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An investigation into the effect of changing the concentration of the sodium thiosulphate solution on the rate of reaction between sodium thiosulphate and hydrochloric acid.

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Introduction

An investigation into the effect of changing the concentration of the sodium thiosulphate solution on the rate of reaction between sodium thiosulphate and hydrochloric acid. PLAN * Introduction I am going to investigate the effect of concentration on the rate of reaction in an experiment between hydrochloric acid and sodium thiosulphate. The rate of reaction is a measure of how fast a reaction takes place. The rate of reaction can be found using the equation: Rate = measurable change ? time taken for that change. The speed of the reaction can be measured in three ways: 1. Precipitation - when the product of the reaction is a precipitate which clouds the solution. 2. Change in mass (usually a gas given off) - a reaction that produces a gas and a difference in mass can be measured. 3. The volume of gas given off - a gas syringe is used to measure the volume of gas given off. I am going to measure the rate of reaction in an experiment with sodium thiosulphate solution and hydrochloric acid. I am going to observe the length of time it takes for a cross to disappear when acid is added to sodium thiosulphate. The cross will disappear when a precipitate of sulphur makes the solution cloudy. I am going to change the concentration of the sodium thiosulphate by adding water to it and using a stop clock to see how long it takes for the cross to disappear. o The equation for this experiment is: Na2S2O3 (aq) + 2HCl (aq) 2NaCl + S (s) + SO2 (g) + H2O (l) o The ionic equation is: S2O3 2- (aq) + 2H+ (aq) S (s) + SO2 (g) + H2O (l) * Hypothesis I predict that the higher the concentration of the sodium thiosulphate, the faster the reaction will be, so the cross will disappear in a shorter time. ...read more.

Middle

3. I will measure out the amounts of the solutions needed into three separate, labeled measuring cylinders. 4. I will then add the sodium thiosulphate to the conical flask. 5. To the conical flask, I will also add the amount of water required (if any). 6. I will take the temperature of the sodium thiosulphate (and water). The constant temperature for my experiment is 22?C. * If the temperature is more than 22?C, I will place the conical flask into an ice-bath. I will do this by adding ice to an appropriately sized beaker, and placing the conical flask into this. I will keep taking the temperature of the sodium thiosulphate (and water) until it reaches the desired temperature (in this case, 22?C). Then I will remove it from the ice-bath and continue as normal. * If the temperature is less than 22?C, I will place the conical flask into a warm-water bath. I will do this by adding warm water to an appropriately sized beaker. If necessary, I will heat the water over a Bunsen burner, standing the beaker on a tripod and gauze. Then I will add the conical flask to this. I will keep taking the temperature of the sodium thiosulphate (and water) until it reaches the desired temperature (in this case, 22?C). Then I will remove it from the warm-water bath and continue as normal. 7. Then I will add the hydrochloric acid to the solution in the conical flask. 8. As soon as the hydrochloric acid has been added, I will start the counter on the stop clock and swirl the conical flask slightly to ensure the solutions are all mixed. 9. I will look into the conical flask from directly above, using one eye only, and stop the stop clock immediately the cross disappears from view. 10. I will note the time taken, and then empty the contents of the conical flask into a sink. ...read more.

Conclusion

This would decrease the percentage error, but it could be impractical to use very large amounts. I think my method was suitable and was performed as accurately as possible. However, some changes in apparatus or method might make it more accurate if I were to do my experiment again. For example, a burette instead of a measuring cylinder for the sodium thiosulphate, water and hydrochloric acid would have been more accurate. A burette has an error of only ? 0.05 cm3 compared to one of ? 0.5 cm3 for a 25 cm3 measuring cylinder, and ? 1 cm3 for a 50 cm3 measuring cylinder. Another change that could be made to the method would be to swirl the conical flask in the same direction, i.e. clockwise, every time it is swirled. This may seem like a small detail, but it may make some difference. One solution to the problem of knowing when to stop the stop clock might be to have some sort of light sensor attached to it that would stop it when no light is let through, i.e. when the cross would no longer be visible because the solution would be cloudy. This would be a better way of finding the time, because eyesight is unreliable. I could also improve the temperature error by using a controlled water bath with a thermostat on the front. This would eliminate the possibility of the temperature changing during the experiment and affecting the results. I could have extended the range of concentration to gain more evidence and more results to make the investigation more clear. So the evidence I obtained, having so many errors, is not entirely accurate. However, there are only a few slight anomalies on the graphs and, despite these, it is still sufficient to support the conclusion because my results were basically, but not exactly, correct. So, in conclusion, I have proved my hypothesis correct by using the results of my experiment and explaining them with the particle theory, and I think that although there were some inaccuracies in my experiment, it has nevertheless been a success. 2 ...read more.

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