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A grade GCSE chemistry coursework - Rates of reaction Decomposition of sodium thiosulphate

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This is an investigation about the decomposition of sodium thiosulphate. There are many factors (variables) which determine how fast or slow the reaction can take place (rate of reaction). These are:

  • Initial volume of sodium thiosulphate.
  • The initial volume of hydrochloric acid.
  • The initial volume of water.
  • The temperature of the reaction mixture.
  • The concentration of the sodium thiosulphate.
  • The concentration of hydrochloric acid.
  • Changing the pressure

In this particular investigation I am going to find out how heat affects the rate of reaction in the reaction of sodium thiosulphate with hydrochloric acid. Sulphur forms a colloidal precipitate which gives the solution a cloudy yellow appearance. The rate of reaction can be found by measuring the time for the solution to become cloudy. I will measure the amount of time for the solution to become completely cloudy by drawing a cross on a piece of paper and place it under the beaker. As the reaction takes place, the solution becomes cloudy and the cross gradually disappears. The equation for this reaction is:

Sodium thiosulphate + hydrochloric acid sodium chloride + water + sulphur dioxide + sulphur

Na2S2O3(aq) + 2NaCl(aq) 2NaCl(aq) + H2O(l) + S(s) + SO2(g)

Preliminary work

I have done preliminary work to work out the right amounts/concentrations etc to use and also to decide the best range results.

Below is a table to show my results:


From the table above I can see that, as the temperature increases, the time taken for the cross to disappear decreases. This means that the rate of reaction increases because of the fact that there are more collision and so more successful collisions. Also, for every increase of

10°C, the time taken for the cross to disappear is roughly halved. It is not exactly halved each time, for example at 10 °C the time taken for the cross to disappear is 195 seconds and at 20°C the time taken for the cross to disappear is 107, which is roughly half of 195 but not quite.

I think the apparatus I used was good, however the range of my results could be bigger so in the main investigation I will increase the temperature by 5°C each time.

Main investigation


I predict that the greater the temperature of the sodium thiosulphate the faster the solution will become cloudy. This is because the greater the temperature, the faster the particles will move, therefore increasing the amount of collisions and successful collisions taking place. Similarly, when the temperature is decreased there will be less energy, so there will be less collisions and successful collisions taking place so the rate reaction will decrease. From my preliminary results I have come up with a theory that for every 10 °C of temperature increased, the time taken for the cross to disappear is roughly decreased by half, but only in some cases.

This is because the particles will have more energy due to the increase in temperature and there will be twice as many collisions, resulting in twice as many successful collisions and therefore the decomposition of the sodium thiosulphate will happen twice as fast when the temperature is increased every 10°C.

In the main investigation I will see if this applies to my results.



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The apparatus I will use in this experiment are as follows:

  • Safety goggles
  • Bench mat
  • Teat pipette
  • Thermometer
  • Stop clock
  • Gauze
  • Square of paper (to draw cross on)
  • 250cm3 conical flask
  • 40cm3 measuring cylinder
  • 5cm3 measuring cylinder
  • 5cm3 2M hydrochloric acid
  • 40cm3 0.1M Sodium thiosulphate
  • Tripod


Firstly I will draw a small dark cross on a piece of paper and place it on a bench mat. I will then put 40cm3 of sodium thiosulphate into a conical flask and place it on a tripod and gauze. I will heat the solution for a few minutes until the desired heat is reached, (i.e. 5 degrees for the first time). Next I will place the conical flask on top of the cross, pour 5cm3 of hydrochloric into the hot sodium thiosulphate solution and time how long it takes for the cross to disappear. I do this experiment ten times, each time heating the sodium thiosulphate 5°C hotter each time. On each experiment I will take three readings and work out an average from the data I have obtained.

Below are diagrams to illustrate and explain my method


Fair Test

To ensure that this experiment is fair I will control the variables, i.e. I will keep everything constant apart from the variable I am changing-temperature. Therefore, I will make sure that I only change the temperature of the reaction mixture and keep the volume and concentration of the sodium thiosulphate, the volume and concentration of hydrochloric acid and the pressure of all the mixtures the same.

Furthermore I will measure out the volumes of both the sodium thiosulphate and hydrochloric acid to the greatest degree of accuracy I can manage. I will also try to make my timing and the heating of the sodium thiosulphate solution as accurate as possible.


There are different variables which can be changed. I am, however due to my line of enquiry, going to change the temperature of the solution. The independent variables will be the initial volume and concentration of sodium thiosulphate, and the initial volume and concentration of hydrochloric acid. Below is a table to show the effect of changing different variables.



I will make sure I handle the sodium thiosulphate solution and hydrochloric acid with care. I will also be careful when heating up the solutions and will at all times wear safety goggles and also remain standing throughout the experiment. Furthermore I will be careful not to spill anything and will take extra care when doing the experiment.


