12.3

Question 1

12.3 Investigating the reaction between potassium manganate(VII) and ethanedioic acid

Answer 1

In this activity, you will carry out some research on the theory behind the reaction between potassium manganate(VII) and ethanedioic acid. You will investigate the effect of carrying out this reaction at acidic pH.

Question 2

Chemicals

Answer 2

You will be provided with:
• aqueous potassium manganate(VII), KMnO4(aq), of concentration 0.0200 mol dm–3
• aqueous ethanedioic acid, H2C2O4(aq), of concentration 0.500 mol dm–3
• aqueous sulfuric acid, H2SO4(aq), of concentration 1.00 mol dm–3
• distilled or deionised water.

Question 3

Part 1 Neutral conditions

Answer 3

Under neutral conditions, potassium manganate(VII), KMnO4(aq), reacts with ethanedioc acid, H2C2O4(aq), to produce a brown colour.

The rate equation for this reaction is:

rate = k[KMnO4(aq)][H2C2O4(aq)]

The rate of this reaction can be followed by observing the formation of the brown colour.
From the start of each experiment, you will time how long it takes for the brown colour to appear. The initial rate of the reaction can be determined from this time.

1. Combine the H2C2O4 solution and water in one container.
2. Add the KMnO4 solution to a second container.
3. Combine the reactants, and record the time it takes for the solution to turn yellow/brown.
4. Repeat each experiment as required.
5. Calculate 1/t for each experiment.
6. As rate  1/t, you should be able to confirm the orders with respect to KMnO4(aq) and H2C2O4(aq) by inspection. Hence you should be able to confirm the rate equation.

Question 4

Part 2 Acidic conditions

Answer 4

In acid conditions, the reaction taking place is different and the colour change is different.

1. Repeat the process in part 1 under acidic conditions using sulfuric acid, H2SO4(aq), as the source of H+(aq) ions. Hydrochloric and nitric acids are unsuitable for use.
2. You will need to select appropriate mixtures of the three solutions and carry out enough experiments to be able to see the effect of the concentrations of KMnO4(aq), H2C2O4(aq) and H+(aq) on the rate of reaction.

Question 5

Background Research

Answer 5

1. Find out the different reactions under neutral and acidic conditions and write the equations.
2. What are the changes in oxidation numbers in each reaction.
3. The reaction under acidic conditions is an example of ‘autocatalysis’. Find out what autocatalysis is and find other examples of autocatalysis.

Question 6

KMnO4(aq)

Answer 6

Currently no hazard classification at this concentration

Question 7

H2C2O4(aq)

Answer 7

Aqueous ethanedioic acid (oxalic acid), H2C2O4(aq), of concentration 0.500 mol dm–3.

This solution is made up by dissolving 63.0 g of H2C2O4•2H2O in distilled or deionised water and making up to 1.00 dm3 of solution.

Each learners will require about 100 cm3. WARNING
Causes serious eye irritation

Question 8

H2SO4(aq

Answer 8

) Aqueous sulfuric acid, H2SO4(aq), of concentration 1.00 mol dm–3.

Each learner or group will require about 100 cm3. Causes skin irritation
Causes serious eye irritation

Question 9

Equipment

Answer 9

Learners will be selecting appropriate apparatus for the experiment. It is likely that they will require:
• safety spectacles
• burette (or volumetric pipettes)
• clamp stand, with boss and clamp (for supporting the burette)
• measuring cylinders (various sizes up to 50 cm3)
• beakers
• conical flasks
• test tubes
• glass rod
• dropping pipettes
• stop clock or stop watch