How does changing the Concentration of Sodium thiosulphate affect how fast it reacts with an acid?

Variables:

1. The ratio of sodium thiosulphate [Na2S2O3] to water. Depending upon this ratio will depend upon the concentration of sodium thiosulphate [Na2S2O3] and therefore, will change the probability of particles colliding in the solution because; there will be more particles in one particular area. This should be the only variable that will change because; it is the variable that I am investigating and therefore the test will be fair if I change it.
2. Changing the molarity of hydrochloric acid [HCl] and the volume of hydrochloric acid [HCl] must not be changed because; the acid is just there to make a reaction happen, not there to be investigated. Concentration of hydrochloric acid [HCl] and the volume of hydrochloric acid [HCl] must stay the same because; I need to have an equal probability that the particles in the solution will collide. If this were disregarded then the investigation would not be fair.
3. Constant temperature will need to stay constant because, it will affect the rate of a reaction in two ways. The temperature will make the particles move faster and give them more energy. So the probability of collisions exceeding the activation energy will be higher. Therefore, for the investigation to be fair the temperature must stay constant.

Accuracy:

1. The same pair of eyes must look at the cross because, different people have different quality eye. Therefore, some one with good eye sight will need to look for a longer period than a partially sighted person. So, if the same pair of eyes are not used accuracy will not be achieved.
2. Measuring cylinders, these must be used for measuring the volume of a substance because; they are more accurate than the conical flask’s approximation volume. If accuracy is to be achieved to a certain level, flasks have a greater percentage of in accuracy that a measuring cylinder would.
3. The person who watches the cross must operate stop clock because, it will be a quicker reaction time and create less accurate times. If some one were to be told to stop the clock then the reaction time would be slower and affect accuracy.
4. Distilled water must be used at all times, not tap water because, tab water has impurities within it, and these impurities may affect the rate of reaction and would create inaccuracies within my results.
5. Also, the same cross must be used all the way through the experiment because, other crosses may be of different thickness and shade so the cross may appear to disappear faster or slower in comparison to my other results.

Equipment:

• Sodium thiosulphate [Na2S2O3] 3x175cm3
• Hydrochloric Acid [HCl] [1Molarity solution] 3x25cm3
• Distilled Water 3x75cm3
• Conical Flask x1
• Measuring Cylinder x1
• Paper and Pen x1

Method:

• Draw a cross on the paper using the pen. [To be done once only]
• Place it under the conical flask.
• Measure 45cm3 of sodium thiosulphate [Na2S2O3] using the measuring cylinder and place in the conical flask.
• Measure 5cm3 of distilled water using the measuring cylinder and place in the conical flask.
• Measure 5cm3 of hydrochloric acid [HCl] using the measuring cylinder and place in the conical flask. [Giving a total of 55cm3 always]
• As soon as the previous process has been completed activate the stopwatch immediately.
• Stop the stopwatch when the solution becomes too cloudy for the cross to be seen.
• Record the results in a table.
• Repeat the method above five more times, but each time decrease the volume of sodium thiosulphate [Na2S2O3] by 5cm3 and increase the amount of distilled water each time by 5cm3.
• Finally, repeat the whole method two more times, making sure all information in the brackets is complied.

Analysis:

        In this experiment I discovered that as the concentration of sodium thiosulphate [Na2S2O3] increased the time taken for the reaction to occur and the cross to disappear decreased. Therefore, as the concentration increased the rate of the reaction must also increase. According to the graph I produced, the line of best fit is straight, for that reason the concentration is directly proportional to the rate of the reaction. Hence, my prediction is proved.

        To conclude, the line of best fit shows that as the particles in the sodium thiosulphate [Na2S2O3] solution become closer together or more concentrated. The rate of the reaction will increase because, there will be a greater probability that the particles will collide with the hydrochloric acid [HCl] particles and more collisions will be effective. Therefore, the time taken for the reaction to occur will decrease and the rate of the reaction will increase. Including the anomalous result, would also      re-enforce (re- pref. Attachable to almost any verb or verbal derivative, denoting once more, again, anew, afresh, repeated, back.) my prediction because, the points give a positive correlation, which also proves my prediction. This correlation happens because; the graph follows a specific scientific trend, the line of best fit is straight so the rate of the reaction is directly proportional to the concentration of a solution. Therefore, the rate of the reaction must be directly proportional the concentration of sodium thiosulphate [Na2S2O3]. In addition, as the reaction occurs faster the sulphur [one of the end products] will diffuse around the solution quicker and the cross will disappear quickly.

Evaluation:

        My plan worked well excluding the anomalous result because, I was able to use my plan effectively to get results and create a graph, of which I proposed to do in my prediction. The volumes and molarity of my chemicals did not make my results go out of proportion, if that had happened my graph would have lots of anomalous results. The rate of the reaction did not occur to fast or to slow, that I would not be able to react at the right time. Either I would not be able to de-activate the stop clock fast enough or either I would not be able to determine when to de-activate the stop clock. The first point on my graph is anomalous because, it did not react fast enough is comparison the other results. This could have happened because; the hydrochloric acid [HCl] did not enter the solution fast enough and would therefore make the stop clock run for longer than expected, creating a slower reaction rate. To stop this happening in future experiments I could have used a syringe to eject the hydrochloric acid [HCl] into the solution at a faster rate, which could be an improvement for my plan. Temperatures of chemicals could also be kept the same temperature before the reaction starts to take place. The reason being, temperature affects rates of reactions in two ways so even the slightest alteration in temperature will create anomalous results. So if this investigation were to be repeated, I would keep my chemicals in a water bath so that the chemicals would definitely be at the same temperature before adding them to the conical flask. Swirling the solution is a good method of starting the reaction off but it was difficult to create the same swirling motion each time an experiment was set up. Keeping the cross dry was also difficult; the ink started to run and it was difficult to judge when to stop the stop clock. If the experiment were to be repeated it would be a good idea to laminate the cross, then it could be wiped free of liquid after each experiment. Considering that the anomalous result was not far off the line of best fit and all other points fit the line of best fit well so that I am able to conclude, the results that have been recorded should give enough evidence in order to support my conclusions. Further experiments could be planned in order to research how temperature affects the rate of a reaction, the molarity and volume of sodium thiosulphate [Na2S2O3] and hydrochloric acid [HCl] because, that would give an equal probability of particles colliding. The temperature of sodium thiosulphate [Na2S2O3] would be varied at regular intervals before the acid was added at each varied temperature. Results would be recorded and graphs drawn accordingly. Finally, conclusions and evaluations would be considered in order for other experiments to be planned and researched.