Fair Test
- I will have the same person measuring the end point each time as different people will be looking for different points of the cross disappearing.
- I will not stir the solution as I cannot accurately judge how much this will affect the rate of reaction because I cannot make each stir as accurate as the last.
- The Conical flask will be washed out after every experiment to check that there is no build up of sulphur on the bottom of the flask as this would affect timing.
- I will ensure that the person looking at the cross and waiting for it to disappear is only looking at the cross and not staring because if they were to stare they could get an imprint of the cross on their retina and this would make the test very unfair as the time would be too long.
- The concentration of the acid must be kept the same because if it was changed this would alter the results every time the experiment was preformed and we are not testing how the acid concentration alter the results only how the concentration of sodium thiosulphate changes the results.
Variables
The one thing I will change each time will be:
The things I will keep the same are:
- Volume of acid
- Concentration of acid
- Volume of Sodium Thiosulphate
- No stirring
- Temperature
- The person looking at the X
Pre – Testing
Concentration of Time
Sodium thiosulphate (Seconds)
The Pre-Testing definitely helped me with my decisions or rather re-enforced them. The Pre-Testing went very well and it was quite simple to achieve these results.
I have carried out the pre-testing using 0.2M of Sodium thiosulphate. I tried a stronger solution of 0.4M but the reaction was so fast it made timing very difficult.
The lowest concentration I will use is 20% because at 15% it took over 400 seconds and the point may never be reached where enough sulphur was produced to block out the cross.
Safety
- Wear goggles to protect eyes especially when looking at the solution.
- Do not breathe in the fumes when looking into the conical flask to see if the cross has disappeared.
- Tie long hair back and tuck ties in as they could get caught on something or get some acid on them.
Prediction
My prediction is that the higher the concentration of sodium thiosulphate the quicker the cross will disappear. I will use scientific knowledge to prove my prediction should be correct. The easiest way to explain that my prediction has actually been backed up with scientific knowledge id the collision theory.
The collision theory – the rate of reaction depends on how often and how hard the particles collide with each other.
There is a collision theory also very specific and relevant to this experiment.
The collision theory based on CONCENTRATION – If the solution is made more concentrated it means there are more particles of reactant knocking about between the water molecules which make collisions between the important particles more likely, therefore if there is a high concentration of sodium thiosulphate then there are loads of these particles in the solution and therefore the likelihood of collision between sodium thiosulphate and hydrochloric acid is increased.
Quantative Prediction
I expect to see the rate of reaction double when the concentration of sodium thiosulphate is doubled as there will be twice as many sodium thiosulphate particles, therefore there it is likely that there will be twice the number of collisions.
Accuracy/Precision
I will maximise accuracy/precision in the test by using burettes to measure the amount of sodium thiosulphate as this is a very accurate way of measuring out small amounts.
I will also keep all of my units of measurement the same e.g. seconds, percentage.
I will repeat the experiment once more and make a results table including experiment 1, 2, 3 and the average, a third experiment will be carried out if the first and second experiments are not close.
Evaluation
If I did my experiment again then I would improve it by putting the beaker with the chemicals in, in a water bath to keep the temperature the same throughout the experiment. Even though the temperatures of the chemicals in my experiment were very similar this could affect results slightly.
