Example 1
A profile has reactants 20, peak 80 and products −10 kJ mol−1. Find forward Ea, reverse Ea and ΔH.
Forward Ea=80−20=60. Reverse Ea=80−(−10)=90. ΔH=−10−20=−30 kJ mol−1.
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Year 12 · SACE Stage 2
Energy profiles and equilibrium curves
Read numerical energy barriers and time-dependent equilibrium data.
Method and worked examples
On an energy profile, the forward activation energy is peak minus reactant energy. Reverse activation energy is peak minus product energy. Reaction enthalpy is product minus reactant energy, so exothermic products lie lower. A catalysed profile has a lower peak but the same initial and final energies for the same reaction.
On concentration–time data, equilibrium is indicated by an unchanged plateau under fixed conditions; it does not require equal concentrations. Read the earliest tabulated time at which values stop changing if that is the requested resolution. An instantaneous rise in only one species at unchanged volume can indicate addition of that species. Subsequent gradual changes show the response toward a new equilibrium.
Example 1
Example 2
For A ⇌ B,[A] is 0.80,0.55,0.40,0.40 at 0,20,40,60s. When does the first observed plateau begin?
The first tabulated value followed by no further change is at 40s. This is the plateau onset to the resolution of the supplied data, not an assertion of equal forward and reverse rates at every earlier time.
Watch out: That endpoint difference is ΔH. Activation energy is measured from the starting state to the pathway peak.