Activation Energy from Rate Constants at Different Temperatures
Arrhenius Plots
- Arrhenius plots for two reactions with different activation energies can be drawn on the same graph
Arrhenius plots for two reactions with different activation energies
- The reaction with a steeper gradient has the higher activation energy, Ea
- This indicates that the rate constant, and therefore rate, will change quicker with temperature changes
Calculating the Activation Energy
- The activation energy, Ea, can be calculated using rate constant values, k1 and k2, for two given temperatures, T1 and T2
- This requires the use of the following equation that is given in the data booklet:
Worked Example
Hydrogen iodide decomposes in the gas phase to form hydrogen and iodine
2HI (g) → H2 (g) + I2 (g)
At 283 oC, the rate constant is 3.52 x 10-7 mol dm-3 s-1At 508 oC, the rate constant is 3.95 x 10-2 mol dm-3 s-1Calculate the activation energy, Ea, for the reactionAnswer
- Convert the temperatures from oC to K:
- T1: 283 + 273 = 556 K
- T2: 508 + 273 = 781 K
- Write the appropriate Arrhenius equation from the data booklet
- Substitute the values
- Evaluate the equation to get the activation energy, Ea
