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How Concentration affects the rate of reaction.

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How Concentration affects the rate of reaction I am investigating the rates of reaction and how the different variables can affect it. I know that the temperature, a catalyst, concentration, the surface area, light and pressure of a gas can affect the rate of reaction. In this particular experiment I am going to investigate on how the concentration of one of the reactants affects the rate of reaction. To show that the concentration does affect the rate of reaction I am going to carry out the reaction of sodium thiosulphate and hydrochloric acid. The reactant I am going to change in concentration for each experiment/reading is sodium thiosulphate. Word Equation Sodium + Hydrochloric � Sodium + Sulphur + water + Sulphur Thiosulphate acid Chloride dioxide Chemical Equation Na2S2O3(aq) + 2HCl(aq) -> 2NaCl(aq) + S(s) + H2O(l) + SO2(g) Equipment Sodium Thiosulphate (NaSO) of different concentrations and volumes 5cm� Hydrochloric acid (HCl) Water of different volumes Paper marked with a cross 2 Measuring cylinders Conical Flask Stopclock I will set the equipment up as shown in the diagram below Method I am going to measure out the right amount (50cm�) and concentrations of the sodium thiosulphate and hydrochloric acid. I will vary the concentration of the sodium thiosulphate but will still keep the volume of the acid the same. I will keep the volume (5cm�) and concentration (2.0 moi/dm�) of the hydrochloric acid the same. The table below shows the amount of water I will add to the sodium thiosulphate to vary the concentration in each experiment. Experiment Number Volume of 0.15 mol/dm� sodium thiosulphate (cm�) Volume of water (Cm�) Concentration sodium thiosulphate solution (mol/dm�) 1 50 0 0.15 2 40 10 0.12 3 30 20 0.09 4 20 30 0.06 5 10 40 0.03 I will pour the right amount and concentration of sodium thiosulphate (varies in each experiment No.) in the conical flask, which is on the cross that is marked on the paper. ...read more.


By doing 4 sets of results I could spot anomalous results and I can find out why they had happened. 2 sets of the results were done on the same day and the other 2 were done on 2 other days. Each set of results had 5 readings. The range between each reading was the 10cm� dilution of water added to the sodium thiosulphate solution. This is good range, as it has not got a too bigger gap from the last reading or too minute gap that I cannot detect any changes in the time for the cross to be obscured. The calculations I used to calculate the rate of reaction was: 1/time for the cross to be obscured. The 4 sets of results are on the next page. After having compared the results, I realised that the 2 sets of results done on the same day had quicker times for the cross to be obscured, which has also made the reaction faster. I repeated the other 2 results again but the times were very similar to the original results. I think the 2 sets of results produced on the same day had warmer water, which gave the particles of the solution more energy, this made them move faster and collide more frequently and more effectively to give a faster rate of reaction. I noticed that the other two experiments that were done on two other days had used colder water. So, this was probably what had caused the time difference between the results. I am going to use the 2 sets of results that were done on the same day to draw my conclusion, find averages and plot graphs because they are more reliable. - They were done on the same day, and the water and room used was roughly the same temperature. All these factors make the results of these 2 particular sets of results reliable and trustworthy. ...read more.


From these results I will average them to get a single reliable reading. I will also work out the rate of reaction (1/time). I will then plot 2 graphs showing the rate of reactions and how it affects the time taken to collect the gas and how the concentration of hydrochloric acid affects the time taken to collect the gas. From the graph and results I should be able to draw conclusions and see whether if my prediction is correct. Even though my method of finding how the concentration of sodium thiosulphate affects the reaction rate is good, I have researched another method which can be used for the same experiment but produces more accurate results. It involves a computer with special light sensitive equipment. It records how much light passes through the solution as the Sulphur makes it more and more cloudy. This method also involves machinery that put the reactants together for you. When you choose to do an experiment, the acid is added to the flask containing the sodium thiosulphate solution (the flask is partly covered to stop any stray light affecting the measurements). After a short time, the reaction starts and you will see the light intensity reading on the computer screen change as the Sulphur is formed. The lower the reading, the less light is being allowed through, the more Sulphur is being formed. I also had a problem when I plotted my graph. I found it quite difficult to join the points with a line of best fit. If I have to plot a graph in future I am going to use a computer as the curves will be smoother and therefore the line of best fit will be more accurate. All in all I think this was a good experiment but it could have been improved if I used more of the resources available and had more time to do the experiments. The results supported my predictions and they seem to be fairly reliable results. Sheena Lam 10H ...read more.

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