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Investigating the reaction between Sodium Thiosulphate and Acid.

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INVESTIGATING THE REACTION BETWEEN SODIUM THIOSULPHATE AND ACID Sahir Kara 10Hill Experiment Aim: To investigate how the concentration of acid affects the rate of reaction with Sodium Thiosulphate. Background Knowledge: The reaction that will be set-up will occur as follows: Sodium Thiosulphate (aq) + Hydrochloric Acid (aq) Sodium Chloride (aq) + Water (aq) + Sulphur (s) + Sulphur Dioxide (g) Na2S2O3 + 2HCl 2NaCl + H2O + S + SO2 During a reaction, bonds between atoms must be broken before new molecules can be made. This means that for every chemical reaction a minimum amount of energy is needed to go into breaking these bonds. This is called the 'activation energy'. The molecules also have to collide in order for a reaction to take place, and the collisions between particles have different amounts of energy. If colliding molecules have enough energy, then it is successful and a reaction occurs. The reactions are fastest if the energy of the molecules is greater than the activation energy of the reactants. (RESOURCE 2) Different factors can affect the rate of reaction, as explained below: - Concentration In solutions of higher concentrations, particles are closer together, and have a greater chance of colliding. There are consequently more collisions, and the rate of the reaction is higher (as concentration of a solvent is increased). - Temperature At higher temperatures, particles are moving faster. Therefore, at a higher temperature, there are more collisions, and the collisions are also more energetic. More collisions therefore have energy greater than the activation energy, meaning that the rate of reaction is faster when temperature is increased. - Presence of a catalyst Catalysts generally lower the activation energy needed for a reaction to take place. This means that more collisions are likely to be successful (because more then have energy greater than the activation energy), and the rate of reaction will increase. - Surface area This factor is of a solute in a reaction, and it can affect the rate of reaction, but this cannot be changed in this reaction, because sodium thiosulphate and hydrochloric acid are both liquids. ...read more.


Therefore, they need to be carefully controlled in this experiment, as they are not being used as input variables. Light intensity & the person judging when the cross has 'disappeared' are also variables that should be controlled, as they affect the results of time taken for the cross to 'disappear'. Prediction: I predict that as the concentration of the hydrochloric acid is increased, the rate of reaction also increases (they are directly proportional to one-another). Therefore, as the concentration of acid is increased, the time taken for the cross to disappear will decrease. Prediction Reasoning: The reasoning for this is that as concentration increases, the amount of hydrochloric acid in a given volume (cm3) of that substance increases. Therefore, more particles of hydrochloric acid will be present in a given volume, and consequently more collisions between hydrochloric acid particles and sodium thiosulphate particles will occur (when the sodium is added to the hydrochloric acid, and the reaction begins). If more collisions between particles occur, the rate of reaction will then increase directly to the concentration of acid increasing. (RESOURCE 2) Using the above predictions, I have come to the following expectations for the graphed results to be produced: Apparatus list: The following apparatus / equipment will be required, to be used in the experiment: * 2 x 10cm3 measuring cylinder * 2 x pipettes * 1 x conical flask * Supplies of: o Hydrochloric acid o Sodium Thiosulphate o Water * Stopwatch / time measuring equipment * Safety goggles Plan: Firstly, goggles will be worn, and all apparatus to be used will be gathered, and arranged as the diagram shown below indicates. Next, using a 10cm3 measuring cylinder, 10cm3 sodium thiosulphate will be measured out. Then, using a different 10cm3 measuring cylinder, 10cm3 of 4.0M hydrochloric acid will be measured out. A cross will then be drawn on a piece of white card / paper, and placed under a conical flask. ...read more.


were actually found to be anomalous, these are those highlighted in bold. This is because when incorporating error from when obtaining results, two of the previously 'anomalous results' actually could have been following the 'pattern' indicated by the other results, and the results predicted. The reasons for including error bars of the certain heights shown were due to the fact that the 'time taken' results obtained could have been �2 seconds. This then affected the rate of reaction directly, because rate of reaction was calculated by 1 / seconds. The error bars on the graph showing the affect of concentration on the rate of reaction were then worked out by calculating the rate of reaction of the values shown on the error bars from the other graph. In order to improve the accuracy of the evidence further, the following changes could be made to the overall experiment: * Electronic measuring devices, and precise data logging could have been used to measure when the sulphur produced in the precipitate reaction had completely blocked-out all light coming through the solution produced. * Electronic measuring devices could also have been used to measure-out precisely the hydrochloric acid and sodium thiosulphate used in each reaction. * New measuring cylinders could have been used to measure-out each new amount of hydrochloric acid or sodium thiosulphate used, reducing the effect of contamination on rate of reaction obtained. * To overcome the temperature differences present with the experiment, the reactions could have taken place in a temperature-controlled water bath, or by using a Bunsen burner to heat the hydrochloric acid and sodium thiosulphate before the reaction had begun. To extend the experiment with new lines of enquiry, I could change the concentration of sodium thiosulphate instead of the hydrochloric acid, to analyse and compare the results obtained with the results obtained from this experiment. Also, different variables such as temperature or presence of a catalyst could be used as input variables, other than concentration of the acid used. Page 1 ...read more.

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