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Aim: To determine how the concentration of each species in a reaction affects the rate of reaction

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Aim: To determine how the concentration of each species in a reaction affects the rate of reaction Plan Introduction In this coursework, the rate of reaction between two reactants will be investigated. Rate of reaction can be defined as the time during which a reactant is lost or a product forms during a chemical reaction. This is calculated by dividing the value of concentration by the time in seconds. There are factors other than concentration that affect the reaction rate, which are temperature, surface area and a catalyst. However, the effect of concentration is the only factor being investigated, meaning that the other factors need to remain constant, and in the absence of any catalyst. Theory Increasing the concentration of a solution increases the reaction rate. This is because there are more particles in the solution, making molecular collisions more likely. Therefore, more collisions between particles take place. The reaction that will be examined in order to fulfil the aim is the reaction between sodium thiosulphate solution and dilute hydrochloric acid: 2HCl(aq) + Na2S2O3(aq) 2NaCl(aq) + SO2(g) + S(s) + H2O(l) The experiment is carried out by constructing a large cross on a piece of paper, and placing this beneath the reaction mixture. Hydrochloric acid is then mixed together with sodium thiosulphate. The concentration of one of the reactants would change, while the other remains constant, to determine exactly how the concentration affects the rate of reaction. As soon as the two reactants are mixed together, a stopwatch would be started and be stopped as soon as the cross is no longer visible (becomes opaque). This time recorded represents the time taken for the cross to disappear. As both the concentration and reaction time are known, the rate of reaction can be calculated by dividing the concentration by the reaction time. This is achieved by constructing a concentration - time graph, where the gradient at the different concentrations represents the rate of reaction at those concentrations. ...read more.


This enables accurate results to be produced, and both an accurate and reliable conclusion to be drawn. The following methods are going to be used in order to ensure a fair test: * Using only one variable per test e.g. changing the concentration of sodium thiosulphate but keeping the concentration of hydrochloric acid constant - this enables an accurate conclusion to be drawn * Glassware containing the solution will be washed both before use and after measurements - this ensures that results do not become damaged by eliminating ions from the previous trials * Three trials will be undertaken for each mixture - this enables an accurate average time to be calculated, which reduces the effect of any anomalous results * Using separate glassware for different reactants - this ensures that the reactants aren't mistakenly mixed together which would produce flawed results * Resetting the stopwatch after each use - prevents accidentally writing down the incorrect time after the cross disappears * Keeping the total volume of the solution at 75cm3 as different volumes could produce different results * Starting the stopwatch as soon as the reactants are in the beaker Producing different concentrations In order to determine how each species affects the reaction rate, the concentration of each species needs to be changed, making this a variable. This is achieved by altering the volume of each reactant. However, to accurately determine the effects of concentration, only one reactant can be a variable at a time. By not doing this, the concentrations of both Na2S2O3 and HCl are variables, then their effects can not be determined, as it becomes difficult to distinguish between the two variables. Therefore, in the experiments, the following concentrations will be used, by use of the following volumes: Experiment 1 Total volume = 75 cm3 In Experiment 1, the concentration of Na2S2O3 is constant, while the concentration of HCl is the independent variable. Starting concentration of HCl = 2 mol dm-3 Concentration of HCl (mol dm-3) Volume of HCl (cm3) ...read more.


one person would be in charge of determining when the cross disappears Foreign ions damaging the results One person would be in charge of washing apparatus after use Temperature could have affected the results as it is not controlled Carry out all of the experiments on the same day to ensure constant conditions i.e. room temperature Errors in measurement Errors in measurement Modification to method Producing accurate and correct times from the stopwatch Ensure that the stopwatch functions properly, reset the stopwatch before use, slowly copy the time on the stopwatch after the reaction is complete and then compare the time written down with the time on the stopwatch Using accurate volumes of the reactants Slowly measure the amount of reactant that is required, take the meniscus into account as the starting point, crouch down for a view from table level to ensure the correct volume and ensure this is poured directly into the reaction beaker and not onto the sides of the beaker The main source of error in my investigation was obviously measuring the time taken for the cross to disappear. I found it difficult trying to start the stopwatch immediately after the reactants were mixed. Also, it was surprisingly a challenge to determine when the cross had completely disappeared as in some cases a small part of the cross was still visible from some angles. Although I feel that measurements taken have been quite accurate, these factors clearly affected the accuracy of my results. In order to reduce the effect of all these errors, I would do the following if repeating this experiment: * Have five trials instead of three - in order to produce more accurate average times * Use different beakers for the reaction mixture if there was a greater amount of equipment available - in order to avoid effect of foreign ions * Extensive cleaning of apparatus before use - the apparatus could have been contaminated before use which would produce inaccurate results ?? ?? ?? ?? 1 ...read more.

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