Chemistry / Unit 9 Chem 1698 worked example, 60 to 90 seconds
Determining Activation Energy From Rate Constants at Two Temperatures
The idea
Determining activation energy from rate constants measured at two temperatures means substituting both temperature-rate constant pairs into the two-point form of the Arrhenius equation, then solving for the activation energy term.
In practice
HI decomposition has k = 3.52 × 10⁻⁷ L/mol/s at 555 K and k = 3.95 × 10⁻² L/mol/s at 781 K. Substituting both pairs into the two-point Arrhenius equation and solving isolates an activation energy near 1.8 × 10⁵ J/mol, or 180 kJ/mol, consistent with the anchor's own worked value for this decomposition.
Where people slip
Using Celsius instead of kelvin for either temperature in this equation, or swapping which rate constant and temperature form "point one," produces an activation energy far from the correct magnitude or of the wrong sign.
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