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Investigation exploring rates of reaction, using the reaction between sodium thiosulphate and hydrochloric acid as a model.

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Introduction

Investigation exploring rates of reaction, using the reaction between sodium thiosulphate and hydrochloric acid as a model Aim To investigate the factors affecting rates of reaction, to gain an understanding of the science involved with changing the rate of a reaction. Research What is meant by 'rate of reaction'? It is the rate of disappearance of a reactant or the rate of appearance of a product. In a previous investigation, the rate of reaction of calcium carbonate and hydrochloric acid was measured by recording the rate of appearance of carbon dioxide. It is obvious that reactions happen at different rates. For example, Iron + oxygen --> iron oxide ...takes place extremely slowly, especially if the iron is dry. Meanwhile, silver nitrate + hydrochloric acid --> silver chloride + nitric acid ...takes place instantaneously. Why are rates of reaction important? The rate of a reaction matters to manufacturers. In industry, a slow reaction means slow production, and this often means less profit. Chemical engineers have to understand the factors that affect rates of reaction so that they can use them to make the product form more quickly. An example of an important reaction in industry is the Haber process. Haber process The reaction between nitrogen and hydrogen is used to make ammonia, which is an important material in the fertiliser industry. However, under normal conditions, the reaction between nitrogen and hydrogen is very slow. Why is the reaction between nitrogen and hydrogen slow under normal conditions? It is because the triple bond holding the two nitrogen atoms together in an N2 molecule is very strong. This means it takes a lot of energy to break the bonds, and so the activation energy of the reaction is high. Nitrogen + hydrogen --> ammonia N2 (g) + 3H2 (g) --> 2NH3 (g) There are 4 ways to speed up a reaction: 1. Use a catalyst: iron is used - the container that the reactants are in is filled with short lengths of iron tube. ...read more.

Middle

This is because 1/Time (is proportional to) Concentration I will be able to use the graph I produce with my results to find out whether the experiment was accurately carried out. The graphs showing time taken for x to disappear will be curved, as the time taken will not increase/decrease in direct proportion to either the temperature or the concentration of the sodium thiosulphate. Explanation If the temperature of the environment a reaction is taking place in is raised, particles will gain energy and will move around quickly. This means they will collide more often, and the collisions will take place energetically. Because particles with high energy levels are more likely to overcome the 'activation energy barrier', in order for a reaction to take place, raising the temperature increases the rate of reaction. Increasing concentration of reactant solutions means there are more particles in a certain volume, so collisions are more likely to happen. In order to react, particles of reactants must collide, with sufficient energy or force to overcome the activation energy barrier. Only a small percent of all collisions result in a reaction. The minimum energy that a particle must have to overcome the barrier is called the activation energy, or EA. The size of the activation energy varies from reaction to reaction. If the frequency (number) of collisions is increased, by increasing concentration, surface area or pressure of the reactants, the r. of r. will rise. However, the fraction of successful collisions remains constant. I would expect the graph showing 1/Time plotted against temperature to be a curve. See research for more theory about the rate of reaction. Accuracy The experiment relies on the person observing the 'x' through the solution, which immediately makes the results less accurate. The measurements of the hypo and the HCl in the measuring cylinder cause an error of � 1ml, and the hypo and HCl concentrations may also allow for a small error as they may vary slightly when made up. ...read more.

Conclusion

The beaker is kept partly covered to stop any stray light affecting the measurements. The reaction starts quickly and the light intensity reading on the computer screen changes as the sulphur is formed. The lower the reading, the less light is let through and therefore the more sulphur is formed. I used a computer simulation to collect the following readings: Results Using 2M hypo Time (seconds) Light intensity (%) Light intensity (%) (repeat) Average light intensity (%) 0 100 100 100 4 70 69 69.5 8 54 55 54.5 14 36 35 35.5 25 21 20 20.5 37 12 11 11.5 49 5 7 6 61 1 2 1.5 74 0 1 0.5 Using 1M hypo Time (seconds) Light intensity (%) Light intensity (%) (repeat) Average light intensity (%) 0 100 100 100 4 95 94 94.5 8 86 87 86.5 14 75 76 75.5 25 57 56 56.5 37 40 39 39.5 49 26 27 26.5 61 15 14 14.5 74 7 6 6.5 87 2 1 1.5 101 0 1 0.5 Using 0.5M hypo Time (seconds) Light intensity (%) Light intensity (%) (repeat) Average light intensity (%) 0 100 100 100 4 96 97 96.5 8 90 91 90.5 14 83 84 83.5 25 69 68 68.5 37 55 55 55 49 45 46 45.5 61 32 33 32.5 74 24 23 23.5 87 12 13 12.5 101 6 5 5.5 As you can see the results are very accurate and there are no anomalies. This is probably an accurate method to use. Although I have not included the graphs for these results, they show a clear curve. Use of a spectrophotometer A spectrophotometer is often used to measure colour concentration. The word 'spectrophotometer' means 'light measurer making use of part of the spectrum. In practice the spectrophotometer can measure how much light of a particular wavelength can pass through a liquid or gas. This could be useful for this experiment because it is an accurate way of recording the rate of a reaction. ...read more.

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