Reaction profiles for exothermic and endothermic reactions
Energy changes • Exothermic and endothermic reactions
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Key concepts
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Basic structure of a reaction profile
A reaction profile plots energy (vertical axis) against reaction progress (horizontal axis). The starting point shows the energy of the reactants. The finishing point shows the energy of the products. A curve between the two points rises to a peak that represents the highest-energy arrangement along the reaction pathway.
Activation energy (Ea)
Activation energy is the energy difference between the reactants and the highest point (peak) on the profile. Activation energy represents the minimum energy required for reactant particles to form products. Larger activation energy causes a slower reaction rate because fewer particles have sufficient energy to reach the peak.
Overall energy change (ΔH) and its sign
The overall energy change equals the energy of the products minus the energy of the reactants and is called the enthalpy change (ΔH). A negative ΔH indicates that products lie lower in energy than reactants (energy released). A positive ΔH indicates that products lie higher in energy than reactants (energy absorbed). The magnitude of ΔH equals the vertical distance between reactants and products.
Identifying exothermic and endothermic reactions
Exothermic reactions show products at lower energy than reactants, producing a negative ΔH and releasing energy. Endothermic reactions show products at higher energy than reactants, producing a positive ΔH and absorbing energy. Identification follows directly from comparing the relative vertical positions of reactants and products on the diagram.
Drawing conventions and limitations
Diagrams use a single energy axis and a schematic reaction progress axis; neither axis shows exact units unless specified. Labels include reactants, products, activation energy (Ea) and enthalpy change (ΔH). Reaction profiles on this level remain qualitative: relative heights convey direction and sign of ΔH but not precise energy values. Avoid assuming axis scales or exact times from the shape of the curve.
Key notes
Important points to keep in mind