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How does the concentration of the reactants affect the rate of reaction when sodium thiosulphate and hydrochloric acid are combined?

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How does the concentration of the reactants affect the rate of reaction when sodium thiosulphate and hydrochloric acid are combined? Introduction: A chemical reaction is a process in which one set of chemical substances, known as the reactants, are changed into another substance or set of substances, called the products. The factors that affect the rate of chemical reactions include: * Particle size. * The use of a catalyst * Concentration. * Pressure. * Temperature. For my investigation I have chosen to analyse the way in which the rate of reaction is altered by the concentration of the reactants. When the reactants, sodium thiosulphate and hydrochloric acid, are combined they produce 4 different products as shown in the following equation: Sodium Thiosulphate + Hydrochloric Acid --> Sodium + Water + Sulphur + Sulphur Chloride Dioxide Na2S2O3 (aq) + 2HCl (aq) --> 2NaCl (aq) + H2O (l) + SO2 (g) + S (s) In my experiment I intend to change the concentration of the sodium thiosulphate and measure the effect this has on the rate of the reaction. The rate will be indicated by how much light is obscured by the precipitated sulphur at certain time intervals. The actual time taken for the reaction is measured by the time taken for the sulphur to be precipitated. The sulphur becomes a milky yellow colour, which obscures a cross, drawn on a piece of paper placed below the conical flask. Background: The formation of a colloid is very important in this experiment because the reaction between Sulphur and hydrochloric acid actually forms one. When this happens the solution turns 'cloudy'. When you use a higher concentration the amount of colloids colliding goes up which means the rate of reaction speeds up. The thing that triggers the combination of sulphur and hydrochloric acid to actually turn cloudy though is the fact that they the two substances cannot mix. ...read more.


Beaker size * Volume of hydrochloric acid * Concentrations of sodium thiosulphate * Temperature of ingredients * Order in which reactants are added * Environment in which experiment is performed * Concentration of acid * Piece of paper with cross on * Overall volume * Safety: To avoid injuring others or myself around me, I will have to follow the safety precautions, which are shown below: * I will at all times be wearing scientific safety goggles to protect my eyes against any unpredictable splashes of hydrochloric acid. * All spillages and leakages will be immediately cleaned up from any surfaces. * Equipment will have to be replaced after I finish using them * I will move carefully around the room. * To prevent disorder any items that are not necessary will be stored in a safe place before I start the experiment. * Pouring of chemicals needs to be performed slowly and carefully. * The same applies to the carrying of apparatus. Results: Volume of sodium thiosulphate (cm�) Volume of water (cm�) Concentration of sodium thiosulphate (%) Times in which it takes for the cross to disappear from view (s) Average reaction time (s) Molar Mass Reaction rate (per sec) 50 0 100 50 42 41 44 0.25 0.023 40 10 80 62 65 51 59 0.2 0.017 30 20 60 78 63 75 72 0.15 0.014 20 30 40 110 109 112 110 0.1 0.009 10 40 20 265 272 263 267 0.05 0.004 Reaction rate = 1 Molar mass = amount of sodium thiosulphate Average time Conclusion: Looking back at the overall results I can confidently say that they prove that when the concentration of the reactant, sodium thiosulphate, is at its highest the reaction rate is at its fastest. My results seem to support my prediction that with increasing concentration the rate of reaction speeds up. I also predicted that by doubling the concentration of sodium thiosulphate eg. ...read more.


I was very careful measuring the chemicals out, so I believe this is why I had a majority of results that seemed more relative to the rules of the collision theory and the effects of concentration on a reaction. I would have been able to improve the experiment by using a light sensor instead of visual judgement to determine more accurately the exact moment the cross disappeared from view. The light sensor would measure accurately when the solution reached the same degree of cloudy-ness in all repetitions. This would make the experiment much fairer because the same would apply throughout. This would have made my results more accurate. It was a relatively fair test though, because my only variable was the concentration of the Sodium Thiosulphate solution. I would have liked to repeat the experiment again to confirm my results if I had had more time to do so. Also, if we had had more time I would have liked to use a wider range of concentrations to see if the patterns that have developed continued. For example, I could do an experiment where I varied the concentrations of hydrochloric acid and used a fixed volume of 5cm� for the sodium thiosulphate. This might not be as effective though because as hydrochloric acid is far more reactive than sodium thiosulphate (which is why I used such a small amount of HCl for my main investigation), it would mean the reaction rate would be too fast and more difficult to record. I could also test much higher volumes of sodium thiosulphate so I could compile a larger set of results to observe the extent that my lines of best fit reach to and to see, if ever, where the curve on the reaction rate graph develops into a horizontal line. Altogether though I think my results were quite reliable because I kept to the safety and fair test regulations, and they have been proven reliable by the same trends appearing in sequence rather than results with little correlation. ...read more.

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