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Rates of reaction 1 - What affects the rate of reaction?

A chemical reaction can be fast or slow. It just depends on what you do to it. In this activity you will try to measure how a chemical reaction is affected by temperature.

When sodium thiosulfate and acid are mixed they produce a precipitate of sulphur. The mixture goes cloudy and we can measure how quickly it does so by using a light sensor connected to a computer.

##### What to do

Set up your apparatus as shown above. Place 20ml of sodium thiosulfate in the flask. Do the following test run before your real experiment.
Get the computer ready to measure the light level over a period of a few minutes.

1. Start the computer recording.
2. Add 4ml acid to the flask. Stir quickly and take the temperature.
3. Watch the trace on the computer while the reactants form a precipitate.
4. Stop the computer recording and wash the flask.
• find the point when the reaction began
• find a point when the reaction was working steadily - perhaps 30 or 60 seconds later.
• get the computer to work out the average gradient of the graph between these two points.

Now think about the experiments you will do to show the effect of changing the temperature on the reaction rate. You need to do at least 4 different experiments at 4 different temperatures. Tell your teacher about your experiment plan. Do your experiments.

Questions
• Look at one of your graphs. Write a sentence describing what was happening in this reaction.
• Use the software to calculate the average gradients of each of the graphs you obtained. Then, draw a scattergraph (or x-y graph) of temperature against the gradient. Give your graph a title.
• What does your scattergraph tell you about the effect of temperature on the rate of this reaction?
• Why does a higher temperature increase the rate of a reaction?
• Use your graph to find out how much faster the reaction goes when you double the temperature.
Extra

Describe an experiment to show the effect of sodium thiosulfate concentration on the rate of this reaction. Make a special point of mentioning the ways you can make this a fair test. Do your experiment if time permits.

Activities in this section adapted from The IT in Science book of Data logging and Control. © IT in Science and may be reproduced as needed for use within your school.

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