PAG 10.3: Rates – Activation Energy
The Ethanedioic Acid and Potassium Manganate(VII) Reaction
- A less common experiment to determine the activation energy of a reaction is the reaction between potassium manganate(VII) solution and ethanedioic acid
2MnO4– (aq) + 5H2C2O4 (aq) + 6H+ (aq) → 2Mn2+ (aq) + 8H2O (l) + 10CO2 (g)
- In this reaction, potassium manganate(VII) with ethanedioic acid in the presence of sulfuric acid
- The key point in this experiment is the colour change of the manganese which changes from purple to brown
Method
- Using a different / clean measuring cylinder for each chemical, transfer 10.0 cm3 of each of the following into a separate boiling tube
- 0.1 mol dm-3 potassium manganate(VII) solution, KMnO4 (aq)
- 2.0 mol dm-3 sulfuric acid, H2SO4 (aq)
- 0.25 mol dm-3 ethanedioic acid, H2C2O4 (aq)
- Warm / heat all three boiling tubes in a water bath
- Place a test tube in a test tube rack
- Measure and record the temperature of all three solutions
- Add 0.5 cm3 of the potassium manganate(VII) solution to the test tube
- Add 0.5 cm3 of the sulfuric acid to the test tube
- Then add 1.0 cm3 of the ethanedioic acid to the test tube
- Immediately start the stopwatch and swirl the mixture
- Stop the stopwatch when the purple colour changes to pale brown and record the time
- Repeat the experiment for a minimum of four more / different temperatures
The Ethanedioic Acid and Potassium Manganate(VII) Reaction
Specimen Results
Temperature, T (K) | 1 / T | Time, t (s) | 1 / t | ln (1 / t) |
297.0 | 0.003367 | 63 | 0.015873 | -4.1431 |
310.3 | 0.003223 | 34 | 0.029412 | -3.5264 |
316.9 | 0.003156 | 26 | 0.038462 | -3.2581 |
323.6 | 0.003090 | 22 | 0.045455 | -3.0910 |
335.3 | 0.002982 | 16 | 0.062500 | -2.7726 |
- The temperature of the reaction is converted to 1 / T by calculating the reciprocal value
- The rate is found by calculating the reciprocal of time, t
- This is then converted into the natural logarithm function to give ln(1 / t) or ln(rate)
- A graph of ln(rate) versus 1 / T is then plotted:
Analysis
An ln(rate) versus 1 / T graph for the Ethanedioic Acid and Potassium Manganate(VII) Reaction
- From this graph, we can see that the line of best fit is a straight line
- The equation for a straight line is y = mx + c
- The Arrhenius equation can be rearranged to ln k = ln A - Ea/RT
- Therefore, the gradient of the line is -Ea / R
- This means that we can use the gradient from the graph to calculate the activation energy for this reaction
- Gradient = can be rearranged to Ea = –gradient x R
- Ea = –x 8.31 = 30616 J mol-1
- Ea = = 30.616 kJ mol-1