Rates - Activation Energy (OCR A Level Chemistry)

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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

1-4-6-ethanedioic-acid-and-potassium-manganatevii-experiment

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

1-4-6-arrhenius-plot-for-ocr-pag

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 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 = –begin mathsize 14px style fraction numerator negative 1.4 over denominator 0.00038 end fraction end stylex 8.31 = 30616 J mol-1
    • Ea = 30616 over 1000 = 30.616 kJ mol-1

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Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.