Science Course Work

The Method

 For this experiment I will use the precipitation method (Disappearing Cross) which involves first measuring out 50cm (cubed and measured at the meniscus) of sodium thiosulphate solution and pouring it in to a conical flask. Then drawing a cross on a piece of scrap paper, putting the flask on top of the cross and checking to see whether you can see the cross looking down through the top of the flask. After that carefully measuring out 5cm (cubed and measured at the meniscus) of 0.5 molar hydrochloric acid, pouring it into the flask containing the sodium thiosulphate and starting the stopwatch straight away. Looking down at the cross, watching the solution turn cloudy, and timing how long it takes for the cross to disappear. Then repeating the same experiment but changing the concentration of the hydrochloric acid being at 1.0 molar, 1.5 molar, 1.7 molar and finally 2.0 molar (the amount of sodium thiosulphate must be kept the same!).

Measurements Observations and Recording

 So in this series of experiments I will measure the amount of both Sodium thiosulphate and Hydrochloric acid from the bottom of the meniscus and using a pipette. To make sure that it is accurately consistent through out all the experiments (the sodium thiosulphate will be at 10ml, and the hydrochloric acid at 5ml all the time!). I will observe the solution of sodium thiosulphate and hydrochloric acid until it turns cloudy and I am unable to see the cross through it. Then I will stop the stopwatch and record my results in a table, according to the concentration of hydrochloric acid that I have just used. After that I will do the same experiment 2 more times so that my results are accurate and reliable I chose to do each concentration 3 times because I thought that doing my experiment 3 times instead of just one would give me better and more reliable results. Then I will work out the average time in (sec’s) it takes for that particular concentration of hydrochloric acid to react with Sodium thiosulphate and make the cross disappear, by adding up the 3 times I have and dividing them by 3. Moving on I will do the exact same series of experiments to all the different concentrations of hydrochloric acid (0.5 molar, 1.0 molar, 1.5 molar, 1.7 molar, and 2.0 molar). After I have all 5 average times, for all the different concentrations of hydrochloric acid, taken to react with Sodium thiosulphate until I could not see the cross anymore. All I would do now is turn all the average times into rates of reaction by using the formula “1/average time” and writing down the answer to 3 decimal places. Finally I will also measure the temperature before and after each experiment because this is a factor that could affect the rate of resistance, and I am only suppose to be looking at the concentration. (Example of what table my result table will look like on page 3).

 Why I Chose This Method

 I chose this method because it is a really good, pretty effective and quick way to determine how fast the reaction takes place and to what extent the concentration of hydrochloric acid affects the rate of reaction. As at the start we have two liquids (Sodium thiosulphate and Hydrochloric acid) and we end up with a solid! (Sulphur, which makes the solution cloudy and therefore we can find out how long it takes for the solution to turn cloudy using different concentrations of hydrochloric acid).

Safety Precautions and Fair test

The safety precautions I would take are wearing safety goggles to make sure I don’t get any of the acid in my eyes. I would also take care not to touch any of the acid with my hands and not to breath in any of the sulphur dioxide produced in this reaction.

 

 Also to make it a fair test I will make sure that there is only one person looking at the cross until they cant see it anymore because of the solution turning cloudy and then stopping the stopwatch. This is because if there was more then one person looking at the cross and deciding when they cant see it any more and therefore stopping the stopwatch. Then there eyesight might be slightly better or worse then the other person that observed the cross before or would after him/her resulting in him/her being able to see the cross for slightly longer/shorter. Therefore meaning that the investigation would not be fair as the results would not be accurate, as they were not taken by only one persons judgement. Some other things that I would do to make it a fair test are that I will use the same volume of solutions in every experiment (10ml of sodium thiosulphate and 5ml of hydrochloric acid). I will also use the same cross every time because different crosses may have different visibility and therefore will give me unreliable results. Finally I will make sure that the same person stops the stopwatch in all of the experiments, this is because again different people have different judgements and so if more then 1 person stops the stopwatch then my results will be unreliable.

