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Investigation of some of the factors affecting rates of reaction.

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Investigation of some of the factors affecting rates of reaction Introduction The rate of a chemical reaction is the speed at which that reaction proceeds. The speed at which reactions occur is of great importance to all living things, as their cells use chemical reactions to grow and reproduce. It is also very important in chemical industry to understand the speed of chemical reactions. The rate of reaction can be measured by either following the speed of formation of the products or the speed of decrease of the reactants. We normally choose which is easier to follow, for example if a solid is formed as a product it will be easier to follow the formation of the product. As the rate is the speed of formation or depletion, it is calculated by measuring the time taken to form the product and: Rate = 1___ Time taken Therefore, the rate has units seconds-1. Any chemical reaction involving reactants and products requires the particles of the reactants to collide with each other before reaction can take place. Therefore we can decide which factors are likely to affect the rate of the reaction based upon collision theory: 1. Temperature. This will affect the rate of the reaction in that heat energy causes particles to move faster and have more kinetic energy. So, as temperature is increased, then the particles will be colliding more frequently and also with more force or energy. As the energy increases, the proportion of molecules with sufficient energy to react successfully will increase. This is due to the distribution of energies of all molecules being like that in the following sketch graph: This graph shows that when the temperature increases the distribution shifts upwards in terms of energy. The number of particles with the maximum energy is less, but the number of molecules at any higher energy is greater. This is called the Maxwell-Boltzman distribution. ...read more.


Looking at the results in more detail, it can be seen that as the concentration of sodium thiosulphate doubles, then the time for the cross to disappear drops by half. For example, as the concentration goes from 0.03moldm-3 to 0.06moldm-3, the time drops from 161s to 84s. The same trend can be observed from 0.06moldm-3 to 0.12moldm-3. In the second part of the investigation, temperature was used as a variable. The sodium thiosulphate was heated to the required temperature using a Bunsen, and the acid was added. The timings were made as above. The results for this section are shown in Table 3. Temperature (?C) Time taken (s) Average time (s) 22 167, 161 164.0 32 84, 81 82.5 42 47, 49 48.0 52 27, 26 26.5 62 14, 15 14.5 Table 3. Results of the temperature part of the investigation. As can be seen from the table and from the graph which follows (Figure 3), the time taken for the cross to become invisible decreases as the temperature increases. The time taken almost drops by half every time the temperature rises by 10?C. Conclusion The investigation was designed to determine how two variables, concentration and temperature, affected the rate to the reaction between hydrochloric acid and sodium thiosulphate. 1. Concentration as a variable The first part of the experiment changed the concentration of the sodium thiosulphate and recorded the time taken for a red cross under the flask to become invisible. The results are shown in Table 2 and the graph in Figure 2. Also from these results it is possible to calculate the rate of reaction using the formula in the introduction. The following table (Table 4) shows the rates of reaction calculated for each of the five concentrations of sodium thiosulphate. Concentration (moldm-3) Rate of reaction (s-1) 0.03 0.0062 0.06 0.012 0.09 0.023 0.12 0.025 0.15 0.037 Table 4. Rate of reaction as effected by concentration. The following graph (Figure 4) shows how the rate of reaction varies with the concentration. ...read more.


The main sources of error in this experiment are the heating arrangements and the fact that the acid was not heated. The sodium thiosulphate was heated directly over a Bunsen flame, using a thermometer to detect when it was at the correct temperature. This method of heating is not accurate as it is uneven and not all the apparatus will be at the same temperature. As this is the case, it will rapidly cool and so the experimental observation will not be at the same temperature as was intended. Instead, it would be better and more accurate, not to mention safer, to warm the flask and its contents in a water bath. Also, a method could be devised to ensure that the desired temperature was maintained throughout the observation. This could be insulation or even doing the experiment in a water bath. The acid was not heated for safety reasons, because the acid could not have been heated directly over a Bunsen flame without danger of boiling or hot acid splashing out. The fact that the acid was at room temperature would not have affected the results that much because there was only 5cm3 of acid and there was a much larger volume of sodium thiosulphate, 50cm3. However, the effect would be more pronounced at the higher temperatures. It may be predicted that if the acid was also heated that the graph in Figure 5 would look the same shape but would slope more steeply. This is because the point at 22?C would be the same, but those at higher temperatures would have ever increasingly higher reaction rates. For example, that point at 62?C probably represents a reaction occurring at around 55?C as the thiosulphate has cooled after removal from the Bunsen flame and the acid added is also cooler so further reducing the temperature. In summary it can be stated that the results in this experiment, while not absolutely accurate, show reliable and reproducible trends which provide convincing evidence for the points proposed in the hypothesis. ...read more.

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