One temperature
If k, A and T are given, use the exponential equation directly. Put Ea into joules when R is in J K−1 mol−1.
Arrhenius equation & plots
Calculate with the Arrhenius equation, turn experimental data into a straight-line plot, and recognise the graph and unit mistakes that cost easy marks.
k = Ae−Ea/RT · ln k = −Ea/R(1/T) + ln A
Choose the form that matches the data
Several temperatures
Plot ln k against 1/T. The gradient is −Ea/R, so a steeper negative line means a larger activation energy.
OCR A intercept
The y-intercept is ln A. If the plotted x-axis does not reach 1/T = 0, the visible left edge is not the intercept.
Four traps worth recognising on sight
J and kJ
R is in joules. Convert Ea to J mol−1 before it goes into the exponential, then convert the final Ea back to kJ mol−1 if required.
Kelvin
T means absolute temperature. A temperature quoted in °C must be converted before substitution.
Natural logarithms
The straight-line equation uses ln k. Using log10 changes the gradient and gives the wrong Ea.
What temperature changes
Higher temperature increases k. It does not lower Ea; a catalyst changes the activation-energy pathway.