 Equipment list (Apparatus)

 

• Sodium thiosulphate solution
• Dilute hydrochloric acid
• A Pipette (for accurate and reliable measurements)
• Conical flask
• Scrap piece of paper (to draw a cross on)
• Stopwatch
• Goggles  

Example of what my result table will look like

My Results Table

Interpretation of results and Conclusion

 My result show that as the concentration of hydrochloric acid increased so did the rate of reaction. This is because as I increased the concentration of hydrochloric acid the amount of hydrochloric acid particles also increased. As the amount of hydrochloric acid particles increased so did the chance that they would hit the sodium thiosulphate particles more often. If the hydrochloric acid particles hit the sodium thiosulphate particles more often, then it takes less time for them to exceed the activation energy and for the reaction to take place. Therefore as I increased the concentration of hydrochloric acid, the rate of reaction also increased. Also the temperature of all the experiments before and after was the same this tells us that no heat was given off in the reaction (it was not an exothermic reaction). So if there was no heat given off then that means that concentration of the hydrochloric acid was my ONLY variable (that affects the rate of reaction). Meaning that my results are fairly accurate and reliable.

 I predicted that as the concentration of hydrochloric acid increases so will the rates of reaction and according to my results my prediction is correct. The results show that as the concentration of hydrochloric acid increases so do the rates of reaction. This is because of the collision theory and the fact that as the concentration of hydrochloric acid increases so do the amount of particles, and as the amount of hydrochloric acid particles increase so does the chance of them hitting sodium thiosulphate particles more often. And if the hydrochloric acid particles hit the sodium thiosulphate particles more often then it takes a shorter amount of time for them to exceed the activation energy and for the reaction to take place.

Evaluation

 So by using the precipitation method I have proved that as the concentration of hydrochloric acid increases from (0.5 to 2.0 molar) the rate of reaction between hydrochloric acid and sodium thiosulphate also increases. Meaning that the higher the concentration of hydrochloric acid from (0.5 to 2.0 molar) the quicker the reaction takes place between hydrochloric acid and sodium thiosulphate.

 Overall the quality of my results is pretty good and all of my results “fit the pattern” meaning that all my results back up my prediction which was “as the concentration of hydrochloric acid increases so does the rate of reaction”. However there were two anomalies with my “rate of reaction” against “concentration of hydrochloric acid” graph, that was because even though they did follow the pattern they didn’t follow it steadily. Meaning that the rate of reaction did increase as the concentration of hydrochloric acid increased but it did not increase steadily. In other words the rate of reaction did not increase the same amount each time the concentration went up by a certain amount. I also had an anomaly on my “average time (seconds)” against “hydrochloric acid concentration (M)” and again it did follow the pattern meaning that as the concentration of hydrochloric acid increased the average time taken decreased. The reason for the anomaly was the same as for the other graph, it was because as the hydrochloric acid increased by a certain amount the average time decreased but not steadily. Therefore not fitting the line of best fit and being anomalous.

  I think my investigation was fair as I followed my plan exactly. And I made sure that there was only one person looking at the cross through out all the experiments, I made sure that I used the same volume of solutions in every experiment (10ml of sodium thiosulphate and 5ml hydrochloric acid). I used the same cross for all the experiments and finally I made sure that there was only one person stopping the stopwatch through out all the experiments. So overall I think keeping my investigation fair was a success and my results reflect that.

 I have collected enough evidence to prove that as the concentration of hydrochloric acid increases from (0.5 to 2.0) molar so does the rate of reaction, providing that the concentration of hydrochloric acid is the only input variable. But my investigation only proves that the rate of reaction increases as the concentration of hydrochloric acid from (0.5 to 2.0) molar. I could have carried out the experiment with higher concentrations of hydrochloric acid to see for how long the rate of reaction continued to increase with the concentration. If I could use higher concentrations of hydrochloric acid then I could have made a more in depth statement like “as the concentration of hydrochloric acid increases so does the rate of reaction until the concentration gets to… molar”.