Investigating the reaction between manganate(VII) and ethanedioate ions

Class practical

Potassium manganate(VII), KMnO₄, is a deeply coloured purple crystalline solid. It is a powerful oxidising agent. In acidic solution, it undergoes a redox reaction with ethanedioate ions, C₂O₄^2-. The MnO₄^- ions are reduced to Mn²⁺ and the C₂O₄^2- ions are oxidised to CO₂.

2MnO₄^-(aq) + 16H⁺(aq) + 5C₂O₄^2-(aq) → 2Mn²⁺(aq) + 8H₂O(l) + 10CO₂(g)

In this experiment students use a continuous monitoring method to investigate how the rate of the reaction changes with the concentration of MnO₄^- ions. They then go on to investigate the effect of the addition of a small amount of Mn²⁺ on the reaction.

Lesson organisation

This practical activity takes around 1 hour.

Apparatus	Chemicals
burette (50 cm3) 50 cm3 volumetric flask / measuring cylinder (×5) colorimeter with cuvettes test tubes with rubber bungs 10 cm3 measuring cylinder pipettes	potassium manganate(VII) solution, 0.002 mol dm-3 (no hazard) acidified ethanedioic acid solution (CORROSIVE – causes severe skin burns and eye damage) (Note 1) manganese(II) sulfate solution, 0.02 mol dm-3 (no hazard) distilled water Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Students should wear eye protection when carrying out this experiment. 

1 This solution must be 0.125 mol dm^-3 with respect to ethanedioic acid and 1.5 mol dm^-3 with respect to sulfuric acid. 100 cm³ of this mixture can be made up by dissolving 1.125 g of ethanedioic acid (MODERATE HAZARD – harmful if swallowed, harmful in contact with the skin) in 25 cm³ of distilled water and making up to 100 cm³ with 2 mol dm^-3 sulfuric acid (CORROSIVE – causes severe skin burns and eye damage).

Procedure

Creating a calibration curve

a Prepare the following solutions of KMnO₄ containing varying concentrations of MnO₄^- ions.

Using a burette, carefully transfer the required amount of the KMnO₄ solution (0.002 mol dm^-3) into a 50 cm³ volumetric flask or measuring cylinder and make up to 50 cm³ with distilled water.

Solution	Volume of 0.002 mol dm-3 KMnO4 solution added / cm3	Concentration of MnO4- ions in final solution / mol dm-3
1	10.0	4.0 × 10-4
2	7.5	3.0 × 10-4
3	5.0	2.0 × 10-4
4	2.5	1.0 × 10-4
5	1.0	0.40 × 10-4

b Place a cuvette containing distilled water into a colorimeter and using a suitable filter adjust to 0% absorbance.

c Place each of the solutions 1 to 5 into the colorimeter in turn and read off the corresponding absorbance.

d Plot a graph of absorbance (y-axis) against concentration (x-axis) - the calibration curve.

The investigation

e Place 2.0 cm³ of a 0.002 mol dm^-3 solution of potassium manganate(VII) in a test tube. Fit the tube with a rubber bung.

f Zero a stop clock ready for use.

g Using a small measuring cylinder, add 8.0 cm³ of the acidified ethanedioic acid solution to the test tube containing the potassium manganate(VII) solution.

Quickly stopper the tube, invert the tube to mix the contents and start the stop clock.

h Using a teat pipette, quickly transfer some of the mixture from the test tube to a cuvette and place the cuvette in the colorimeter. Measure the absorbance of the mixture in the cuvette every 20 seconds until the absorbance drops to 0.01.

i Using the calibration curve, convert the absorbance values obtained into concentrations of MnO₄^-.

j Plot a graph of time against concentration of MnO₄^- ions.

k Repeat steps e to j, but this time add 1 drop of the 0.02 mol dm^-3 solution of manganese(II) sulfate to the acidified ethanedioic acid solution before mixing.

l Compare the two graphs. What effect does the addition of 1 drop of Mn²⁺(aq) have on the reaction?

Teaching notes

The reaction between manganate(VII) ions and ethanedioate ions at room temperature is fairly slow initially but quickens as the reaction proceeds. Manganese(II) ions, Mn²⁺, formed as the reaction proceeds act as an autocatalyst. Once a small amount of Mn²⁺ ions have formed, they can react with MnO₄^- ions to form Mn³⁺ ions as an intermediate species. These then react with the C₂O₄^2- ions to reform Mn²⁺.

4Mn²⁺(aq) + MnO₄^-(aq) + 8H⁺(aq) → 5Mn³⁺(aq) + 4H₂O(l)

2Mn³⁺(aq) + C₂O₄^2-(aq) → 2CO₂(g) + 2Mn²⁺(aq)

In the reaction carried out in the absence of manganese(II) ions, the reaction initially proceeds slowly. Then as a few Mn²⁺ ions are formed, the rate increases and proceeds following first-order kinetics with respect to the manganate(VII) ions. Adding Mn²⁺ ions at the beginning of the reaction results in a smooth decrease in concentration with time from the outset.

As an extension task the students can go on to use their plots of concentration of MnO₄^- ions against time to determine the rate of the reaction for different concentrations of MnO₄^- ions. A plot of log(rate) against log[MnO₄^-] should then be a straight line with a gradient of 1 indicating that the reaction is first order with respect to manganate(VII) ions.

Health & Safety checked, September 2016

Page last updated October 2016