๐ In-Depth Theory
What is a Reaction Profile?
A REACTION PROFILE (also called an energy profile diagram) shows how the ENERGY of the system changes as a reaction progresses.
The x-axis shows PROGRESS OF REACTION (from reactants to products).
The y-axis shows ENERGY (in kJ or kJ/mol).
Key features on every reaction profile:
REACTANTS โ the starting energy level (left).
PRODUCTS โ the final energy level (right).
ACTIVATION ENERGY (Ea) โ the energy barrier that must be overcome for the reaction to start โ shown as the PEAK of the curve above the reactant level.
OVERALL ENERGY CHANGE (ฮH) โ the difference in energy between reactants and products.
Exothermic and Endothermic Profiles
EXOTHERMIC REACTION PROFILE:
Products are at a LOWER energy level than reactants.
The curve rises to a peak (activation energy) then falls below the reactant level.
ฮH is NEGATIVE (products lower than reactants โ energy released).
The difference between the peak and the REACTANT energy level = activation energy.
ENDOTHERMIC REACTION PROFILE:
Products are at a HIGHER energy level than reactants.
The curve rises to a peak (activation energy) then falls โ but levels off ABOVE the reactant starting point.
ฮH is POSITIVE (products higher than reactants โ energy absorbed).
The difference between the peak and the REACTANT energy level = activation energy.
ACTIVATION ENERGY:
All reactions need activation energy to get started โ even exothermic ones.
Activation energy = minimum energy needed to break existing bonds and start the reaction.
Low activation energy โ fast reaction.
High activation energy โ slow reaction (even if very exothermic once started).
Effect of Catalysts on Reaction Profiles
A CATALYST provides an ALTERNATIVE REACTION PATHWAY with a LOWER ACTIVATION ENERGY.
On a reaction profile, adding a catalyst:
LOWERS the peak of the curve (lower activation energy hump).
Does NOT change the energy levels of reactants or products.
Does NOT change ฮH โ the overall energy change is the same.
This means:
More molecules have enough energy to react โ faster reaction rate.
The same overall energy is released or absorbed per mole of reaction.
Why catalysts are important:
Without catalysts, many industrially and biologically important reactions would be too slow to be useful.
In biology, ENZYMES act as biological catalysts โ lowering activation energy of specific reactions.
In industry, transition metals (iron, platinum, nickel) are common catalysts.
A lower activation energy hump on a reaction profile diagram represents a CATALYSED reaction.
โ ๏ธ Common Mistake
The activation energy is measured from the REACTANT level to the PEAK โ NOT from the reactant level to the product level. The overall energy change (ฮH) is from reactants to PRODUCTS. These are two different measurements on the same diagram. Students often confuse them.