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An investigation into how changing one variable influences the rate of reaction between sodium thiosulphate and dilute hydrochloric acid.

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Introduction

An investigation into how changing one variable influences the rate of reaction between sodium thiosulphate and dilute hydrochloric acid. Theory on which the investigation is based - collision theory Collisions are important in a reaction because in order for a reaction to occur the particles of sodium thiosulphate and hydrochloric acid must: * Collide with each other * The collision must have enough energy. If there is not enough energy in the collision the bonds of sodium thiosulphate will not break, therefore new bonds with the hydrogen cannot be formed. A successful collision requires old bonds to be broken and new ones formed. In our case this would involve Hydrogen molecules forming covalent bonds with the oxygen molecules leaving a precipitate of Sulphur. If our reaction were unsuccessful then no Sulphur would be formed. List of variables, which could affect the rate of reaction * The concentration of the dilute hydrochloric acid. * The concentration of sodium thiosulphate. * The temperature at which the reaction takes place. If I increase the concentration of the dilute hydrochloric acid then there will be more hydrochloric acid molecules in the beaker. This will increase the rate of reaction because the more molecules of hydrochloric acid there are, the greater the chances of more successful collisions. Increasing the temperature of a reaction causes the particles to take in energy, this causes them to move faster and collide more often. ...read more.

Middle

* Pour the dilute hydrochloric acid into the solution of sodium thiosulphate, simultaneously start the stopwatch * Time how long it takes for the cross to disappear. * Repeat the experiment, each time changing the concentration of the sodium thiosulphate. Accuracy of the pilot study During the pilot study I used varying concentrations and volumes of sodium thiosulphate. My first reading was taken using a twenty percent concentration of sodium thiosulphate in a 25ml solution. This reaction was not very successful as it took over four minutes for the cross to be obscured. I decided that 529 seconds was too long and tried the same concentration in a 50ml solution. I found that this reaction only took 151 seconds. I then used a volume of 50ml to test a suitable range of concentrations and my results were as follows: Concentration Volume Time taken (s) 20% 25ml 529 100% 25ml 35 20% 50ml 151 40% 50ml 61 60% 50ml 43 80% 50ml 29 100% 50ml 25 During the pilot study I took the readings twice and then took the average of the two. During the real investigation I have decided to take the readings three times in order to gain a more accurate average. Safety Precautions: Safety glasses are the only safety precaution that I will be taking. Table Volume/cm3 Concentration of sodium thiosulphhate Reading one/s Reading two/s Reading three/s Average/s Rate of reaction 50 20% 175 182 167 174 0.006 50 ...read more.

Conclusion

this error may have been caused either by inaccurate measuring or by inaccurate timing. The results shown in the table all appear to be reliable as the rate of reaction increased at the same rate as the concentration. The fact that there are no obvious anomalous points is significant as it shows that the readings taken were very accurate. Accuracy and improvements The main problems that I encountered during the investigation were accurately measuring the sodium thiosulphate solution and the dilute hydrochloric acid and accurately timing the reaction. It was quite difficult to time the reaction because it was hard to judge when you could see the cross any more. During the preliminary investigation I used a cross, drawn on a piece of paper, this can easily become smudged, impairing your vision and affecting the results. I improved on that in the main investigation by using a bold, laminated cross, this meant that it could always be seen clearly. Another possible problem is the temperature at which the reaction happened. It is impossible to keep the temperature of the room completely constant as it can easily be affected by factors such as central heating and a window being closed or opened. The investigation could be extended by increasing the number of variables that you change, such as the concentration of the dilute hydrochloric acid or the temperature. You would always compare the variables to the rate of reaction to see if there is a correlation. [O. M.1] ...read more.

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