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The effect of concentration on the rate of reaction between sodium thiosulphate and dilute hydrochloric acid

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Introduction

The effect of concentration on the rate of reaction between sodium thiosulphate and dilute hydrochloric acid This investigation is about rates of reaction and what affects them. In this case I am going to look at hydrochloric acid and sodium thiosulphate which is a precipitation reaction causing the solution to go 'cloudy'. They react as in the equations below: sodium thiosulphate + hydrochloric acid -> sodium chloride + sulphur + sulphur dioxide + water Na2S2O3(aq) + 2HCl(aq) -> 2NaCl(aq) + S(s) + SO2(g) + H2O(l) A reaction will only occur where the particles of the reactants meet and combine. This is called the collision theory. For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy, or Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same. An increase in the rate of reaction can be achieved by increasing the frequency of collisions. Therefore to increase the rate of reaction it is necessary to cause more particles to collide harder and collide more often. There are several ways to do this and these make up the factors for this experiment. They are listed below along with predictions as to their affect on the reaction. Possible Factors To make sure I carry out a fair test I will only change the concentration of sodium thiosulphate. ...read more.

Middle

This increases the probability of reactant particles colliding with each other meaning the rate of reaction will be quicker. Plan Apparatus Pipette, burette, measuring cylinder, sodium thiosulphate (0.01 mol/dm�), dilute hydrochloric acid (2.00 mol/dm�), distilled water, stopwatch, conical flask, paper with cross marked on, goggles. Method I will measure out the required volumes of sodium thiosulphate and water (see measurement table) using a burette. I will transfer these to the flask . I will then stand the flask on a piece of paper with a cross marked on it with a pen or pencil. I will measure out the acid using a pipette, as this is highly accurate for measuring, and add it to the flask at the same time I start the stopwatch. I will then record the time for the cross to be no longer visible. I will repeat the whole procedure for more reliable and average results. If after plotting a graph I may come across some anomalous results. I will repeat the certain amounts and see what results I get the second time. If the results fit in better with the line of best fit this will tell me that a mistake was made in the first set which affected the rate of reaction. THIO (cm�) HCl (cm�) WATER (cm�) 50.00 10.00 0.00 45.00 10.00 5.00 40.00 10.00 10.00 35.00 10.00 15.00 30.00 10.00 20.00 25.00 10.00 25.00 20.00 10.00 30.00 Measurement table Diagram Conical flask Thio, HCl, water solution Cross marked on paper Predicted graphs Graph 1 Graph 2 time 1/t conc. conc. The time taken for the cross to As the concentration increases disappear decreases as the concentration time decreases increases Inverse proportionality conc = 1 time Graph 1 - This graph shows the time taken for the cross to disappear. ...read more.

Conclusion

One example would be judging the cloudiness of the solution. It all depends on your eyesight whether or not you think the cross has disappeared and with different people doing the same experiment, the results are going to vary slightly depending on how good or poor your vision is. One way of getting round this situation is to use a device which detects the cloudiness of the solution. The device can be set up as follows: firstly a beam of light is directed through the solution and on to a photocell at the other side. The photocell is linked to a voltmeter. The solution starts to get cloudy as the reaction takes place. When the solution gets so cloudy that the beam of light no longer reaches the photocell the voltmeter stops reading the voltage and a timer stops automatically. Using such a device enables you to get more accurate and fair results because the elapsed time will be recorded at the same degree of cloudiness for each test. I think the range of results I had was satisfactory, although maybe a bigger range of measurements would make it more reliable. However I don't think this was necessary when looking at my graph, because the range of results I obtained clearly indicates the effects of concentration on the rate of reaction. Apart from two anomalies, which were repeated and confirmed , my experiment was generally accurate. I can confirm this is true because the line of best fit on both my graphs is similar to that on the predicted graph which demonstrates accuracy. I believe it was a fair test. I gathered enough evidence to support my prediction and therefore can say that my experiment went well. ...read more.

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