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An investigation into concentration and rate of reaction

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Introduction

An investigation into concentration and rate of reaction Aim: to determine the effect on rate of reaction of changing the concentration of sodium thiosulphate in the reaction sodium thiosulphate + Hydrochloric acid -> Sodium Chloride + sulphur dioxide + Sulphur + water. Scientific background A reaction is a change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances. The particular reaction of sodium thiosulphate + Hydrochloric acid -> Sodium Chloride + sulphur dioxide + Sulphur + water or Na2S2O3 + 2HCl -> 2NaCl + SO2 + S + H2O. This is a precipitate reaction in which a precipitate (in this case sulphur) is formed, because of this we can tell when the reaction is complete by judging when a certain amount of sulphur is produced. The collision theory states that for a reaction to take place particles have to "bump" into each other with sufficient force. This theory helps to explain what effects certain conditions will have on the rate of reaction. For example, a higher temperature means that the particles will be moving more quickly and with more energy so are more likely to "bump" into each other and cause a reaction. ...read more.

Middle

35g/l and 45g/l of sodium thiosulphate are not available to use as they are so they will be made by mixing 5cm3 of two other concentrations to get those. Fairness in testing It is important that the same people are used to do each thing every time to ensure there are no differences and that it will be a fair test. The same concentration of hydrochloric acid must be used and the same size conical flask used. The only variable to be changed should be the concentration of sodium thiosulphate. However, the temperature cannot be controlled so the temperature at the time of each test should be recorded and shown if there are any strange results this will probably be the cause. Safety Acid is being used so care should be taken to avoid spillages. Goggles should also be worn to prevent harmful substances entering the eyes. Acid and sodium thiosulphate should be poured into beakers before being measured due to the thin rims of the measuring cylinders so as to avoid spillages. No one should eat or drink while the experiment is going on in case of contamination. ...read more.

Conclusion

It was up to a single person to decide when the cross was no longer visible. A better way to do that would be to use a computer with colour recognition that could tell when the cross was no longer visible or ideally to have a computer that can identify how many molecules of sulphur had been formed. A computer would have been better to time with as well because human reaction times are slow in comparison. Another experiment to further the results of this would be to vary the concentration of acid instead of sodium thiosulphate to prove conclusively that it is not just a one-off case. A reaction involving a gas and pressure such as the reaction between steam and iron would also be good to use as it shows this is not just for solids and liquids. The reaction between steam and iron would have to be measured n a different way though as a precipitate is not formed. The weight could be taken because in the reaction hydrogen is given off and so the weight would decrease and when it reached a certain point, such as 5g of hydrogen given off, then the experiment time could be recorded to see if it matches with the results from this. ...read more.

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