Variables
In my investigation I could change:
- Concentration of sodium thiosulphate
- Temperature of solution
- Concentration of acid
- Type of acid
- Volume of thiosulphate
- Volume of acid
I am going to change the concentration of acid.
I will keep all the other variables the same. The independent variable will change the dependant variable- the rate of reaction.
Prediction
I predict that the less concentrated the acid is then the longer it will take for the sodium thiosulphate to react.
If you increase the temp, increase concentration, add a catalyst or the solid reactant is crushed up into smaller bits then the rate of reaction will always increase.
Apparatus
- Sodium thiosulphate
- Hydrochloric acid
- Measuring cylinder
- Beaker
- Paper
- Pencil
- Distilled water
- Stop watch
- Conical flask
Diagram
Safety
Goggles should be worn to prevent any of the solution from going into the eye.
Everyone should behave sensibly to prevent accidents.
Collision theory
The rate of reaction depends on how often and how hard the reacting particles collide with each other. The particles have to collide hard enough in order to react. The rate of reaction can be increased by increasing the temperature, making the solution more concentrated, size of solid particles and by adding a catalyst. They all increase the number of collisions.
Fair testing
- To make this a fair test I will only change one variable.
- Each experiment will be done three times so an average can be found.
- The same equipment will be used in each experiment.
Diagram for rate of reaction
Method
- Measure out 50ml of thiosulphate and add it to the conical flask.
- Draw a cross on a piece of paper and place the conical flask on it.
- Measure out 10ml of acid and add it to the conical flask and start timing.
- Look into the top of the conical flask and when the cross can no longer be seen stop the stop watch.
- Do the experiment again but measure out 8ml of hydrochloric acid. Then add 2ml of distilled water. The ratio for each experiment will therefore be 10:0, 8:2, 6:4, 4:6 and of acid to water
Conclusion
I conclude that whilst carrying out this investigation I found out that the more acid was in the solution the cross on the paper disappeared quicker. This shows that the more concentrated the acid is then the quicker the reaction time. My research about the collision theory was correct and the more concentrated the solution is then the number of collisions will be increased. If the numbers of collisions are increased then the reaction time will also be increased.
My prediction ‘that the less concentrated the acid is then the longer it will take for the sodium thiosulphate to react with the acid’ is correct because experiment number 5 with the least amount of acid in the solution took the longest time to stop reacting (the cross to disappear) and had a rate of reaction of 0.21 (x10) which is the lowest. All my results supported my original prediction.
Evaluation
I cannot rely entirely on the results of my experiment as it wasn’t carried out with a high level of accuracy, but overall the results do agree with the prediction and scientific research I have. Some of the results are slightly different to what they should be e.g. in the time 2 column in my table for experiment 2 and 3 the time only increases by 1 second which is a small increase as the others increase by 3 or 4 seconds. This may have occurred as for that particular experiment the measurements weren’t quite as accurate and maybe the acid concentration was lower.
I cannot rely on my results for another reason as well because the cross drawn on the paper may have not fully disappeared when the clock was stopped. It could vary from time to time and the result may therefore not be accurate and shouldn’t be relied on.
When all the results were placed onto the graph they seem evenly spaced out and close to the line of best fit. There is one particular result which is anomalous and doesn’t seem to be correct. In my results table the time between each result is usually 3 or more seconds but in the second time column experiment 2 and 3 only have a 1second difference in time. These results are too close together and cannot be reliable.
These results aren’t adequate for a firm conclusion as it has a few anomalous results and on the average times they don’t have a large enough gap between them e.g. 34 and 36. The experiment should be carried out more times for a more accurate result.
The procedure used was suitable but produced quite unreliable results as the cross on the paper may have not fully disappeared when the other person was told to stop the timer as this is what we did. If the person looking at the cross and observing when it disappeared changed then the person’s opinion on when it has disappeared may make it an inaccurate test so it would have to be kept the same person all the time.
If I did the experiment again I would probably take more time measuring out the solutions and it would probably be a better idea for the person observing to see when the cross can no longer be seen to have the stop watch so they can stop it at the exact time they can no longer see it. It could also be improved if the experiment was carried out more times as a better average could be produced. This would make the results more accuarate.
