What does a potential energy diagram show, and what is activation energy?
Potential energy diagrams and activation energy: interpret a potential energy diagram, identify the activation energy and the energy change, and explain the effect of a catalyst.
A focused Virginia SOL Chemistry answer on energy diagrams under CH.6: reading a potential energy diagram, identifying the activation energy, the energy of the products versus reactants, and how a catalyst lowers the activation energy.
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What this topic is asking
Standard CH.6 asks you to read a potential energy diagram, to identify the activation energy and the energy change, and to explain how a catalyst affects the diagram. The diagram shows energy along the path from reactants to products, and the SOL tests whether you can locate the activation energy and the enthalpy change and predict the effect of a catalyst.
Reading a potential energy diagram
The two key features to read off are the activation energy (the climb from reactants to the peak) and the enthalpy change (the difference in level between reactants and products). The shape of the curve tells you both at a glance.
Activation energy and the activated complex
A high activation energy means few collisions have enough energy, so the reaction is slow; a low activation energy means more collisions succeed, so the reaction is faster. Activation energy is always a climb that must be overcome, even for an exothermic reaction, which is why some reactions need a spark to start.
Exothermic versus endothermic on the diagram
If the products are lower than the reactants, energy was released and the reaction is exothermic (negative enthalpy change). If the products are higher than the reactants, energy was absorbed and the reaction is endothermic (positive enthalpy change). The activation energy is a separate feature (the peak height) and does not by itself tell you whether the reaction is endothermic or exothermic; you compare the start and end levels for that.
The effect of a catalyst
On the diagram, adding a catalyst lowers the peak (the activation energy) while leaving the reactant and product levels untouched. This is why a catalyst speeds both the forward and reverse reactions without altering how much energy is released or absorbed overall.
Try this
Q1. On a potential energy diagram, what does the height from the reactants to the peak represent? [1 point]
- Cue. The activation energy, the minimum energy needed to start the reaction.
Q2. Does a catalyst change the enthalpy change of a reaction? [1 point]
- Cue. No; a catalyst lowers the activation energy only, leaving the reactant and product energies (and so the enthalpy change) unchanged.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SOL (multiple choice)1 marksOn a potential energy diagram, the activation energy is the energy difference between the reactants and (A) the products (B) the activated complex (peak) (C) the x-axis (D) the lowest pointShow worked answer →
The answer is (B) the activated complex (peak).
The activation energy is the minimum energy needed to start a reaction; on a potential energy diagram it is the height from the reactants up to the peak (the activated complex or transition state). The energy difference between reactants and products is the enthalpy change, a different quantity.
The trap is reading the activation energy as the reactant-to-product difference; that is the enthalpy change, while activation energy is the climb up to the peak.
SOL (tech-enhanced, fill in the blank)2 marksA potential energy diagram shows the products at a higher energy than the reactants. (a) Classify the reaction. (b) State what happens to the activation energy when a catalyst is added.Show worked answer →
A 2-point diagram-reading item.
(a) Endothermic (1 point): the products are higher in energy than the reactants, so energy was absorbed.
(b) Catalyst (1 point): adding a catalyst lowers the activation energy (the height of the peak), giving the reaction an easier pathway, but it does not change the energy of the reactants or products (so the enthalpy change is unchanged).
Markers reward reading the relative heights for the classification and stating that a catalyst lowers activation energy without changing the enthalpy change.
Related dot points
- Endothermic and exothermic reactions: distinguish endothermic and exothermic processes by the direction of energy flow and the sign of the enthalpy change.
A focused Virginia SOL Chemistry answer on reaction energy under CH.6: the difference between endothermic and exothermic reactions, the direction of energy flow, the sign of the enthalpy change, and how temperature change signals each type.
- Reaction rates and collision theory: explain reaction rate using collision theory, including effective collisions, orientation and the activation energy.
A focused Virginia SOL Chemistry answer on collision theory under CH.6: what reaction rate measures, why particles must collide with enough energy and the correct orientation, the role of activation energy, and the meaning of an effective collision.
- Factors affecting reaction rate: describe how concentration, temperature, surface area, a catalyst and the nature of the reactants change the rate of a reaction.
A focused Virginia SOL Chemistry answer on rate factors under CH.6: how concentration, temperature, surface area, catalysts and the nature of the reactants change reaction rate, each explained with collision theory.
- Chemical equilibrium and Le Chatelier's principle: describe dynamic equilibrium in a reversible reaction and predict the shift when concentration, temperature or pressure changes.
A focused Virginia SOL Chemistry answer on equilibrium under CH.6: reversible reactions and dynamic equilibrium, and using Le Chatelier's principle to predict how an equilibrium shifts when concentration, temperature or pressure changes.
- Phase changes and heating curves: name the phase changes and their energy changes, and interpret a heating or cooling curve including the plateaus.
A focused Virginia SOL Chemistry answer on phase changes under CH.4: the names and energy direction of melting, freezing, vaporization, condensation and sublimation, and how to read a heating curve, including why temperature stays constant during a phase change.
Sources & how we know this
- 2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
- Chemistry Curriculum Framework — Virginia Department of Education (2018)