The determination of a rate equation.

                        S2O32-(aq) + 2H+(aq) → SO2(g) + S(s) + H2O(l)

Overall equation:

                        Na2S2O3 (aq) + 2HCl(aq) → 2NaCl(aq) + SO2(g) + S(s) + H2O(l)

From the equation, we can see that sulphur dioxide is produced and sulphur is precipitated as products. Therefore bubbles should be produced and a yellow cloudy solution should be observed.

The product becomes cloudy once the reaction starts, so it is very difficult for human eye to judge when the end point of the reaction is. Therefore actual time taken for the reaction to complete is not easy to obtain accurately. A cross can be drawn on a piece on white paper and put under the beaker where the reactants are reacting. Record the time taken for the cloud to form until the cross disappears. In this case we can record the time taken for the cloud to get to the same density for each concentration of solution.  

Input variables:

• Varying concentrations of sodium thiosulphate and hydrochloric acid

Output variables:

• Time taken for complete reactions

Controlled variables:

• Total volume of solution
• Temperature
• Pressure

Apparatus

• 50.0 cm3 pipettes
• 5.0 cm3 pipettes
• Burettes
• Boiling tubes
• 250 cm3 volumetric flask
• Glass rods
• Beakers
• Filter funnels
• Droppers

Reason of the choice of apparatus:

Burette:

• This can give a very precise volume of the solution that you need.
• Accuracy

Pipette

• Accurate measurement of a fixed amount of solution can be obtained easily and accurately.
• Increase the reliability.

Volumetric flask        

• Provide a very thorough concentration when diluting solution.

5 cm3 pipette  

• Measure small volume very accurately

Chemicals needed:

• 0.60, 1.00, 1.32, 1.6 and 2.00 mol dm-3 hydrochloric acid
• 0.104, 0.20, 0.24 0.304 and 0.40 mol dm-3 sodium thiosulphate
• Distilled water

Solutions provided:

• 0.4 M sodium thiosulphate Na2S2O3(aq)
• 2 M hydrochloric acid HCl(aq)

Safety:

• 0.4M sodium thiosulphate Na2S2O3(aq)

• have minimal hazards but maybe harmful if ingested in quantities

• 2M hydrochloric acid HCl(aq)

• labelled as irritant solution and the vapour is very irritating to the respiratory system

•     may  cause burns
• handle it with care
• If they do come into contact with the skin, wash off with plenty of water.

• always wear eye protection and lab coat during the experiment
• Sulphur dioxide (formed in reaction)

•     toxic and corrosive by inhalation
• dispose each solution immediately after taking readings
• Pour solution down fume cupboard

Calculations for the chemicals

The volume of the 0.4M solution of S2O32- for dilution to 0.200 mol dm-3

0.4 x V/1000 = 0.2 x 250/1000

               V = 125 cm3

The volume of the 0.4M solution of S2O32- for dilution to 0.104 mol dm-3

0.4 x V/1000 = 0.1 x 250/1000

               V = 65 cm3

The volume of the 0.4M solution of S2O32- for dilution to 0.240 mol dm-3

0.4 x V/1000 = 0.24 x 250/1000

               V = 150 cm3

The volume of the 0.4M solution of S2O32- for dilution to 0.304 mol dm-3

0.4 x V/1000 = 0.304 x 250/1000

               V = 190 cm3

The volume of the 0.2 solution of HCl for dilution to 1.60 mol dm-3

2.0 x V/1000 = 1.5 x 50/1000

               V = 40 cm3

The volume of the 2 solution of HCl for dilution to 0.60 mol dm-3

2.0 x V/1000 = 1.5 x 50/1000

               V = 15 cm3

The volume of the 2 solution of HCl for dilution to 1.32 mol dm-3

2.0 x V/1000 = 1.4 x 50/1000

               V = 33 cm3

The volume of the 2 solution of HCl for dilution to 1.00 mol dm-3

2.0 x V/1000 = 1.0 x 50/1000

               V = 25 cm3

Preparation of the chemicals

For sodium thiosulphate solution:

1. Wash the burette with distilled water and then with thiosulphate solution. Fill the burette with the solution.
2. Using the burette, measure out exact amount of thiosulphate needed for the dilution.
3. Add the solution into the volumetric flask. With the help of the filter funnel fill the flask with distilled water until it reaches the graduation mark (250.0 cm3).
4. Replace the stopper and invert several times.

