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

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Introduction

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 30cm� of sodium thiosulphate and 20cm� 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 cm� of hydrochloric acid and timed how long it took for the cross to disappear. We recorded our results and then drew a graph. Volume of sodium thiosulphate, cm� Volume of water, cm� Volume of hydrochloric acid, cm� Temperature, �C Time taken for cross to disappear (seconds) 30 20 5 30 29.06 30 20 5 40 20.75 30 20 5 50 9.58 30 20 5 60 7.32 30 20 5 70 3.82 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. ...read more.

Middle

I will do the same thing for five different concentrations of sodium thiosulphate so that I can see how the concentration of sodium thiosulphate affects the time taken for the cross to disappear. Here are the measurements I will use: Volume of sodium thiosulphate (cm�) Volume of water (cm�) Volume of hydrochloric acid (cm�) Concentration of sodium thiosulphate (%) 10 40 10 20 20 30 10 40 30 20 10 60 40 10 10 80 50 0 10 100 I will ensure my results are accurate by using the smallest possible measuring cylinder to measure the solutions. The gaps between the numbers are bigger on the smaller cylinders so it is easier to read and will therefore be more accurate. When calculating the average time taken for the cross to disappear, I will record my results to 1dp so that it is more accurate than rounding to a whole number. When calculating the rate of reaction I will record my results to 3dp as the change will be very small. I will ensure my results are reliable by repeating the whole experiment 3 times and then finding an average of the time taken for the cross to disappear for the different concentrations of sodium thiosulphate. I will also make sure that my experiment is a fair test. Obtaining evidence Safety precautions- In my experiment I had to take safety precautions so that neither me or any of my classmates were in danger. * I wore safety goggles to protect my eyes from harmful chemicals. * When using a Bunsen burner in my preliminary experiment, I made sure that it was on the safety flame when we weren't heating anything. * I kept my stool under the bench at all times. The stool would obstruct the pathway and this could be dangerous. If this was not carried out, a person passing by could have tripped over the stool whilst holding a piece of equipment, which may endanger themselves or people around. ...read more.

Conclusion

We made sure that our results were reliable by repeating the experiment 3 times and then finding an average. If I was to do the experiment again I could improve it by using a light sensor to detect when the cross had disappeared. Even though we used the same person to determine when it had disappeared, it would still be more accurate to use a light sensor as you know for certain the cross has definitely disappeared. I would also check the sensitivity of the stopwatches we used. Some stopwatches are more sensitive than others and we should have checked all of the stopwatches to make sure that we had a sensitive one as this would be more accurate. Finally, I would use a thermometer to measure the temperature of the solution so that I could make sure it was kept constant so that it wouldn't affect the time taken for the cross to disappear, as this could be the reason I got my anomalous result. I believe that even though my results could have been more accurate by doing the points above, the reliability of my evidence is still sufficient enough to support my conclusion. By doing the experiment 3 times and finding the average it eliminated any human errors that I may have made as it took all of the data into consideration, and didn't just rely on one result for each concentration of sodium thiosulphate. I could have done further work to provide additional relevant evidence for my investigation. I could have repeated the whole experiment but changed a different variable. For example I could have changed the temperature of the reactants; I could have changed the concentration of the hydrochloric acid; I could have changed the surface area of the reactants or I could have used a catalyst. I could then draw another graph and see if the different variable would have the same affect on the time taken for the cross to disappear. I could compare this to my other graph and then analyse the differences or similarities between them. ?? ?? ?? ?? Rachael Hallas 11A 1 ...read more.

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