Rate-dependent factors

Introduction

This is one of the classic experiments. I think it is worth doing as it is tried and tested and gives students a good understand both of what is meant by reaction rate and how increasing either the concentration or the temperature increases the rate of reaction. It can also be used to cover the mandatory laboratory components listed under Topic 6.1 Collision theory & rates of reaction "Investigation of rates of reaction experimentally and evaluation of the results".

The experiment relies on the fact that when thiosulfate ions react with acid a precipitate of sulfur is formed. 

S2O32-(aq) + 2H+(aq) → H2O(l) + SO2 (g) + S(s)

This reaction is followed by measuring the time it takes for the sulfur formed to obscure a cross which has previously been marked on a tile (or a piece of white card).

It is worth discussing with students whether this time is actually the rate of the reaction. Strictly speaking it is not as it is simply the time taken for a fixed amount of sulfur to appear. The concentration of the thiosulfate is known at the start but the concentration of the thiosulfate when the cross disappears is unknown. However it does seem a reasonable approximation to assume that it is the rate that is measured even though it is clearly not the initial rate.

Teacher’s notes

This is a good practical to aid the teaching and understanding of Topic 6. Students will plot the graphs of concentration against time and temperature against time but they are unlikely to plot the graph of concentration against one over time (1/t) without prompting. They are also unlikely to realise that that the time taken for the cross to disappear is not actually the rate of the reaction. I have written the worksheet below to include the questions I like to give to aid this understanding.

What should emerge from their data is that a plot of concentration against 1/t gives a straight line. Doubling the concentration doubles the rate (i.e. halves the time taken for the cross to disappear). Standard Level students do not need to know about order of reaction but if you are doing this with Higher Level students then it is clear that the reaction is first order with respect to the concentration of thiosulfate ions. The results also tend to support the rule of thumb that increasing the temperature by about 10 oC roughly doubles the rate of the reaction.

For Higher Level students you might like to take this practical further to determine the activation energy (see Determining Ea for a reaction ). 

It is a good idea to tell them to rinse the flask out immediately the cross disappears to prevent a build up of sulfur on the glass at the bottom of the flask. It is also important to keep the cross and the underneath of the flask dry so that it is the same cross each time that is being used.

Standard Level Higher Level Student worksheet

FACTORS AFFECTING THE RATE OF A CHEMICAL REACTION

INTRODUCTION

Thiosulfate ions react in acid solution to produce a precipitate of sulfur according to the equation:

                       
                                    S2O32–(aq) + 2H+(aq) → H2O(l) + SO2(g) + S(s)
 

The rate of the reaction can be followed by measuring the time taken for a fixed amount of sulfur to be produced. This practical first examines the effect on the rate of the reaction when the concentration of thiosulfate ions is altered at a fixed temperature. Then it examines how the rate varies with temperature for a fixed concentration of thiosulfate ions.

ENVIRONMENTAL CARE:

Sodium thiosulfate is known as photographers 'hypo' for fixing developed films and prints. To minimise pollution the concentrations of thiosulfate ions have been kept low and only very small amounts of sulfur dioxide are evolved. The residues can be disposed of down the sink.

SAFETY:

There are no particular hazards associated with this practical except that you should avoid breathing in any sulfur dioxide that is evolved while you are waiting for the cross to disappear.

PROCEDURE:

1. Concentration. Measure 50 cm3 of 0.20 mol dm-3 sodium thiosulfate solution into a conical flask. Place the flask onto a white tile marked with a cross. Measure out 5 cm3 of 2.0 mol dm-3 hydrochloric acid into a measuring cylinder and then add the acid to the thiosulfate starting a stop-clock at the same time. Swirl the contents of the flask and then allow the flask to remain still on the tile. Look down vertically through the solution and time how long it takes for the cross to just disappear. Rinse the flask out immediately but take care not to get the cross on the tile wet. Then put 40 cm3 of the 0.20 mol dm-3 sodium thiosulfate solution into the flask and add 10 cm3 of water. Add 5 cm3 of the hydrochloric acid and time as before. Repeat the experiment four more times using 30, 20, 15 and 10 cm3 of the thiosulfate solution making the total volume up to 50 cm3 with water each time.

2. Temperature.  Measure 50 cm3 of 0.040 mol dm-3 into the conical flask and warm until the temperature is one or two degrees above 20 oC. Place the flask on the tile marked with a cross and add 5 cm3 of 2.0 mol dm-3 hydrochloric acid, timing as before. Record the temperature of the mixture after the acid has been added. Repeat the experiment using fresh portions of the 0.040 mol dm-3 thiosulfate solution each time at temperatures of approximately 30, 40, 50 and 60 oC.

QUESTIONS:

1. Why do you think the flask should be rinsed immediately after each experiment?
2. Why is it important not to get the cross on the tile wet?
3. Using your results from experiment (1) plot a graph of molarity of the thiosulfate solution against time. Plot another graph of molarity against the reciprocal of time (1/t).
4. How does the rate of the reaction change with concentration? Suggest an explanation.
5. Using your results from experiment (2) plot a graph of time against temperature. Plot another graph of the reciprocal of time against temperature.
6. How does the rate of the reaction change with temperature? Suggest an explanation.
7. Apart from concentration and temperature state two other factors which can influence the rate of a chemical reaction.

This worksheet can also be downloaded from:

  Rate-dependent factors

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