Pipette Hydrochloric acid
Stopwatch Sodium thiosulphate
Spotting tile Water
Conical flask
Funnel
Burette
Method
1. Set up apparatus as shown in the diagram.
2. Measure 25cm3 of hydrochloric acid and pour into conical flask.
3. Add 25cm3 of 0.2 mol dm-3 of Na2S2O3 to hydrochloric acid and immediately start stopwatch.
4. When the cross disappears stop the stopwatch.
5. Record the results.
6. Repeat steps 1-5 three more times.
7. Repeat step 1-6 for 0.4, 0.6, 0.8, 1.0 mol dm -3 Na2S2O3.
Diagram
Variables-independent, dependent and control
Independent variable
This is the variable that I change during the experiment. I am going to change the concentration of the sodium thiosulphate solution by adding water.
Dependent variable
This is the variable that is measured during the experiment. I will measure the time taken for a cross to disappear. I am going to use a stopwatch.
Control variable
I will make sure that I carry out a fair test by keeping the following the same:
-measurement
-amount of chemical
-temperature
-the equipment
-same amount of sodium thiosulphate
-same amount of hydrochloric acid
-same surface area
Chemical Equation
Word:
Hydrochloric acid + sodium→ sodium + sulphur dioxide + water + sulphur
Symbol:
2HCL + Na2S2O3 → 2NaCL + SO2 + H2O + S
Dilutions Table:
Analysis
My results tell me that as I used more sodium thiosulphate the concentration increased and the time taken for cross to disappear decreased.
The solution with the greatest concentration was 1.0 mol dm -3 because if I look at the graph or results table then I can see that the fastest time was in concentration 1.0 mol dm -3.
The disappearance of the cross was quickest when it was 1.0 mol dm -3 because there was more substance.
As the concentration increased the particles moved with more collisions – there are more collisions per second.
I did not get an outlier but if I did this could be because:
-it’s difficult to judge cloudiness of solution.
-spillages.
-timing.
-measured liquid wrong.
My results agree with my prediction because my prediction was that as the concentration of the sodium thiosulphate is increased then the rate of reaction will increase. So by looking at my graph I can tell that my results did agree with the prediction.
There was a real difference between results because if I take time away of 1.0 from 0.2 which shows me the difference which is 234 seconds and that is a real difference.
Evaluation
My method was suitable because I got the reliable results which support my prediction.
My results were suitable and reliable because I have repeated the experiment 5 times and also there was wide range of concentration.
My results were accurate. However, the error bars overlap with nearest concentration.
I got accurate results by using accurate equipment, used burette to measure liquids.
To improve the accuracy of the experiment I could’ve used a colorimeter, which is a meter that measures how much light it can absorb.
I did not get an outlier. This is because of spillages, wrong timing or wrong results.
My equipment had various levels of precision. The tolerances of each piece of equipment are:
-Burette – 0.1cm3
-Measuring cylinders – 0.1cm3
-Pipette – exact to 25cm3
They helped me to measure substances accurately.
I could improve my experiment by using a colorimeter, making more repeats of the experiment and look at different concentrations.