SME
Grade 10 Science · Problem type
What a catalyst changes
Use this for how much a catalyst lowers the activation energy, forward and back.
Diagram
The products are lower: the reaction gives off 40 kJ.
Exothermic.
A catalyst lowers the hump, not the levels: ΔH stays the same.
Drop in activation energy: 40 kJ
Reverse activation energy with the catalyst: 90 kJ
This is an example. Type your own values to start a new problem.
Hᵣ
Energy of the reactants
example · kJ
Hₚ
Energy of the products
example · kJ
Eₐ
Activation energy
example · kJ
Eₐ,cat
Activation energy with the catalyst
example · kJ
ΔH
Enthalpy change
calculated · kJ
d
Drop in activation energy
calculated · kJ
Eₐ′
Reverse activation energy
calculated · kJ
Eₐ,cat′
Reverse activation energy with the catalyst
calculated · kJ
Formulas
ΔH = Hₚ − Hᵣ
−40 = 60 − 100
d
=
Eₐ
−
Eₐ,cat
40 = 90 − 50
Eₐ′ = Eₐ − ΔH
130 = 90 − (−40)
Eₐ,cat′ = Eₐ,cat − ΔH
90 = 50 − (−40)
Assumptions
•
Energies are per mole of reaction as written, in kilojoules.
•
A catalyst gives the reaction a lower path; it is not used up.
•
It lowers both directions’ barriers alike and leaves ΔH, and K, unchanged.
Step-by-step
WE KNOW
Hᵣ = 100 kJ, Hₚ = 60 kJ, Eₐ = 90 kJ, Eₐ,cat = 50 kJ
FIND
enthalpy change (ΔH), drop in activation energy (d), reverse activation energy (Eₐ′), reverse activation energy with the catalyst (Eₐ,cat′)
Step 1 · Find enthalpy change (ΔH)
Use ΔH = Hₚ − Hᵣ
ΔH is where the reaction ends minus where it starts: negative when energy is given off.
ΔH = 60 − 100
ΔH = −40 kJ
Step 2 · Find drop in activation energy (d)
Use d = Eₐ − Eₐ,cat
The catalyst’s path is lower by the difference.
d
=
90
−
50
d
=
40
kJ
Step 3 · Find reverse activation energy (Eₐ′)
Use Eₐ′ = Eₐ − ΔH
Going back, the climb starts at the products: the forward climb minus ΔH.
Eₐ′ = 90 − (−40)
Eₐ′ = 130 kJ
Step 4 · Find reverse activation energy with the catalyst (Eₐ,cat′)
Use Eₐ,cat′ = Eₐ,cat − ΔH
Going back, the climb starts at the products: the forward climb minus ΔH.
Eₐ,cat′ = 50 − (−40)
Eₐ,cat′ = 90 kJ
Check
Put the numbers back in. Both sides should match.
−40 = 60 − 100 ✓
40 = 90 − 50 ✓
130 = 90 − (−40) ✓
90 = 50 − (−40) ✓
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