Rates of reaction:- sodium thiosulphate + hydrochloric acid

Plan:-

We must produce a piece of coursework investigating the rates of reaction, and the effect that changing one particular variable has on the reaction time. The rate of reaction is the time it takes one or more of the reactants to react or the time it takes to produce a new product from a chemical reaction. To work out the reaction time we use the formula.

1 divided by time taken for reaction

There are five factors which affect the rate of a reaction, according to the collision theory of reacting particles:

temperature of solution,
concentration of solution,
pressure of gases in reaction,
surface area of reactants,
Catalysts.

For my preliminary experiments I have chosen to investigate the effect temperature and concentration have on a reaction. I have chosen to investigate these reactants because the are the most practical for the lab and equipment that I have available to use also it would take to much time to prepare a solid in powdered and unpowdered form, and it is difficult to get accurate readings due to the inevitabilities of human errors, and as gas is mostly colourless it is difficult to gauge a reaction changing the pressure, and if a substance is added to give the gas colour, it may influence the outcome of the experiment. Similarly the use of a catalyst complicates things, and if used incorrectly could alter the outcome of the experiment.

My preliminary experiments showed me that it would me more practical and time saving if I just altered the temperature of the solution as changing the concentration of the solution was very time consuming. However the preliminary experiments also helped me to choose a suitable strength of solution.

Aim:-

To see the effects of a change in temperature on the rate of a reaction. The reaction that will be used is:

Sodium Thiosulphate + Hydrochloric Acid.
Na2S2O3 (aq) + 2HCl (aq) .

As I decided after my preliminary experiments I will only carry out one experiment, temperature change whilst all other variables stay constant. As both, sodium thiosulphate and hydrochloric acid are soluble in water it means that I would be able to alter the concentration of either of them. However I have chosen to vary the sodium thiosulphate as it is available in larger amounts. To alter the temperature of the solution a water bath will be used to heat up the acid, thiosulphate and water to the necessary temperature.

I decided on my range temperatures and the concentration that I ...

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I decided on my range temperatures and the concentration that I would use during my preliminary series of experiments –

I would be using a concentration of 0.25 mole:-5cm³ hydrochloric acid

5cm³ water

10cm³ sodium thiosulphate

I will also be using a range of 5 temperatures:- 30ºC, 40ºC, 50ºC, 60ºC and 70ºC

I have chosen to use these ranges of temperatures and concentration because a weaker concentration took to long and a stronger concentration was to fast. I have chosen the range of temperatures because they give the best range of results.

Using my preliminary experiments I decided on using the following apparatus:

2 thermometer
1 beaker
2measuring cylinders
1 conical flask
1 stop clock
1 water bath
1 piece of card with cross in center

1 pair of goggles

Method:-

First I will measure out 5cm³ of hydrochloric acid into a measuring cylinder. I will then place it in to a pre-heated water bath and place a thermometer into the acid. I will then measure out 10cm³ of thiosulphate and 5cm³ of water into a measuring cylinder. I will then place a thermometer into the diluted thiosulphate and place it in to water bath. While the acid and thiosulphate solution are heating up I will place a conical flask on to a piece of a card which has a large black cross drawn on its center. Once the acid and thiosulphate have reached the temperature of the water bath that they’re in, room temperature -70ºC. I will pour them into the conical flask and start the stop clock. I will look at the solution and when it turns to cloudy to see the cross at the bottom of the conical flask I will stop the stop clock. I will repeat this process three times for each of my six temperatures.

Repeat results and averages will be taken to improve the credibility of the findings, and present solid grounding for the final conclusion. The repeat results will help to iron out any anomalies and the average will give a good summary of the results of the experiment. However if one set of results is entirely different to the other, a third experiment will be performed to replace the anomalous set of results.

Safety :–

A pair of goggles will be worn during the heating part of the experiment in order to protect the eyes. I would ideally like to wear an apron to protect the skin and clothing but none are available for use so I will just have to careful when heating and transferring liquids. When handling hot beakers and measuring cylinders a pair of tongs will be used.

Fair Test :–

In order for my findings to be valid the experiment must be a fair one. I will use the same standard each time for judging when the cross has disappeared. I will make sure that the measuring cylinders for the 2HCl and Na2S2O3 will not be mixed up. The amount of 2HCl will be 5 cm³ each time, and the amount of Na2S2O3 will be fixed at 10 cm³ and the water will be fixed at 5cm³. During the heating stage of the experiment, I will be as accurate as I can using the equipment available to me, however the water baths have a margin of error in using them as it will be human judgment which read the thermometer. All of these precautions will make my final results more reliable and keep anomalies at a minimum so therefore make the entire investigation more successful.

Prediction:-

I predict that as the temperature is increased the rate of reaction will increase. This means that the graphs drawn up in my analysis will have positive correlation, and will probably be curved as the increase in rate of reaction will not be exactly the same as the temperature is increased. This can be justified by relating to the collision theory; when the temperature is increased the particles will have more energy and therefore move faster. They will therefore collide more often and with more energy. Particles with more energy are more likely to react successfully.

Results:-

Temperature (°C) Time (sec) Time (sec) Average Time (sec) Rate of Reaction

(5 d.p)

16 (room) 118 105 111.5 0.00897

21 107 96 101.5 0.00985

30 93 78 85 0.01177

35 42.78 34.9 38.84 0.02575

42 2814 21.1 24.62 0.04062

49 13.07 16.5 14.785 0.06764

Analysis:-

In this experiment I have found that as the temperature is increased the time taken for the reaction to take place decreases. This means that as the rate of reaction increasers the time taken for the reaction is decreased, this means that more reactions can take place in a shorter time. In the temperature experiment the average time taken for a reaction to take place decreased was 19.363 seconds for every 10°C increase in temperature.

The above graphs show that rate against temperature has a negative positive correlation, however time against temperature has a negative correlation. This is because as you increase the temperature the rate of reaction increases meaning that it takes less time for each reaction to take place. This is because when the temperature is increased the particles have more energy and therefore move faster, meaning that the will collide more often with more energy. If the particles have got more energy they are more likely to overcome the Particles with more energy are more likely to react successfully. I wouldn’t expect either of the lines of best fit to go through the origin because the increase in time/rate is not directly proportional to the increase of temperature. There aren’t any obvious anomalous results in my graphs so therefore I believe that my results do support my prediction. Had I done this experiment again I would have used a Bunsen burner instead of a water bath as it is more accurate to get the temperatures that I would have wished. However apart from that I don’t think that there is any thing else that I could have done to improve my test results except have had more time to have done the extra set of results that I need to get a wider range of results.

By Kit Beaumont 11EBa