Rate of Reaction - Sodium Thiosulphate and Hydrochloric Acid Essay

Rate Of Reaction Sodium Thiosulphate And Hydrochloric Acid Essay

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I am going to investigate the effect of varying the concentration of Sodium Thiosulphate solution in a reaction with Hydrochloric Acid. The theory behind this experiment is that ‘Increasing the concentration can increase the rate of the reaction by increasing the rate of molecular collisions.’

  I chose this reaction because it has a definite end point (when the cross disappears on the paper). The equation for the reaction is:

Sodium Thiosulphate + Hydrochloric Acid    È   Sodium Chloride + Water + Sulphur + Sulphur dioxide


Na  S  O  (aq) + 2HCL  (aq) È  2 Na Cl (aq)  + H  O (l) + S (s) + SO  (g)




·         ;2 Measuring cylinders

·         ;Conical flask

·         ;Beaker

·         ;Stopwatch

·         ;Paper with cross marked on it

·         ;Sodium Thiosulphate solution

·         ;Hydrochloric Acid

·         ;Water

·         ;Pipette


I predict that the greater the concentration of Sodium Thiosulphate 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 effects the rate of reaction because ‘the rate of reaction depends on how frequently 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 successful collisions is greater, and the reactions happen faster.’


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

·       ;The wearing of safety goggles to protect my eyes from chemical splashes.

·       ;Standing up to conduct the experiment, therefore reducing the risk of tripping and spilling chemicals.

·       ;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 thoroughly washed my hands.

·       ;Taking care when using glassware to prevent injury.

Variable Control

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). The person timing the experiment will look for the disappearance of the cross, otherwise there would be a time lapse between seeing the cross disappear and telling the other person to stop the clock and then eventually stopping the clock.


I am going to pour 50cm  of Sodium Thiosulphate or Na  S  O   into 10cm  of Hydrochloric Acid or HCL in a conical flask. The concentration of the sodium Thiosulphate is 40g per litre and the concentration of the Hydrochloric Acid is 2 molar. The flask will be placed on a piece of paper with a cross drawn on it. I will start the stopwatch when I add the acid to the Na  S  O  . I will stop the stopwatch when the solution is cloudy enough to prevent me seeing the cross. I will repeat this procedure with the measurements in the table below. Then to gain an average I will repeat all the tests that give a more accurate result.


I will record my results in a table and then work out an average afterwards to gain a more accurate result.


I conclude that the more concentrated a reactant 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 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 a secondary 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.


My experiment went according to plan but there were flaws in it. For example, I only obtained two sets of results and then worked out an average. I could have done more tests to gain a more reliable average.

I think there is also a human error factor involved when you are measuring liquids and looking for an end point in the reaction. Although the reaction I chose had a fairly definite end point it was still 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. A better standard of measuring cylinders and pipettes could be used.

There was one result that was not really an anomaly as it still followed the pattern but was slightly different to the other result of the other test. When the amount of Sodium Thiosulphate was 10 cm, the time of the first test was 3065 compared to 279 in the second test. This could be human error, which is the reason for doing more tests.

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