How does a potential energy diagram show activation energy, the heat of reaction and the effect of a catalyst?
Potential energy diagrams: interpret potential energy diagrams to identify activation energy, the activated complex and the heat of reaction, and show how a catalyst changes the diagram.
A focused Regents Chemistry answer on potential energy diagrams: reading the activation energy, the activated complex and the heat of reaction (delta-H), distinguishing exothermic from endothermic reactions, and how a catalyst lowers the activation energy without changing delta-H.
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What this topic is asking
The Core Curriculum asks you to interpret potential energy diagrams: to identify the activation energy, the activated complex and the heat of reaction (), to tell exothermic from endothermic reactions, and to show how a catalyst changes the diagram. This is a regular Part A and Part B-2 graph-reading task that connects collision theory to thermochemistry.
Reading the diagram
The activated complex is the temporary, high-energy, unstable species at the top of the barrier, where bonds are partly broken and partly formed. It exists only momentarily at the peak. The activation energy is the minimum energy a collision must supply to reach this peak, which is exactly the energy threshold from collision theory.
Heat of reaction, exothermic and endothermic
To classify a reaction from the diagram, compare the heights of the products and reactants. Products lower than reactants means energy was released to the surroundings (exothermic); products higher means energy was absorbed (endothermic). Table I in the Reference Tables lists heats of reaction for some common reactions, and the sign tells you the type.
The effect of a catalyst
This is the diagram form of the catalyst idea from the previous page: only the height of the barrier changes, not the start or end points. A common Part B-2 task is to draw a dashed lower curve to show the catalyzed pathway while keeping the reactant and product levels the same.
Try this
Q1. State what the peak of a potential energy diagram represents. [1 point]
- Cue. The activated complex, the unstable high-energy arrangement at the top of the energy barrier.
Q2. A reaction has products at higher potential energy than the reactants. State whether it is exothermic or endothermic. [1 point]
- Cue. Endothermic ( positive); energy is absorbed.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Part B-2 style)3 marksA potential energy diagram is drawn for a forward reaction. (a) Describe how to determine the heat of reaction (delta-H) from the diagram. (b) State whether a reaction is exothermic or endothermic if the products have lower potential energy than the reactants. (c) Describe how the diagram changes when a catalyst is added.Show worked answer →
A 3-point constructed-response item reading a potential energy diagram.
(a) Heat of reaction (1 point): subtract the potential energy of the reactants from the potential energy of the products ().
(b) Classification (1 point): if the products are lower in energy than the reactants, energy is released, so the reaction is exothermic ( is negative).
(c) Catalyst (1 point): a catalyst lowers the activation energy (the peak is lower), but the energies of the reactants and products, and therefore , are unchanged.
Markers reward the products-minus-reactants method for , identifying the exothermic case, and noting that a catalyst lowers only the activation energy.
Regents (Part A style)1 marksOn a potential energy diagram, the activation energy of the forward reaction is the difference in energy between the reactants and the (1) products (2) peak of the curve (activated complex) (3) lowest point (4) catalystShow worked answer →
A 1-point Part A item on reading the diagram. The answer is (2) peak of the curve (activated complex).
The activation energy of the forward reaction is the energy needed to go from the reactants up to the top of the energy barrier, which is the activated complex (the highest point on the curve). The heat of reaction, by contrast, is the difference between products and reactants, measured at the start and end, not at the peak.
Markers reward identifying activation energy as the climb from reactants to the activated complex.
Related dot points
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A focused Regents Chemistry answer on reaction rates and collision theory: why effective collisions need enough energy and the right orientation, and how concentration, temperature, surface area, the nature of the reactants and a catalyst change the rate.
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- Heat and calorimetry: calculate heat changes using q = mC(delta-T) for temperature changes and q = mH for phase changes, with constants from Table B and formulas from Table T.
A focused Regents Chemistry answer on heat and calorimetry: the q = mC(delta-T) equation for warming or cooling, q = mH for melting and boiling, the water constants on Table B, and the difference between exothermic and endothermic changes.
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Sources & how we know this
- Physical Setting/Chemistry Core Curriculum — New York State Education Department (2002)
- Reference Tables for Physical Setting/Chemistry, 2011 Edition — New York State Education Department (2011)