How can a graph show the energy change and the energy barrier of a reaction?
Interpret a potential energy diagram to identify activation energy, the energy change of reaction, and the effect of a catalyst, and classify the reaction as exothermic or endothermic (MA STE HS-PS1-4 and HS-PS1-5, energy and rate).
A standard-level answer on potential energy diagrams and activation energy for Massachusetts high school chemistry: reading the reactant and product energy levels, the activation energy barrier, the energy change of reaction, and how a catalyst lowers the barrier, grounded in HS-PS1-4 and HS-PS1-5.
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What this topic is asking
A potential energy diagram is the picture that ties together the energy ideas (HS-PS1-4) and the rate ideas (HS-PS1-5) of this module. A Massachusetts high school chemistry course expects you to read one: to identify the activation energy, the energy change of reaction, whether the reaction is exothermic or endothermic, and how a catalyst changes the diagram.
Reading the diagram
The horizontal axis is the progress of the reaction; the vertical axis is potential energy. Three features matter:
- The reactant level (left) and product level (right) are the energies of the starting and finishing substances.
- The peak between them is the transition state, the highest-energy point particles must pass through.
- The difference between product and reactant levels is the overall energy change.
Activation energy
The activation energy is the energy "hill" particles must climb before they can become products. It is why some reactions need a spark or heat to start, even if they release energy overall: the particles must first be given enough energy to reach the transition state. This is the same barrier that collision theory refers to in reaction rates and collision theory: only collisions with at least the activation energy succeed.
Exothermic versus endothermic on the diagram
The shape tells the story at a glance. An exothermic diagram falls from reactants down to lower products (the extra energy is released). An endothermic diagram rises from reactants up to higher products (energy was absorbed and stored). The overall energy change of reaction is product energy minus reactant energy, the bond-energy result from bond energy and reaction energy shown as a graph.
The effect of a catalyst
A catalyst provides an alternative path with a lower activation energy, so the peak on the diagram is lower. Crucially, the catalyst does not change the reactant or product energy levels, so the overall energy change of reaction stays the same; only the barrier between them shrinks. That lower barrier lets more collisions succeed, which is how a catalyst speeds the reaction without being consumed.
Try this
Q1. On a diagram, the products are higher than the reactants. Is the reaction exothermic or endothermic? [1]
- Cue. Endothermic (products higher means energy was absorbed).
Q2. A catalyst lowers the peak of a potential energy diagram. Does it change the energy change of reaction? [1]
- Cue. No; only the activation energy (the barrier) is lowered. The reactant and product levels are unchanged.
Exam-style practice questions
Practice questions written in the style of MA DESE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
MA Chemistry (style)3 marksOn a potential energy diagram, the products are lower in energy than the reactants. (a) Classify the reaction. (b) Define the activation energy. (c) State how the energy change of reaction is found from the diagram.Show worked answer →
A 3-point diagram-reading item.
(a) 1 point: exothermic (products lower than reactants means energy is released).
(b) 1 point: the activation energy is the minimum energy needed to start the reaction, the height from the reactants up to the peak.
(c) 1 point: the energy change of reaction is the difference in energy between the products and the reactants (products minus reactants). Markers reward the lower-products rule and the two distances on the diagram.
MA Chemistry (style)2 marksA catalyst is added to a reaction. (a) State its effect on the potential energy diagram. (b) State its effect on the energy change of reaction.Show worked answer →
A 2-point catalyst-on-diagram item.
(a) 1 point: the catalyst lowers the activation energy peak (a lower barrier), giving an alternative lower-energy path.
(b) 1 point: it does not change the energy change of reaction; the reactant and product energy levels stay the same, so only the barrier is lowered. Markers reward lowering only the peak, not the start or end levels.
Related dot points
- Explain that breaking bonds absorbs energy and forming bonds releases it, and use bond energies to decide whether a reaction is exothermic or endothermic (MA STE HS-PS1-4, energy from changes in total bond energy).
A standard-level answer on bond energy and reaction energy for Massachusetts high school chemistry: why breaking bonds absorbs energy and forming bonds releases it, using bond energies to find the net energy change, and deciding whether a reaction is exothermic or endothermic, grounded in HS-PS1-4.
- Use collision theory to explain how temperature, concentration, surface area, and catalysts affect the rate of a reaction (MA STE HS-PS1-5, effect of temperature and concentration on reaction rate).
A standard-level answer on reaction rates and collision theory for Massachusetts high school chemistry: how collision theory explains rate, and the effects of temperature, concentration, surface area, and catalysts, grounded in HS-PS1-5.
- Classify reactions as exothermic or endothermic, describe energy transfer as heat, and apply the conservation of energy to chemical and physical changes (MA STE HS-PS3-4(MA), thermal energy transfer).
A standard-level answer on energy changes in chemical reactions for Massachusetts high school chemistry: exothermic and endothermic reactions, energy transferred as heat, the conservation of energy, and the link to temperature change, grounded in HS-PS3-4(MA).
- Describe dynamic equilibrium in a reversible reaction and use Le Chatelier's principle to predict the effect of changing concentration, temperature, or pressure (MA STE HS-PS1-6(MA), shifting equilibrium to increase product).
A standard-level answer on chemical equilibrium and Le Chatelier's principle for Massachusetts high school chemistry: dynamic equilibrium in a reversible reaction and predicting the shift when concentration, temperature, or pressure changes, grounded in HS-PS1-6(MA).
- Name the phase changes, interpret a heating curve, and explain why temperature stays constant during a change of state (MA STE supporting content, energy and changes of state).
A standard-level answer on phase changes and heating curves for Massachusetts high school chemistry: naming the six phase changes, reading the flat and sloping sections of a heating curve, and explaining why temperature is constant during melting and boiling, grounded in the framework's energy and matter content.
Sources & how we know this
- Massachusetts Science and Technology/Engineering Curriculum Framework (2016) — Massachusetts Department of Elementary and Secondary Education (2016)
- Science and Technology/Engineering (STE) Test Design and Development — Massachusetts Department of Elementary and Secondary Education (2024)