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What factors affect the rate at which hydrochloric acid reacts with sodium thiosulphate?

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What factors affect the rate at which hydrochloric acid reacts with sodium thiosulphate? Planning: This investigation is about rates of reaction and what affects them. In this case I am going to look at hydrochloric acid and sodium thiosulphate which is a precipitation reaction. They react as in the equations below: Sodium thiosulphate + hydrochloric acid -> sodium chloride + sulphur + sulphur dioxide + water Na2S2O3(aq) + 2HCl (aq) -> 2NaCl (aq) + S(s) + SO2(g) + H2O(l) For a reaction to occur the reactants must a) collide b) Collide with sufficient energy in order to make a successful collision. The collision theory is useful when carrying out this experiment because it explains why rates of reactions differ. The concentration of a solution effects the rate of reaction because the rate depends on how frequently the molecules of the reacting substances collide. A more concentrated substance has more molecules in a given volume than a more dilute substance. Therefore the frequency of successful collisions is greater, and the reactions will occur more quickly. The 4 factors that control the rate of a reaction are: Temperature, concentration, surface area and the use of a catalyst. Variables: Temperature- I will conduct my experiments at room temperature as an increase could speed up the reaction. This is my controlled variable. Concentration- I will be altering the concentration of Sodium thiosulphate and this will be my independent variable. ...read more.


The measures of Hydrochloric acid will all be the same (5cm) and measured with droppers at eye level to get the most accurate readings possible. I will also use the same method for mixing all the solutions, rinsing all the conical flasks out after each experiment in case some of the liquid is left, making the volumes irregular. I will also make sure that I add both the water and the sodium thiosulphate at exactly the same time (into the conical flask with the hydrochloric acid in it), or it could affect the results of the experiment. I am going to start the stopwatch at the exact time as I put the water and the sodium thiosulphate into the conical flask. To do this it is a lot easier if there are two people doing the experiment, so one person can put the two substances in, and one person can start the stopwatch. Measurements I am going to take a range of readings, ranging from 50cm of sodium thiosulphate (and 0ml of water) to 10cm of sodium thiosulphate (and 40ml of water.) In my preliminary experiment I stopped taking readings as 5cm of sodium thiosulphate but I found that the reaction took over 10 minutes and that it would be more sensible to stop at 10cm of sodium thiosulphate. I will also take 2 readings for each concentration to make sure my results are more reliable. ...read more.


Before conducting my final experiment I did some preliminary work in order to familiarize myself with the project. This helped to improve the accuracy of my results because I was then able to make any changes in the way I conducted the experiment. My results do seem, on the whole, to fit my conclusion and despite the anomaly I was able to produce a smooth line of best fit. I think that I have obtained sufficient evidence in which to support my conclusion, although taking more than 2 readings would have helped to increase their reliability. For example from 16g/dm- 32g/dm the rate almost doubled by 2.2, which shows that although they do tend to imply that as the concentration doubles, the rate also doubles, it would have been more informative to take the average of 5-6 readings and see if the results changed. If I were to extend this investigation and carry out any further work I could look into the affects of catalysts (and other variables) on reaction times. I could also carry out the same experiment using different substances or by varying the amount of acid instead. It would also be interesting to substitute the 'piece of paper with a black cross on it' for a light sensitive detector which would go out when the reaction had completely clouded over. Therefore my times would be more accurate and my graphs would be more precise. Overall I think my experiments were successful and I believe my evidence was sufficient enough in which to support my hypothesis. ...read more.

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