the mass and type of calcium carbonate used, as well as the volume of hydrochloric acid, would be kept the same for each experiment and the concentration of hydrochloric acid would be changed for each experiment.

As the volume of carbon dioxide gas is still being monitored, the results would be processed in exactly the same way. A graph of the volume of carbon dioxide gas against time would be produced and a tangent drawn to calculate the rate of the reaction.

Changes in temperature

Another way in which the rate of a reaction can be measured is by recording the time taken for a product to be formed. This method can be used if the amount of product being formed is the same each time.

One example of such a reaction occurs when sodium thiosulfate is added to hydrochloric acid. These two chemicals react in a precipitation reaction to form sulphur dioxide, water and a yellow sulphur precipitate.

As the aim of this experiment is to investigate the effect of changes in temperature on the rate of reaction, the volumes and concentrations of the sodium thiosulfate and hydrochloric acid should be kept constant in each experiment. This means that the amount of product formed in each experiment should remain the same.

The solution is colourless to start with. A piece of paper with a black cross drawn on it is placed below the reaction vessel. The black cross can be seen clearly at first but as the reaction progresses, and the yellow precipitate is formed, the black cross becomes more difficult to see. The amount of precipitate produced is a constant in each experiment, so the black cross will no longer be seen when enough of the precipitate is formed, which should be the same in each repeat. The aim of the experiment is to record the length of time until the black cross cannot be seen.

The temperatures of the hydrochloric acid and sodium thiosulfate must be the same as each other for each experiment. To make sure that this is the case, both reactants should be placed in separate test tubes into the same water bath to be heated before they are mixed. To investigate the effect of temperature changes, at least four different temperatures should be used. The method for this experiment is as follows:

1. Add 10cm³ of sodium thiosulfate solution into a conical flask along with 40cm³ of water. Heat the solution in the flask if necessary by placing it in a water bath set at a specific temperature.
2. Place the conical flask on top of a piece of paper with a black cross drawn on it.
3. Measure out 5cm³ of dilute hydrochloric acid in a small measuring cylinder.
4. Add the dilute hydrochloric acid to the solution in the conical flask, swirl the flask to mix the solutions and immediately start the timer.
5. Place the conical flask back onto the black cross and when the cross can no longer be seen, stop the timer.
6. Record the temperature of the solution.
7. Repeat steps 1-6 using water baths set at different temperatures. You may also want to use an ice bath to reduce the temperature of the sodium thiosulfate solution below room temperature.

The rate of reaction for each temperature can then be calculated by dividing 1 by the time taken for the cross to disappear. The higher the temperature the less time should be taken for the cross to disappear and therefore the higher the rate of reaction should be.

Use of a catalyst

A common reaction used to investigate the effect of catalysts is the catalytic decomposition of hydrogen peroxide solution. At room temperature, hydrogen peroxide solution decomposes very slowly to produce water and oxygen gas, as shown by the equation:

To increase the rate of decomposition, a catalyst such as manganese(IV) oxide can be used. The use of a catalyst will cause the hydrogen peroxide to decompose faster, meaning that oxygen gas would also be produced much faster. The catalyst therefore increases the rate of reaction.

The volume of oxygen gas produced can be measured using an inverted measuring cylinder or gas syringe as described previously. The catalyst is added to the hydrogen peroxide solution and the bung quickly replaced to minimise the loss of oxygen gas to the surroundings.

The progress of this reaction can be followed by recording the volume of oxygen at specific time intervals. These volumes can be plotted on a graph of volume of oxygen against time and the rate of reaction calculated by drawing a tangent to the curve of best fit and calculating the gradient.

A catalyst is a substance which increases the rate of reaction whilst remaining chemically unchanged at the end of the reaction. To prove that manganese oxide has acted as a catalyst in this reaction and has not therefore been used up in the reaction, the mass of the manganese(IV) oxide can be recorded before the reaction. The solution can be filtered at the end of the reaction to remove the insoluble manganese(IV) oxide. Once the filtrate has dried, its mass can be measured. The mass of the manganese(IV) at the end of the reaction should be equal to its mass at the start of the reaction.

Using different catalysts for the decomposition of hydrogen peroxide

The effect of using different catalysts on the rate of hydrogen peroxide decomposition can be investigated by using the same method as previously discussed for manganese(IV) oxide but using other solid catalysts such as copper(II) oxide and zinc oxide.

The mass of solid catalyst used in each experiment must be kept constant to ensure the test is kept fair. The graph below shows example results for these three catalysts in the decomposition of hydrogen peroxide.

The graph tells us that manganese(IV) oxide (MnO_{2 (s)}) is the most effective catalyst for this reaction as the gradient for manganese(IV) oxide is steeper than the gradients for copper(II) oxide (CuO_(s)) and zinc oxide (ZnO_(s)). This means that the greatest amount of oxygen gas was produced over the shortest period of time. Zinc oxide is the least effective catalyst for this reaction.