Chemical changes · GCSE Chemistry
Reaction profiles
GCSE Chemistry revision on reaction profiles: activation energy, ΔH, exothermic and endothermic sketches, and how a catalyst lowers the hump but not the enthalpy change.
Reactants, hump, products. If products sit lower, it is exothermic and ΔH is negative. The hump height from the reactants is activation energy. A catalyst cuts the hump, not the ΔH arrow.
The important bits
What you need to know
- 1
A reaction profile (energy level diagram) plots energy against the progress of the reaction. Start with reactants, end with products, with a peak in between.
- 2
Activation energy, Ea, is the rise from the reactant energy to the top of the peak. It is the minimum energy needed for successful collisions.
- 3
For an exothermic reaction the products are lower in energy than the reactants. ΔH is the drop from reactants to products and is negative.
- 4
For an endothermic reaction the products are higher in energy than the reactants. ΔH is the rise from reactants to products and is positive. Ea is still measured from reactants up to the peak, so Ea is larger than ΔH.
- 5
A catalyst provides an alternative pathway with a lower Ea, drawn as a lower peak. Reactant and product energies — and therefore ΔH — do not change.
- 6
Label both arrows: Ea upwards from reactants to peak, ΔH from reactants to products (down if exo, up if endo). Unlabelled diagrams lose marks.
- 7
You can have a large Ea even if ΔH is negative: combustion of coal needs a spark, then it keeps going because the reaction is exothermic overall.
- 8
Do not draw the catalyst as a downhill slide with no peak. There is still an activation energy, just a smaller one.
Quotations worth analysing
Short evidence. Real method.
“Ea: reactants → peak. ΔH: reactants → products.”
Two different arrows. Students put ΔH on the hump or Ea between products and reactants. Separate them and put arrowheads in the correct direction.
“A catalyst lowers Ea but does not change ΔH.”
Same start, same finish, lower peak. That is also why equilibrium yield does not shift: the energy difference between reactants and products is unchanged.
“Exothermic: products below reactants. Endothermic: products above reactants.”
Read the diagram before you read the question text. The relative heights tell you the sign of ΔH even if the word exothermic is missing.
Go deeper
The hump is why fuels need a match
Methane and oxygen can sit mixed at room temperature without reacting because almost no collisions reach Ea. A spark supplies enough energy for some molecules to react. The exothermic ΔH then heats the surroundings and neighbouring molecules, so more collisions succeed and the flame continues. An endothermic profile cannot “keep itself going” in the same way: you must keep putting energy in, as in a lime kiln. That is why the relative heights of reactants and products matter in real life, not only as a sketch. If Ea is tiny, the reaction may be dangerously fast at room temperature (some Group 1 reactions).
Go deeper
Catalysts redraw one line, not the whole graph
Draw the uncatalysed profile first in pencil, then a second peak underneath the first, starting and finishing on the same horizontal levels. If you drop the product line as well, you have invented a different reaction. Industrial diagrams sometimes show two small humps for a two-step catalytic mechanism; at GCSE one lower hump is enough. Enzymes in Biology graphs are the same idea. When a question gives numbers, Ea might be 80 kJ/mol and ΔH −20 kJ/mol: the peak is 80 above the reactants and the products are 20 below. The reverse reaction’s Ea would then be 100 kJ/mol (back up from products to the same peak).
Go deeper
Reverse reactions flip the profile
If the forward reaction is exothermic with Ea(forward) and ΔH negative, the reverse is endothermic with Ea(reverse) = Ea(forward) + |ΔH|. Catalysts lower both humps. That is why a catalyst speeds forward and reverse and does not change the equilibrium constant. Higher-tier equilibrium questions sometimes show this. Even on Foundation, you should be able to look at a profile and say which direction is exothermic. If products are lower, going forwards releases energy; going backwards requires energy in. Label both arrows if numbers are given so the reverse Ea is not a guess.
See the idea in action
A reaction has reactant energy 100 kJ/mol, peak 180 kJ/mol, product energy 40 kJ/mol. Ea = 180 − 100 = 80 kJ/mol. ΔH = 40 − 100 = −60 kJ/mol, so it is exothermic: products are 60 kJ/mol below reactants. A catalyst might lower the peak to 150 kJ/mol: new Ea = 50 kJ/mol, ΔH still −60 kJ/mol. The reverse Ea without a catalyst would be 180 − 40 = 140 kJ/mol.
Exam technique
Turn knowledge into marks
Sketch reactants, peak, products. Label Ea and ΔH with arrows in the right direction. Products lower = exothermic. For a catalyst, only lower the peak. If numbers are given, subtract to show the working.
Common mistakes
Do not give these marks away
- 01
Labelling ΔH as the height of the hump, or drawing Ea from products to the peak on an exothermic profile as if it were ΔH.
- 02
Showing a catalyst changing the energy of the products.
- 03
Drawing an endothermic profile with products below reactants.
How does a catalyst appear on a reaction profile?
AIt lowers the energy of the products
BIt lowers the activation-energy peak; reactant and product energies stay the same
CIt removes the activation energy completely
DIt raises ΔH so the reaction becomes more exothermic
Show the answer
It lowers the activation-energy peak; reactant and product energies stay the same. The alternative pathway has a lower Ea. ΔH is the difference between reactants and products, so it does not change.
Quick questions
If this is the bit you searched
How do you draw an exothermic reaction profile GCSE?
Reactants on a higher line, a peak for activation energy, products on a lower line. Label Ea from reactants to the peak and ΔH downwards from reactants to products.
What is the difference between activation energy and ΔH?
Activation energy is the energy needed to start successful collisions (up to the peak). ΔH is the overall energy difference between reactants and products.
How does a catalyst change a reaction profile?
It lowers the peak (lower Ea). The reactant and product energy levels, and so ΔH, stay the same.
Can an exothermic reaction still have a high activation energy?
Yes. Combustion is exothermic overall but still needs a spark because Ea is high enough that room-temperature collisions rarely succeed.