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Sodium Thiosulphate coursework.

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Introduction

GCSE Chemistry - Sodium Thiosulphate coursework Preliminary Study I am going to investigate the rates of reaction, and the effect different changes and factors have on them. The rate of reaction id the rate I lose a reactant or the rate of formation of a product during a chemical reaction. The rate of reaction is measured by dividing 1 by the time taken for the reaction to take place. There are five factors that affect the rate of reaction; these are - temperature, concentration (of solution), pressure (in gases), surface area (of solid reactants) and catalysts. I have chosen to investigate the effect concentration has on a reaction, the reason being because this is practical to measure. Aim To see the effects of changing the concentration of Sodium Thiosulphate has on the rate of reaction. Here is the word and symbol equation (balanced) of my investigation: Sodium Thiosulphate + Hydrochloric acid --> Na2S2O3(aq) + 2HCL(aq) --> Sodium Chloride + water + sulphur + sulphur dioxide 2NaCl(aq) + H2O(l) + S(s) + SO2(g) My investigation is going to be done by varying the concentration of Sodium Thiosulphate (while everything else remains constant). I chose this one because it is soluble in water, as the other is as well. Also, Sodium Thiosulphate is available in larger amounts as well as the concentration being prepared already. These are the concentrations of Sodium Thiosulphate I will be using first: - 0.5 mole/dm3 of HCL (This will stay the same. It is a Dependant variable). 0.5% - 2.5% of Sodium Thiosulphate (All the concentrations will be tested in turn going up in steps of 0.5%. This is an Independent variable). ...read more.

Middle

Powders react faster than blocks - greater surface area and since the reaction occurs at the surface we get a faster rate. This factor does not apply to my investigation. 4: The presence (and concentration/physical form) of a catalyst (or inhibitor). A catalyst speeds up a reaction, an inhibitor slows it down. This factor can be named an Independent variable as I can add a catalyst, but I will not be doing so. 5: Light. Light of a particular wavelength may also speed up a reaction. E.g. alkanes reacting with Halogens. Light can be names a Dependant variable, as I have no control over the wavelength of light. I've used the term reaction rates, so I should define what we mean by this; the reaction rate is the increase in molar concentration of product of a reaction per unit time or the decrease in molar concentration of reactant in unit time. Rate Law and Reacting Order If we examine the effect on the rate of a reaction by changing the initial concentration of reactants, we may be able to derive the rate law and hence the reaction order. Consider the following reaction: - 2NO2 + F2 --> 2NO2F If the concentration of NO2 is doubled then the rate is doubled, likewise when the concentration of the fluorine is doubled the rate doubles and so we get the following rate law; Rate = k [NO2][F2] The rate law is an equation that relates the rate of a reaction to the concentration of reactants raised to various powers. The rate constant, k, is proportionally constant in the relationship between rate and concentrations. This has a fixed value at any given temperature but varies with temperature. ...read more.

Conclusion

The total amount of moles reacting per second is shown above. Fig. 7 shows that when the concentrations were relatively low (0.5M - 1.0M), the increase of rate x 1000 was also fairly small (increasing from 5.1 to 6.7). There was then a gradual increase in the difference, and between 1.0M and 1.5M the rate more than doubled from 6.7 to 10.2 compared to previous results. This shows that there are far enough collisions at 4.5M than at lower concentrations. For this to make sense, it is necessary if I recap on the collision theory: For a reaction to take place, particles must collide with each other, only a small percent result in a reaction. This is all due to the energy barrier to overcome. Any particle with enough energy to overcome the barrier is called the activation energy, or the Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased, the rate of reaction will increase. However, the percent of successful collisions remains the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area. Like stated previously, most of the results that had been recorded didn't fit the line-of-best-fit. Which could be a result of inaccurate calculation, temperature or even the concentration levels. Looking at the curve on fig.3, it shows the intake of heat energy that results in a faster reaction time - this supports the fact that this reaction is an endothermic reaction and when heat energy is absorbed, the time taken decreases substantially. However, it seems that the HCL had reached its factorial limit, which would have affected the time recorded - therefore, higher concentrations should have been used. This is proved when the results slope horizontally on fig. 3 & 6. Graham Black 11CE Alsop High School ...read more.

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