Not all of these collisions will result in a reaction though. The reactions will only occur if the particles have enough activation energy (Ea). This energy enables the chemical bonds to be broken and the atoms, ions and electrons to be rearranged.
Equipment
- Flask
- Measuring Cylinder
- Stop-Watch
- White Paper
- Red Pen
- Pipettes
- Safety Glasses
Procedure
- Wash all the equipment carefully
- Measure out 50cm³ of Sodium Thiosulphate into the flask
- Measure out 5cm³ of dilute hydrochloric acid into the measuring cylinder
- Add the Hydrochloric acid to the Sodium Thiosulphate in the flask and gently swirl.
- Place the flask on the paper marked with the cross and start the stop-watch.
- Observe the experiment through the top of the flask and stop the stop-watch when the cross can no longer be seen.
- Note down the time
- Wash all the equipment to avoid contamination
- Repeat the experiment with 40cm³ of Sodium Thiosulphate, 10cm³ of Water and 5cm³ of dilute Hydrochloric acid.
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Keep repeating, adding 10cm3 less Sodium Thiosulphate and 10cm3 more water each time.
When all this is done I will repeat the entire experiment again so that my results are more reliable.
Results
Set 1
Set 2
Average Set
Conclusion
My results agree with my prediction because I predicted that the lower the concentration of the Sodium Thiosulphate the longer the reaction would take.
My graph shows this because it curves downwards indicating that as one factor decreases so does the other.
This is because two particles have to be in contact before they can react with each other. In a lower concentration the particles are less likely to meet so reactions will be less often as the particles won’t meet as often. In the higher concentrations the particles are closer and more likely to come into contact with each other and react.
Therefore as I decrease the concentration of the Sodium Thiosulphate the reaction will take longer.
Evaluation
The second try of 20cm³ of Sodium Thiosulphate, 30cm³ of Water and 5cm³ of dilute Hydrochloric acid is an anomalous result. The reaction takes more time than the result after it which has lower concentration. This contrasts with all the other results and also my predictions. It is also noticeable in the graph where it is shown as the out of place dip in the line.
This could have been caused by a number of things such as:
- Measuring – Measuring of chemicals in liquid form is difficult to do accurately because of the movement in the liquid and the accuracy of the measuring cylinder.
- The person watching could have been mistaken as to when the cross disappeared.
- If the same person watching the reaction was operating the stop-watch then errors could of occurred in their coordination.
These errors could be removed by:
- Using electrical measuring apparatus or failing that using a burette as these are still more accurate than a measuring cylinder.
- Using a light sensor to detect when the cross is no longer visible. The experiment would be connected to a light sensor and timer and the sensor would detect when there was no light coming through the solution. Although this would increase accuracy dramatically it would also involve the use of expensive equipment that is not always available.
Other than that one anomalous result I think that the experiment is fairly accurate.
Bibliography
CGP Revision Guide – Science
Chemistry in Context
Notes from lessons taught by Ms Owen
The BBC Bite size revision website
Prior knowledge
Sam Holloway The Effect of Concentration on a Reaction Rate p.