Below is a table showing my results from the main investigation


I can see from observing the table that the first part of my prediction was correct. As the temperature increases the rate of reaction increases too, due to more collisions and more successful collisions, caused by the increase in temperature.

However, the second part of my prediction that for every increase of 10°C the rate of reaction is roughly halved, is only true in some places. For example, when the temperature is 40°C the reaction took 28.7 seconds and when the temperature was 50°C the reaction took 14 seconds which is roughly half the time. In contrast, when the temperature was 5°C the reaction took 290 seconds, and when the temperature was increased by 10°C to 15°C the reaction took 128.7 seconds, which is not half or even roughly half 290.

I am going to draw a graph analyse by results properly.


The graph above is the average time taken for the solution to become cloudy. It also has a line of best fit to show the consistency and accuracy of the results. There are no anomalous results which shows that the experiment was very accurate. Furthermore, the graph is a perfect curve

It is obvious that there is a general trend in the results. As the temperature increases, the time taken for the sodium thiosulphate solution to become cloudy decreases. This is because as the temperature increases, the reactant particles move more quickly. In addition to this, more particles have activation energy. This means more of the particles collide and more of the collisions result in a reaction, so the rate of reaction increases.

My results do support the first part of my prediction.


All of my results were accurate and fitted the pattern of results. I did not have any problem whilst carrying out my investigation apart and do not believe I could have done it any better without specialist equipment with plenty of time.

The quality of my evidence is good. There were no points on my scatter graph that were not on or close to the line of best fit. The range of my results were also suitable to make good observations and conclusions

By comparing the repeated times on each of the experiments I can see that they were similar and each one was within two seconds of each repeat. From this evidence I believe my results were very reliable.

My prediction that for every increase of 10°C, the time taken for the sodium thiosulphate to become cloudy is halved was only true for some of the temperatures, and therefore I cannot say that my prediction was correct. This might have occurred for several different reasons, such as the fact that the temperature control might not have been exactly the same or that my judgement of when the cross may have disappeared might not have been exactly the same for each one. However, I don’t think I could have controlled any of these variables any better than I had done without specialist equipment which I did not have.

In order to provide additional evidence and extend my investigation to find out more and back up my conclusion I could perhaps repeat the experiment using specialist equipment. I could also change the initial volume of either the hydrochloric acid or sodium thiosulphate solution and see if I get a similar pattern of results. If the results were consistent and accurate I could use them to back up my original theory. To further investigate the decomposition of sodium thiosulphate I could change the concentration of hydrochloric acid and sodium thiosulphate or use a catalyst in the reaction.

Overall I think that my investigation was successful in obtaining reliable results.


will smith on June 30, 2014:

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pizza123 on June 30, 2014:


shawn on June 12, 2014:

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darren on June 11, 2014:

ooooooh sorry si

Liam on June 11, 2014:

Please stop having a convo - I WANT TO REVISE

darren on June 11, 2014:

hello jamie

darren on June 04, 2014:

i am from allertonshire and this will help me a lot

ghandi on June 04, 2014:

hello you guys are awesome

geoffrey on May 09, 2014:

hi guys!!! thanks so much!!! love you guys you saved me on my chem hehe chem is my fave!!! lol i put a testube up my botty

Declan on May 05, 2014:

Thanks so much for this! You saved my butt! Well written and very informative! Thanks again :)

Charlotte on April 27, 2014:

This really helped me with my chemistry coursework and I used the physics one for my physics a coursework and I got an An overall thank you so much!!!!

Nico on March 11, 2014:

Great article which really helped me with my experiment. Thank you so much!!!!

James on March 04, 2014:

Don't post that wrong web page sorry :( great website though using to too help with my aqa Isa

MB on February 18, 2014:

I have done the same experiment and my results are far off from what you have done, does that mean I have done it wrong.

Also, is that all you need to talk about for the preliminary results

Luno2012 (author) from United Kingdom on February 04, 2014:

@ezra - To make it a fair test you need to heat the solutions up to room temperature...If you did not, then you would need to write about it in your conclusion

@Lani - this experiment was done 3 years ago. It can be reproduced now, so if you are doing it now, then you can use the figures in the experiement above.

ezra on February 01, 2014:

What grade was this? does it matter that you didn't heat up both the solutions?? doing the experiment now... need to know ASAP pls!!!!!!

Lani on December 31, 2013:

when did you do this experiment?

JJBSportz1997 on December 04, 2013:

This was a really useful bit of secondary data for some work i had to do. Thanks!

Firoz from India on July 01, 2013:

Useful article on Rates of reaction Decomposition of sodium thiosulphate. Voted up.

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