Investigating rates of reaction

In a nutshell 

Many factors affect the rate of a reaction including changes in surface area, concentration and temperature. You can investigate the effect of these factors on the rates of chemical reactions through experiments which involve measuring gas production and observing changes in colour.

Variable definitions 

Equipment list  

The following equipment list can be used to conduct experiments to measure the rate of reactions. 

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Experiment 1: Investigating the effect of surface area on rate of reaction 

Marble chips/chalk contains calcium carbonate $$(CaCO_3)$$ and reacts with hydrochloric acid to produce carbon dioxide gas in a neutralisation reaction.

​​$$calcium\, carbonate \, + \,hydrochloric \, acid \rightarrow calcium \, chloride\, + \, carbon \, dioxide + \, water$$

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​$$CaCO_3(s) \, + \, 2HCl(aq) \rightarrow CaCl_2(aq) \, + \, CO_2(g) \, +\, H_2O(l)$$​​

Experimental variables

The independent variable is the one that you change to investigate how it alters the rate of reaction. The dependent variable is the one that you measure; it will be affected by changing the independent variable. A control variable is a variable that is unchanged through the experiment.

Safety precautions 

When performing experiments, it is very important you consider safety precautions. This is so you and others do not get hurt. 

1. Marble chips $$(CaCO_3)$$​​ 2. ​$$CO_2$$ gas 3. ​$$HCl$$​

Method

To carry out the experiment follow the instructions below.

Analysis ​​

To observe the effects of surface area on rate of reaction the data is plotted on a graph. 

Your graph should look something like this:

1. Whole marble chips experiment 2. Crushed marble chips experiment 3. Powdered chalk experiment 4. Double quantity of marble chips

Conclusion

When the line on the graph plateaus, it indicates that gas production stops and the reaction has ended. The powdered chalk experiment should have the steepest gradient and end first; indicating the fastest rate of reaction. This is because powdered chalk has the highest surface area. The experiment with whole marble chips should take the longest to end; indicating the slowest rate of reaction.

Evaluation

Once completing the experiment it is important to you check if there are any anomalies in the graph. If there are anomalous result(s), have a think about why this may be and whether they are down to external factors or how you carried out the experiment. If your trends do not match the graph above you may be required to repeat the experiment.

Experiment 2: Investigating the effect of concentration on rate of reaction

Changing the concentration of solutions can also affect the rate of reaction.

Experimental variables 

Method

You should repeat experiment one, but this time you should not vary the size of the solid and should only use whole marble chips. You should then vary the concentration of $$HCl$$HClHCl​ acid in each experiment; use  $$0.1 \,M, 0.2 \,M$$ and $$0.5 \,M$$​ acid.

Analysis

Repeat the same analysis as experiment one. The lines now represent different concentrations of acid instead of different particle sizes.

1. Lowest acid concentration 2. Middle acid concentration 3. Highest acid concentration

Conclusion

The experiment with the highest concentration of acid should have the steepest line and should come to an end first as it has the fastest rate of reaction. 

Evaluation

Carry out the same evaluation to this set of data as you did to experiment one.

Experiment 3: Investigating the effect of temperature on rate of reaction

To investigate the rate of reaction you can carry out a precipitation reaction. Sodium thiosulfate and hydrochloric acid, both colourless solutions, react to form sulfur, a yellow precipitate. The amount of time it takes for the precipitate to form can be used to measure the rate of reaction. 

NaS_2O_3(aq) \, + \, 2HCl(aq) \longrightarrow SO_2(aq) \, + \, H_2O(l) \, +\, 2NaCl(aq)

​$$sodium \, thiosulphate \, + \,hydrochloric \, acid \rightarrow sulphur \, dioxide \, + \, sulphur \, + \, water \, +\, sodium \, chloride$$

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​$$Na_2S_2O_3(aq) \, + \, 2HCl(aq) \rightarrow 2NaCl(aq) \, + \, SO_2(g) \, + \, H_2O(l)+S(s)$$​​

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Experimental variables  

Safety precautions 

​When performing experiments, it is very important you consider safety precautions. This is so you and others do not get hurt. 

​

Method

To carry out the experiment follow the instructions below.

Analysis ​​

To observe the effects of surface area on rate of reaction the data has to be plotted on a graph. 

Your graph should look something like this:

1. Lowest temperature 2. Highest temperature

Conclusion

The experiment carried out at the lowest temperature should take the longest time to cover the cross mark, as it has the slowest rate of reaction. As temperature increases, the rate of reaction increases and the reaction time is shorter.

Evaluation

Look at the graph to see if all the results match the line of best fit. If there is an anomaly, repeat the experiment again at the given temperature. If you have more time it is worth carrying out the reaction at each temperature three times, and plotting the mean time taken for the cross to disappear on the graph.