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To measure the rate of reaction of thiosulphate ions with acid in solution, and find out how the rate of reaction depends on the concentration of both thiosulphate and acid.

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Introduction

Chemistry - CH2 Practical Assessment Written Account of Practical Assessment Aim: To measure the rate of reaction of thiosulphate ions with acid in solution, and find out how the rate of reaction depends on the concentration of both thiosulphate and acid. Hypothesis & Prediction: We will place the reaction vessel over a black cross, drawn on a white background. The black cross will disappear from view, owing to the increased sulphur concentration in the solution, when a fixed amount of reaction has taken place. I predict that the time taken for this fixed amount of reaction is directly proportional to the rate of reaction; that is, if the reaction is fast, the cross will disappear quickly and vice versa. The more concentrated the reagents, therefore, the faster the reaction will be. Background Information & Knowledge: Reactions can only happen when the reactant particles collide, but most collisions are not successful in forming product molecules. The reactant molecules must collide with enough energy to break the original bonds so those new bonds in the product molecules can be formed. All the rate-controlling factors are to do with the frequency of reactant particle collision. If the concentration of any reactant in a solution is increased, the rate of reaction is increased. Increasing the concentration increases the probability of a collision between reactant particles because there are more of them in the same volume. The exact relationship between reaction rate and concentration depends on the reaction "mechanism". This is the process involving elementary reaction steps. The slowest step controls the rate. The nature of the slow step is not obvious from the balanced equation. Only experimental observation reveals the link between concentration and reaction rates. As we have seen, a basic tenet of collision theory is that the reactants must collide in order to react and that the rate of reaction is related to the number of collisions per unit time. ...read more.

Middle

The molar concentration of a solute is by definition the quotient of the amount of solute substance present, in moles, and the volume of solution present, in litres (L or dm3). Subsequently, I determined the reaction rate for all the experimental runs by calculating 1/time for each reaction. In order to make the resulting figures for 1/time easier to plot, I then multiplied them by 10-2. Using the figures obtained from these calculations, I plotted two graphs, drawing the best line through the points on each graph. Please see separate sheets for plotted graphs. Conclusion & Evaluation Precision of measurements & accuracy of experiment Experimental error is always with us; it is in the nature of scientific measurement that uncertainty is associated with every quantitative result. This may be due to inherent limitations in the measuring equipment, or of the measuring techniques, or perhaps the experience and skill of the experimenter. It goes without saying that, when conducting this experiment, I endeavoured to be as careful and as accurate as I could in my measurements. However, there are several areas where precision could be improved: ? I had to measure out my reagents and water using three burettes, which were set up on one of the lab benches. I found these a little difficult to read at times, as they were placed significantly higher than eye level. Therefore, it was not always possible for me to measure out these volumes as accurately as I would have liked. ? I should have started the stopwatch at the exact time I put the water and the sodium thiosulphate into the conical flask containing the acid. This is not always possible when conducting the experiment alone, as it is also important to ensure that the reagents are properly homogenised for the reaction to occur. It is difficult, therefore, to do all these things simultaneously. ? For each run of the experiment, I ensured that the amounts of reagents were diluted within a certain range, so that the speed of reaction was not too fast. ...read more.

Conclusion

I successfully conducted the experiments that enabled me to do this. In addition, I confirmed my prediction that the time taken for the fixed amount of reaction to take place, (the cross disappearing), is directly proportional to the rate of reaction; that is, if the reaction is fast, the cross will disappear quickly and vice versa. In addition, the shape of the lines on my graphs suggested that this is a first order reaction, and I worked this out using my results. As can be seen from my results tables, during the fourth run of the experiment, where the concentrations of nitric acid and thiosulphate were at their highest, the cross disappeared most quickly (in only 19 seconds). Conversely, in the eighth run of the experiment, where the concentrations of nitric acid and thiosulphate were at their lowest, the time taken for the cross to disappear was the longest. This indicates that the higher the concentration of reagents, the faster the reaction will be for this experiment. If we plot a graph of time against concentration, the results show that the cross disappears soonest when the solution is most concentrated. The graph would appear as shown below, (please see separate tables of results for data used): We can see from the graph that: Speed of reaction ? Concentration of thiosulphate If we draw the same type of graph, but this time showing the concentration of nitric acid being systematically varied, we find that the graph looks the same as previously (please see separate tables of results for data used): We can see from the graph that: Speed of reaction ? Concentration of nitric acid Once again, this is due to the collision theory and how it affects reactions rates. I have already expounded the basic tenets of this theory at the beginning of this piece. Kinetic theory may also be relevant to this, as it relates molecular speeds to the Maxwell distribution function, enabling estimation of mean free path and frequency of collisions between molecules. 1 ...read more.

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