Investigate how changing the concentration of Sodium Thiosulphate affects the rate of reaction with Hydrochloric Acid.

Aim:

I am going to investigate how changing the concentration of Sodium Thiosulphate affects the rate of reaction with Hydrochloric Acid. I am going to time how many seconds/minutes it takes until you can no longer see the black cross on a piece of paper under the beaker. Each time I do the experiment, I will change the amount of thiosulphate and water in the reaction mixture. I want to find out if the concentrated thiosulphate makes the reaction go quicker.

Background Information

For a chemical reaction to take place, the particles have to collide with enough energy for something to change.

There are five factors that will affect the rate of reaction that we need to take into consideration, these are:

Temperature - I will conduct all the tests at room temperature (hopefully on the same day) because temperature has an effect on the rate of the reaction.
Concentration – I will try my best to use all the acid from the same bottle, which then makes it more of a fair test as all the acid should be the same concentration.
Surface Area – This will not affect any of my results, as we are not using any solids in the investigation.
Catalyst – This speeds up reactions.
Pressure – The pressure will be the same in each of the experiments, as we will try to use exactly the same amount of acid in each experiment and the same size beaker.

Prediction!

I predict that the greater the concentration of Sodium Thiosulphate in the solution, the faster the chemical reaction will take place. Therefore, the cross will disappear more quickly due to the cloudiness of the solution.

I think that the concentration of a solution affects the rate of reaction because the rate of reaction depends on how much the molecules of the reacting substances collide. A more concentrated substance has more molecules for a given volume than a more dilute substance. Because there are more molecules about, the frequency of collisions is greater, and the reactions happen faster.

Method:

Equipment -

- 2 Measuring cylinders

- Conical flask

- Beaker

- Stopwatch

- Paper with cross marked on it

- Sodium Thiosulphate solution

- Hydrochloric Acid

- Water

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Method:

Equipment -

- 2 Measuring cylinders

- Conical flask

- Beaker

- Stopwatch

- Paper with cross marked on it

- Sodium Thiosulphate solution

- Hydrochloric Acid

- Water

I am going to measure how changing the concentration of Sodium Thiosulphate affects the reaction time between it and hydrochloric acid. Every time I will add 20 ml of hydrochloric acid that is 2 lower to that, I will add 20 ml of a Sodium Thiosulphate and water. By varying the amount of water in the 20 ml of Sodium Thiosulphate solution, I will vary the concentration. I will first put the hydrochloric acid in a beaker with a cross on a piece of paper underneath. Then I will add the Sodium Thiosulphate and will start the stop clock immediately. As soon as the cross disappears, I will stop the timer.

I will use a range of concentrations to get a wide spread of results. The concentrations I will use are:

4 ml sodium thiosulphate + 16 ml water

8 ml sodium thiosulphate + 12 ml water

12 ml sodium thiosulphate + 8 ml water

16 ml sodium thiosulphate + 4 ml water

20 ml sodium thiosulphate + 0 water

Safety!

To conduct my experiment safely I will follow normal science room rules, which include:

Wearing safety goggles to protect my eyes from the acids we are using.

Stay stood up when doing the experiment to prevent spilling the acids on me or work around me.

Taking care when handling chemicals, particularly Hydrochloric acid and Sodium Thiosulphate because they are irritants. I will not touch my eyes or mouth until I have washed my hands.

I will also take care when moving and using glass beakers and test tubes to stop injury.

I will be sensible all the way through the experiment to stop me and people around me from getting injured.

Fair Test!

To make this experiment a fair test I will only vary one thing - the concentration of the Sodium Thiosulphate solution. I will conduct all the tests at room temperature because temperature has an effect on the rate of the reaction. The measures of Hydrochloric acid will all be the same (10cm). I will also try to use all the acid being used of the same bottle, this way it is a fair test. The person timing the experiment will keep an eye on the cross, so that we can be very accurate with the timing, otherwise there may be extra seconds added on to the final time of the experiment which would then not even be worth keeping and would have to be repeated. The reason I am repeating this test 3 times, is to get the most accurate results I can. The more results the better I can judge what good results to work on are and what are irregular results. Also by doing this test 3 times I can get better at doing the test and improve as I go on.

Results:

Test 1

It is hard to see any irregular results in this but I think that it took too long to disappear fully. This could be because the person stopping the clock and not doing it exactly on time, or it could be down to something more scientific. I would need to do more tests to check this.

Test 2

We were not sure if there were irregular results, but we felt that either the 16 ml of Sodium Thiosulphate was too fast a time or the 12 ml Sodium Thiosulphate was too slow a time. Therefore, we decided to test 14 ml of sodium thiosulphate and to re-test the 12 ml.

This proves our prediction that the 12 ml Sodium Thiosulphate took too long but that makes you think whether the results for both the 8 ml of Sodium Thiosulphate and the 4 ml of Sodium Thiosulphate. I need to look at more results to make a accurate decision. Or do this set of results again.

Test 3

I think that these are my best results, there does not appear to be any irregular results and the reaction times are around those I was expecting.

Conclusion

Average results:

The 16 ml of Sodium Thiosulphate is the irregular result in this case. It does not follow the pattern like the results. One problem in using the average results is that one 'bad' result will affect the overall average and will make that result less accurate or reliable.

I conclude that the more concentrated an acid is, the quicker the rate of reaction time will be.

I have come to this conclusion because of several reasons. Firstly, my results give conclusive evidence that as the amount of Sodium Thiosulphate decreases, and the amount of water in the solution increases there are less atoms to collide and therefore less successful collisions causing chemical change so the reaction rate is slower. In a more concentrated solution, there are more atoms to collide so the reaction time is quicker.

My results support the prediction I made because I said 'the greater the concentration of Sodium Thiosulphate the faster the rate of reaction time.’ I believe I was correct and another source states that 'the reaction time will be faster with a more concentrated solution because, the more molecules there are, the frequency of successful collisions is greater and therefore the reaction rate is speeded up'. - Secondary source Britannia Interactive Encyclopedia 1998.

Evaluation

My experiment went according to plan but there were faults in it. Although the reaction I chose had a definite end, it was well hard to tell whether the whole cross had disappeared or not. Instead of using a cross, a light beam could be used and when the beam goes out that is the end point. If The Gilberd had this kind of equipment then I think it would have been a much more accurate investigation.

I think that because my results have a certain amount of inconsistency between them it would be a good idea to repeat them again if I had time

Are my results accurate?

My results are good in their accuracy. Although, I did get some irregular results but I was expecting that. All my 'good' results fitted my prediction. I tested my results three times to make sure that I did not make any mistakes.

Are they reliable?

I feel that my results are accurate apart from the ‘odd’ one. I see them as reliable but if my views then my results are not reliable. I think I would need to look at this investigation more to make my mind up to whether they are or not.

Other things that may have affected the accuracy of the experiment could include, the distance of the persons eyes away from the conical flask and the X underneath. Also before we done the test, we could of tested to see who has the best eye vision, which would therefore help a lot.

To extend this work one could look into the affects of catalysts (and other variables) on reaction times or you could try the same experiment with different substances or by varying the amount of acid instead.

David Mateer