In this investigation I am trying to find out how the concentration of sodium thiosulphate affects the rate of reaction.

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Rachael Hallas 11A                                                                                              

                                         Chemistry coursework

Planning

In this investigation I am trying to find out how the concentration of sodium thiosulphate affects the rate of reaction.

In my experiment I will use sodium thiosulphate and hydrochloric acid to time how long it takes for a cross to disappear. I will also use water to change the concentration of the sodium thiosulphate. The sodium thiosulphate and the hydrochloric acid react together resulting in sodium chloride, sulphur dioxide, sulphur and water being produced. Sulphur is a fine yellow precipitant and the cross disappears when enough sulphur forms to blot it out.

   

   sodium      +  hydrochloric               sodium  +  sulphur  +  sulphur  +  water

thiosulphate          acid                       chloride      dioxide

Na 2S 2O 3 (aq)  +  2HCl (aq)              2NaCl (aq) + SO2 (g)  +  S (s)   +  H2O (l)

Preliminary work

I have first done some preliminary experiments to help me to decide how I am going to carry out the investigation.

The first preliminary experiment was investigating the effect of temperature on rate of reaction. We poured 30cm³ of sodium thiosulphate and 20cm³ of water into a conical flask. We then heated up these reactants using a bunsen burner and a beaker of boiling water to a certain temperature, placed the conical flask on top of a piece of paper with a cross drawn on it, poured in 5 cm³ of hydrochloric acid and timed how long it took for the cross to disappear. We recorded our results and then drew a graph.

From doing this preliminary experiment, I have decided that I am not going to have temperature as a variable. When we had got the temperature to the correct amount, it was hard to maintain this temperature throughout the test. Once the conical flask had been paced on the cross, the temperature started to go down as it was no longer being heated by the bunsen burner, it was in room temperature. My graph shows that I had 2 anomalous results, 1 of them more than 4 ºC off the line of best fit. My results would not be accurate or reliable enough and I would not end up with sufficient evidence to support a conclusion.

     

In the second preliminary experiment, we changed both the colour of the paper that the cross was drawn on and the volume of hydrochloric acid. We did the experiment 3 times- once on green paper, once on white paper and once on pink paper. We poured 30cm³ of sodium thiosulphate and 20cm³ of water into a conical flask. We did the test 5 times on each coloured paper and each time we did it we changed the volume of hydrochloric acid added to the conical flask. We placed the conical flask onto the paper and timed how long it took for the cross to disappear. We recorded our results and then drew a graph.

From doing this preliminary experiment, I have decided to use 10 cm³ of hydrochloric acid. I want the cross to disappear quickly so that I have time to do the experiment 3 times, but not too quickly so that I don’t have time to see it disappear. I can see that the quickest the cross disappeared with 10cm³ of acid was 34 seconds and this is about the right amount of time for the cross to disappear in- not too quick and not too slow.

I have also decided to draw the cross on white paper. It was easiest to see when the cross had disappeared as the black cross stands out the most on the white paper. If I look at my graph, I can see that none of the results are anomalous for the experiment I did on white paper but for the experiment on the green and pink paper, there is one anomalous result on each. My results are likely to be more accurate and reliable if I use white paper.

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There are many variables that I could change in this investigation to see what factors effect the rate of reaction:

  • The concentration of the sodium thiosulphate
  • The concentration of the hydrochloric acid
  • The colours of the paper on which the cross is drawn and the colour of the cross
  • The surface area of the reactants
  • The temperature of the reactants

I could also use a catalyst in this investigation as catalysts work by lowering the activation energy. More collisions would have more energy than the minimum needed to react and therefore the reaction would go faster.

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