Apparatus
I used the following equipment in my experiment:
- Small measuring cylinder
- Large measuring cylinder
- Conical flask
- Stopwatch
Below is a diagram showing how I set up my apparatus.
Method
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Get all of your equipment ready in front of you.
- Put 10ml of hydrochloric acid in a small measuring cylinder.
- Put 50 ml of Sodium Thiosulphate in a large measuring cylinder.
- Put a small piece of paper with a cross on it underneath your conical flask.
- Then carefully pour in your 10ml of Hydrochloric acid.
- After this pour the Sodium Thiosulphate into the conical flask and then quickly start timing with your stopwatch.
- As soon as you see the water going cloudy and you can no longer see the cross under the conical flask, stop timing. Record how long it took for the solution to go cloudy.
- After this you must wash out your conical flask thoroughly.
- When you have completed all of the above you need to change the concentration of the Sodium Thiosulphate by adding water to dilute the solution. Repeat this experiment for every concentration stated earlier in the investigation.
For safety reasons you will need to wear goggles at all times.
Key Factors
After my preliminary work I have decided to change the solution of Sodium Thiosulphate from 10 ml to 15 ml and the concentration of water to 50 ml from 40 ml. I have done this because it took too long so I have increased the amount of Sodium Thiosulphate. This should decrease the time taken for the solution to go cloudy.
The dependent variable in my experiment is the time it takes for the solution to go cloudy. The independent variable in my experiment is the amount of Sodium Thiosulphate I will use in the experiment. I will measure this by using a large measuring cylinder accurate to 1 ml.
To make it a fair test I will keep the amount of Hydrochloric acid the same because this is the control of the experiment. I will measure it with a small measuring cylinder because they are more accurate than the larger measuring cylinders. If I were to change the temperature then it would spoil my results because of the kinetic theory. For reliability I will repeat the test at each concentration three times then take an average of the three results.
Prediction
I predict that as the concentration of Sodium Thiosulphate decreases then the time taken for the solution to go cloudy will increase. I think this because if there are more Sodium Thiosulphate particles then they will have more of a chance of colliding with the particles of Hydrochloric acid so the solution will go cloudy quicker. If the Sodium Thiosulphate is diluted with water then it will take longer for the solution to go cloudy, as there will be less Sodium Thiosulphate particles to react with the Hydrochloric acid particles. I will test my prediction.
Results
Here are the results of my experiment.
Conclusion
Interpretation of Results
From my graph I can see that as the concentration of Sodium Thiosulphate increases, the time taken for the solution to go cloudy decreases. For example, when there was a concentration of 20ml of Sodium Thiosulphate it took 62.66 seconds for the water to go cloudy but when the concentration of Sodium Thiosulphate was at 15 ml it took 101.79 seconds. I used a line of best fit to make it clear that the time was increasing as the concentration of Sodium Thiosulphate was decreasing.
Conclusions
From my results I can tell that what I had predicted earlier in the investigation was correct. I predicted that as the concentration of Sodium Thiosulphate decreased, the time taken for the solution to go cloudy increased. This is because at first when there is no water to dilute the Sodium Thiosulphate there are more particles for the Hydrochloric Acid to react with. That is why it only took 21.97 seconds to go cloudy. The diagram below explains this.
There are a lot of Sodium Thiosulphate molecules and no water molecules so they will crash into the Hydrochloric acid particles more often. Because they will collide more often the reaction will go quicker. However when I poured the water in to dilute the Sodium Thiosulphate the reaction slowed down. This is because there are less molecules of Sodium Thiosulphate to react with the Hydrochloric acid particles.
The diagram below shows this.
That is why the reaction took longer to occur with water in the solution.
Evaluation
My results were quite accurate because the results from the three tests I did were quite close. The biggest range I had all of my tests was 8. I had one result of 96.71 and another result at the same concentration of 104.78. I think this was the best method to test my theories because it was very easy to set up and it was very easy to do. I measured the rate of reaction by putting a cross on a piece of paper and when I could no longer see the cross because the solution was too cloudy I stopped the timer. The time on the stopwatch was the rate of reaction. To improve my experiment I could have changed the concentration of the acid. This would have improved the test because it would give me results and information about the test. The concentration I would use for the hydrochloric acid would be 20ml each time. I would do this to see how it would affect my experiment.
Reliability of the Evidence
My test was fair because I kept the amount of Hydrochloric acid the same throughout the experiment. This was the control of the experiment. I also controlled the independent variable, the Sodium Thiosulphate, by measuring it carefully and accurately. In my experiment I did not have any anomalous results. This is shown as my graph goes down in a curve, which is what it should do. I think that I do have enough data to get accurate results, however if I had done some more tests at different concentrations it would have made it more accurate. If I had done the same tests again I believe I would get similar results. To improve my method and the reliability of my experiment I could have used a thermometer to keep the temperature the same, because temperature affects the rate of reaction.
