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rate of reaction sodium thiosulphate and hcl

Investigating the Rate of Reaction - Sodium Thiosulphate and HCl


To investigate the rate of the reaction between Sodium Thiosulphate and concentrated Hydrochloric Acid while changing the concentration.

Background information:

The experiments done support the hypothesis and prediction made. From the results you get you should see there is a directly proportional relationship between concentration and the rate of the reaction. When increasing the concentration the rate should increase. The only way this reaction is possible is if the particles are close enough to each other and when they collide they must have energy. Heating in this reaction will affect the energy levels of the particles because it remained constant. Only six collisions occur in this reaction. (Website 1, 2, 3, 4)


The higher the concentration of Sodium Thiosulphate, the faster the reaction will be. This will cause the cross to disappear a lot quicker due to cloudiness.



We first got the entire equipment ready, the five 250cm³ cylinders and the different measuring cylinders (10cm³, 25cm³ and 50cm³).

We first measured 8g of Sodium Thiosulphate and put it into 200cm³ of water and mixed it.

We then measured 35cm³ of concentrated hydrochloric acid and poured it into a separate beaker of 65cm³ of water.

We then took a piece of paper and labeled each cylinder with 50cm³, 40cm³, 30cm³, 20cm³ and 10cm³ so we wouldn't get confused.

We then poured those amounts of Sodium Thiosulphate into the different cylinders.

We then measured 40cm³, 30cm³, 20cm³ and 10cm³ water and added this to the cylinders that didn't contain 50cm³ to make all solutions equal.

We then started with the solution of 50cm³ Sodium Thiosulphate by pouring in 5cm³ of the concentrated hydrochloric acid that we had mixed.

As soon as we put 5cm³ of the concentrated HCl we shook the solution, put it over the cross and began to time it took to change colour.

We repeated the above with pouring 5cm³ hydrochloric acid in each, timed it and took results.


Rate of reaction = number of mol sulphur / time

Rate of reaction with Time 1 = 32/180 = 0.17 Rate of reaction with Time 2 = 32/73.2 = 0.44 Rate of reaction with Time 3 = 32/52.4 = 0.61 Rate of reaction with Time 4 = 32/38 = 0.84 Rate of reaction with Time 5 = 32/25.8 = 1.24

[Na2S2O3] = amount of solution * initial concentration / 50

With 50cm³ = 50*50/50 = 50 With 40cm³ = 40*40/50 = 32 With 30cm³ = 30*30/50 = 18 With 20cm³ = 20*20/50 = 8 With 10cm³ = 10*10/50 = 2

HCl (cm³)

Na2S2O3 (cm³)

Water (cm³)

Time (s)




























Discussion of results:

My results show that as the concentration decreases, time taken for the solution to change colour increases. This causes a negative graph from the results I have obtained and concludes that the rate of reaction is the speed at which the chemical reaction takes place. It may have been affected by the concentration of the reactants, the temperature of reactants, and the presence of the catalyst which all have a part in creating this reaction. From the results you can see that increasing the concentration will increase the amount of collisions happening in the container giving you the different times we got.


The data we obtained was as accurate as we could get them. We could have used a light sensor to make our results more accurate. The reactions of the different concentrations were very interesting and I would like to do more practicals similar to this one.


Sodium Thiosulphate increases the rate of reaction therefore causing the time taken to decrease. This happens because as the concentration increases and it increases the amount of reactions that are taking place at a certain time. When you use the catalyst which is water it speeds up the rate of the reaction but because there is less concentration the reaction time increases. (Website 3)


All sights accessed 7 September 2009.






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