For hydrochloric acid:

1. Wash the burette first with distilled water and then the hydrochloric acid. Fill the burette with the acid.
2. Prepare another burette by similar method. Fill this burette with distilled water.
3. Measure the exact amount of distilled water that needed into a beaker.
4. Use the burettes to measure the acid that required into the same beaker.
5. Mix the two solutions thoroughly with glass rod.

Precautions

When diluting the hydrochloric acid, the acid should be added to the distilled water slowly. Hydrochloric acid reacts vigorously with water. If water were added to acid, a lot of heat would be given out at the junction due to the reaction of the two liquids. The sudden and violent expansion of stream would cause acid to burst out of the container.

Reminder

1. When measuring the solutions using burettes, read the mark on the burettes properly at the eye level.
2. Swirl the solution to ensure they mix well.
3. Although it is difficult to keep the temperature and pressure absolutely constant in the laboratory, changes can be minimized by doing experiments under approximately the same conditions.

Method:

1. Draw a cross on a piece of white paper. Put it under the beaker where reaction takes place.
2. Add 50 cm3 of 0.2M sodium thiosulphate solutions to the beaker and 5 cm3 of 2.00M of hydrochloric acid to the beaker. Start timing.
3. Measure the time from the mixing of the solutions to the point when the cross has form a dense cloud and become completely disappeared (precipitate of sulphur).
4. Repeat the experiment at the different concentrations as in table shown below.
5. Write down the result in the following table.

Calculations:

1. Calculate 1/t for each of the experiment.
2. Plot separate graphs of 1/t against the volume of the reactant for the two experiments. Since the total volume of solution is constant, 1/t and volume of reactant is directly proportional to the rate of reaction and concentration of reactant respectively.
3. Deduce the order of reaction with respect to each reactant from the shape of the graph.

• Zero order

The graph would be a horizontal line, which means the respective reactant doesn’t have any effect on the reaction rate.        

rate α [reactant]0        

• First order

A straight line passing through the origin will be obtained, i.e. the rate is directly proportional to the concentration of the reactant. The rate constant, k can be deduced from the slope of the line (rate/[reactant]).

rate α [reactant]1        

• Second order

The rate of reaction is directly proportional to the square of the reactant’s concentration (rate α [reactant]2), i.e. when a square amount of volume of reactant is used, the rate would be double. In this case, we will get a curve with an increasing gradient. However, if we plot a graph of 1/t against the square of volume used, we can get a straight-line graph, which go through the origin.

rate α [reactant]2        

1/t  /10-2 s-1

         2nd 

         1st 

        0

                Volume

4. Write out the rate equation of reaction by combining the reaction rates.

        rate α [S2O32-(aq)]n

rate α [H+(aq)]m

        where n and m are the order of reaction of S2O32-(aq) and H+(aq) respectively.

        The overall reaction rate is:

rate α [S2O32-(aq)]n [H+(aq)]m

        So, the rate equation is:

                rate = k [S2O32-(aq)]n [H+(aq)]m

where k is the rate constant and (n + m) is the overall order of the reaction.

References:

1. A-level Chemistry 2nd edition by E. N. Ramsden (P.388-389)
2. Chemistry by Chris Conoley and Phil Hills (P.580-590)
3. Advanced level Physical Chemistry by A. Holderness (p.254-268)
4. An introduction to reaction kinetics by David Abbott (P.10-19)
5. Advanced Chemistry Student’s Book Page 256
6. Chemistry – A Modern View Page 109