In my investigation, I intend to find out what effect changing the concentration of sodium thiosulphate has on the rate of reaction between hydrochloric acid and sodium thiosulphate.

Range of Values

I chose a range of five values so that I can plot my results on a graph easily because the difference between one number and the next is five, which makes it easy to draw a simple scale to plot it on. I also chose this range of values because it is easy to measure out the sodium thiosulphate and water because they are multiples of five. Finally, this range of values also gives me a clear idea of the pattern that will result when the experiment is conducted.

Controlled Variables

In this experiment I need to have a list of controlled variables so as to keep the experiment fair. My controlled experiments in this experiment are:

1. Temperature
2. Concentration of hydrochloric acid
3. Volume of acid
4. Type of acid
5. Size of and colour of X
6. Same glassware

Dependant Variable

The dependant variable in my experiment will be the time taken for the X to disappear. I will measure this with a stop watch and record the time taken in seconds.

Apparatus

• 3 burettes
• 2 retort and clamps
• 2 conical flasks
• 2 beakers
• Stop clock
• Filter paper
• Sodium thiosulphate (gdm-3)
• Hydrochloric acid (2M)
• Distilled Water

Method

I will be carrying out an experiment for five different values of concentration of sodium thiosulphate. I will repeat each of these five experiments twice. So in total I will be conducting fifteen experiments.

In this experiment, I will mix the hydrochloric acid with the different concentrations of sodium thiosulphate solutions, and then take note of how long it takes until the X on the piece of filter paper underneath the flask to become no longer visible.

The reason I will do duplicate experiments is so that any errors made in recording the first result will be obvious in the second result and therefore can be taken again to fix any mistakes. With the two/three results I take, I will take an average of them so that my final results will be reliable.

Safety Problems:

A number of health and safety issues could arise while carrying out this experiment. To avoid these issues, a number of precautions must be made:

• Wear safety goggles while conducting experiment
• Acids can be corrosive so care must be taken when measuring and pouring.
• If a spillage occurs, it must be cleaned immediately.
• If acid is split on skin, the affected area must be washed immediately.
• The experiment will emit a pungent odour so the formed solution must be poured down the sink in the fume cupboard to avoid irritation to people with respiratory problem e.g. asthma

Method:

1. Collect apparatus
2. Using a burette, measure out 40cm3 of sodium thiosulphate.
3. Pour Na2S2O3 into a conical flask.
4. Draw an X on a piece of filter paper.
5. Place conical flask on paper.
6. Using a burette, measure out 10cm3 of 0.1M hydrochloric acid
7. Pour hydrochloric acid into another conical flask. Add acid to the flask of sodium thiosulphate.
8. Start the stop clock.
9. When the X is no longer visible. Stop the stop clock.
10.  Record results.
11. Repeat above steps for the remaining solutions of sodium thiosulphate.

Strategy for Dealing with my Results

 Prediction

I predict that as the concentration of sodium thiosulphate increases, the rate of reaction will also increase. I think this because the higher the concentration, the higher the amount of ions and the more energy they have. This means that the thiosulphate ions and the hydrogen ions will have the activation energy required to cause lots of successful collisions, and so the more successful collisions, the shorter the reaction time.

I will use my results to find out if my hypothesis is true by plotting a point graph of rate of reaction (x-axis) against the concentration of sodium thiosulphate (y-axis). If my graph is correct then it will show that the rate of reaction is proportional to the concentration of sodium thiosulphate.