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To investigate factors which change the rate of reaction between sodium thiosulphate and hydrochloric acid.

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INVESTIGATING FACTORS WHICH CHANGE THE RATE OF REACTION Planning Aim To investigate factors which change the rate of reaction between sodium thiosulphate and hydrochloric acid. The reaction The reaction that will take place between sodium thiosulphate and hydrochloric acid is as follows: Na2S2O3 (aq) + 2HCl (aq) ==> 2NaCl (aq) + H2O (l) + SO2 (aq) +S (s) In this experiment, a solid insoluble substance, sulphur is produced (so the initially clear solution eventually turns cloudy). This is useful as we can use this to estimate the rate of reaction by timing how long the reaction takes by timing how long it takes for a black cross to be obscured by the solution. As the rate of reaction is defined as the amount of sulphur produced (a constant) divided b the time it takes for the sulphur to obscure the cross, the rate of reaction can be measured. Variables There are several factors, which can alter the rate of a reaction. These are: * Temperature: An increase in temperature will make the reaction occur faster. This happens because the particles of that substance move faster when it is heated. Because the particles move faster, they would collide more often, making the reaction rate faster. Also see collision theory below. * Concentration: The increase in concentration, the greater and faster the reaction. This is because if the solution is more concentrated, it will increase the number of molecular collisions in a given time. ...read more.


For the main experiment, the method used in this experiment would be suitable, but a Bunsen burner and water bath will also be needed to heat the reactants up. A range of temperatures will need to be tested (18oC - 65oC). Main Experiment Apparatus * Sodium thiosulphate * Hydrochloric acid (0.2mol/dm3) * 400ml beaker (and water to act as a water bath) * Test tube with black cross on the back * Thermometer * Gauze * Tripod * Bunsen Burner * Heat-proof mat Diagram The apparatus was set up as below: Method * The apparatus was set up as above and the water in the beaker and sodium thiosulphate in the test tube was heated up to the desired temperature using a Bunsen burner, or left alone to measure 16oC (tap water temperature). * Once the sodium thiosulphate had reached the correct temperature, the Bunsen burner was turned off, 3ml of 0.2 molar hydrochloric acid was added, and the solution was stirred vigorously with the thermometer for 5 seconds. The stopwatch was also started. * When the solution became so cloudy that the black cross at the back of the test tube was not visible, the stopwatch was stopped. The time was recorded * The experiment was repeated for 18oC, 25oC, 35oC, 45oC, 55oC and 65oC. For each temperature, triplicates were done to make the results more accurate. Safety Hydrochloric acid is only slightly corrosive but safety goggles were still worn. ...read more.


or slow it down (if too little reactant is used). To fix this problem, a burette could be used to measure the quantities of acid and sodium thiosulphate out with more accuracy. > The temperature of the reactants could only be kept to a certain degree of accuracy as the Bunsen Burner heated water baths were hard to control. This could be fixed by using an electronic water bath. Heating up the hydrochloric acid to the desired temperature before it is added to the sodium thiosulphate, so that the net temperature of the reactants is not lowered when the acid is poured into the test tube. > It was also hard to keep a constant stirring rate as the method of stirring was not decided upon in the planning. This could be fixed by using an electronic magnetic stirrer which would keep the test tube still but have a fast, efficient and constant stirring speed to produce more accurate results. The experiment can be extended and made more useful by 1) Increasing the number of temperatures measured (for example, every 5oC not 10oC) and perhaps increasing the range of temperatures measured (within safety bounds). This will give a more reliable curve and a larger range of results to see if the curve changes in much higher temperatures. 2) Using a more reliable/accurate method (as suggested above - use measuring equipment with a higher degree of accuracy and a light gate to eradicate human error. 3) Investigate other variables such as concentration of acid, volume of acid, etc. Devesh Parekh 1 ...read more.

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