I’m not using a catalyst because this would affect the results of my investigation.
Both the sodium thiosulphate and the hydrochloric acid are soluble in water, so the concentration of either can be changed. I am going to change the concentration of the sodium thiosulphate because I think the results will be clearer since there will be more or less sulphate to turn the solution yellow.
Prediction and hypothesis
I predict that, as the concentration of the sodium thiosulphate increases the rate of reaction will also increase. This will lead to a decrease in the time taken for the solution to turn “cloudy”.
For a reaction to take place between two particles, they must collide, so if more collisions occur, the rate of reaction increases. However, only some collisions cause a reaction, since not all particles have enough energy to react. Increasing the concentration of a reactant means there are more particles per unit volume which may collide and so react. Therefore collisions between reacting particles are more likely to occur, and so the higher the reaction rate.
Plan
Equipment needed
- 1 beaker
- 3 measuring cylinders
- 1 stopwatch
- 1 laminated cross
- 1 pair of safety glasses
Steps
1 I will measure out 25ml of hydrochloric acid in a 100ml beaker. Using a measuring cylinder I will measure out 50ml of sodium thiosulphate.
2 I will put a laminated cross underneath the beaker and when the 2 solutions mix I will time how long it takes until the solution has formed a precipitate of sulphur and I can no longer see the cross. This interval is related to reaction rate (a fast clouding implies a fast reaction).
3 I will then repeat the experiment varying the concentration of sodium thiosulphate by 5ml each time. I will do this by replacing 5ml of the sodium thiosulphate with distilled water each time.
4 There is naturally a likely human error in this approach (exact disappearance of a cross). I will record repeat results and averages to improve the credibility of my findings. The repeat results will help remove any anomalies and the average will give a good summary of the results.
5 In all cases the experiments will be conducted at room temperature.
Safety
I will wear a pair of safety goggles to protect my eyes
Controlled Variables
- Same amount of hydrochloric acid
- Same total volume of solution
- Same cross
- Same size beaker
- Same stopwatch
- Same person using the stopwatch
- Same person judging how long it takes for the cross to disappear
- Ambient temperature is the same
The controlled variables are to make the experiment a fair test.
Key Factors
Temperature – Must be kept the same as an increase in the temperature will cause the reaction time to increase. This is because the heat energy gives the particles greater activation energy.
Concentration – This is the variable I am going to change. I expect that as the concentration increases the reaction time will decrease. This is because there will be more molecules in the same space, blocking the light.
Preliminary experiments
I estimated which concentrations to use during my preliminary set of experiments and I tried the first and last values. If these times revealed a wide and noticeably measurable range then concentrations in between these values would be able to show sensitivity. However this was to be balanced against the need to complete several “tries”.
At first I tried 25ml of hydrochloric acid and 25ml of sodium thiosulphate. This took 50.81 seconds.
I then tried 25ml of hydrochloric acid with 5ml of sodium thiosulphate and 20ml of water. This took 6.00 minutes.
I concluded this would lead to a lengthy total experiment so I tried another amount.
In my second preliminary experiment I tried 25ml of hydrochloric acid and 50 ml of sodium thiosulphate. This took 32.96 seconds.
I then tried 25ml of hydrochloric acid with 25ml of sodium thiosulphate and 25ml of water. This took 1.07 minutes.
These times, while more compressed than above, suggest a more reasonable experiment length and still provide an adequate interval for measurement and I decided to use these amounts.
Final amounts
Results
Analysis
I converted the amount of sodium thiosulphate into its percentage of concentration using this formula:
Amount of sodium thiosulphate used
50 x 40
(I multiplied by 40 because there was 40g of sodium thiosulphate per litre)
From my graph I can see that as the concentration of the sodium thiosulphate increases the time taken for the solution to precipitate decreases. Because of this the graph has a negative correlation.
I predicted that as the concentration of the sodium thiosulphate increased the time taken for the solution to precipitate would decrease so my graph shows my prediction is correct.
This is because for a reaction to take place between two particles, they must collide, so if more collisions occur, the rate of reaction increases. However only some collisions cause a reaction, since not all particles have enough energy to react.
The rate of reaction will increase if the concentration of one or more of the reactants is increased (in this case sodium thiosulphate) because if the concentration of a solution is increased there are more reactant particles per unit volume. This increases the probability of reactant particles colliding with each other.
More molecules in the same space means more collisions.
Evaluation
The experiment largely produced results in line with expectations. The variables were controlled and the test procedure was straightforward and simple.
Unfortunately, in order to complete the experiment in a reasonable time, it was necessary to conduct all “Tries” within a 2-3 hr timeframe. In practise this meant that “try 3” was completed on a different day. This I believe gives rise to an anomalous set of results (Try 3) since the room temperature changed over this interval (20c to 25c).
It may therefore be noted that since “Try 3” suggests a faster “clouding” than the other sets of experiments and since this occurred when the temperature was higher, then this confirms that increasing the temperature increases the reaction time.
There are several things I would change if I were to do this investigation again.
1 I would monitor room temperature and attempt to perform all three experimental “Tries” in quick succession (or even in parallel).
2 It’s not particularly accurate to judge when the cross has disappeared by eye as a person’s view of when they can't see it can change which will affect the results. Ideally I could use a lamp and light sensor.
I could also use a colorimeter which is a device used to compare or measure colours and their intensities. This would be able to show when the solution had finally precipitated by ‘showing’ when there was no longer any light being let through the solution.
3 As well as more accurate ways of telling when the mixture had fully precipitated there are also more accurate ways of measuring out the liquid such as using burettes/pipettes which measure accurate to 0.1ml instead of the measuring cylinder I used which only measures to 1ml.
Nevertheless I think the evidence I have collected is sufficient to support a conclusion that as the concentration increases the rate of reaction increases.
To extend the enquiry I could do further experiments where I would keep the concentrations the same but change the temperature.
