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# Investigation- To investigate the factors involved in the following reaction:Na2S2O3(aq) + 2HCl(aq) 2NaCl(aq) + H2O(l) + SO2(aq) + S(s)

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Introduction

Caroline Earp Investigation- To investigate the factors involved in the following reaction: Na2S2O3(aq) + 2HCl(aq) --> 2NaCl(aq) + H2O(l) + SO2(aq) + S(s) PLANNING STAGE Variables: The following are variables which will affect my investigation. Temperature, Pressure, Concentration of Na2S2O3, Concentration of HCl, Stirring. The key variables which I'm going to investigate are Concentration of Na2S2O3 and Concentration of HCl. What I'm going to find out: I'm going to find out what effect varying the above stated variables has on the rate at which the reaction between Na2S2O3 and HCl takes place. What I expect to happen: I predict that when I double the concentration of the Na2S2O3, the rate of reaction will double. The relationship will be directly proportional and the graph that I will get will be a straight line thought the origin. The origin will be a point on the graph since if there is no Na2S2O3 then there will be no reaction. The relationship will only be directly proportional up to a certain point, when the rate will level off and increasing the concentration further will have no added effect. I predict that when I double the concentration of the HCl, the rate of reaction will double. Again the relationship will be directly proportional, and the graph a straight line through the origin. If there was no HCl then there would be no reaction and so the origin is a point on the graph. Again the graph will level off when a further increase in the concentration has no more effect. ...read more.

Middle

that I couldn't be sure of much accuracy in my measurements. I scaled the volumes up by 5 (the concentration would stay the same) to compare the rate times which I got. By doing this I had to use a different beaker in which to do the experiment which meant that the depth of the solutions was now 2.6cm, rather than 1.4cm. Therefore the value for x ( the amount of sulphur needed to blot out the cross) was different. I will call this new value y grams of Sulphur. I then decided to try the smaller volumes again but this time doing my measurements with a buiret to see if that made any difference to the level of accuracy. Results, Observations and Measurements: See next page for results tables and graphs Since I did the experiment over several days the temperature was likely to change. Just to see I measured the temperature of all the solutions on each day. I noticed that the temperature of the reactants stayed constant all the time the reaction was taking place. Solution Temp 1(�C) Temp 2(�C) Temp 3(�C) Temp 4(�C) Temp 5(�C) H2O 21.5 20 21.5 23 23.5 Na2S2O3 22 20 21.5 23.5 23.5 HCl 22 20 21.5 23.5 23.5 Reactants (mixed) 22.5 21 22 24 23.5 On the graph showing rate of reaction varying with concentration of Na2S2O3, I have joined all the points and then drawn a best fit straight line through the origin. On the other graph showing rate of reaction varying with concentration of HCl, I have plotted 3 lines. ...read more.

Conclusion

Using this method you could measure exactly the time it took for a known amount of sulphur to be produced. This would have cut out the inaccuracies when I changed my total volume, and began to measure the time taken for y grams of Sulphur to be produced. You could improve even further on how you measured the rate using this set-up by taking values for the concentration of sulphur over a period of time. You could then plot a graph of time against concentration and to find the rate at a particular instant. You would then end up with a different line for each concentration of reactant. From these lines you could find the initial rate of reaction by taking a tangent of the curve where it starts. This would give you very accurate values for the initial rate of the reaction. Even though I tried to measure the initial rate of the reaction what I actually measured was an average rate of reaction. I measured the rate of reaction over a period of time. The further the reaction is to completion the more error will creep in. An alternative method of measuring the rate would be to monitor the pH. This probably would work although I doubt if it would be as effective as measuring the concentration of Sulphur. Monitoring the pH would be measuring the concentration of H+ ions. The results which you would get would be affected by the fact the SO2 (aq) will be slightly acidic, but this shouldn't be too much of a problem. The success of this method will depend on what part the H+ ion plays in the breakdown of the S2O32- ion. ...read more.

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