A reaction diagram goes downhill by 120\ kJ mol^-1 from reactants to products. State H and classify the reaction. [2 points]

United StatesChemistrySyllabus dot point

How does an energy diagram represent the relative potential energies of reactants and products and the enthalpy of a reaction?

Topic 6.2 Energy Diagrams: draw and interpret an energy diagram showing the relative enthalpies of reactants and products and the enthalpy change of the reaction.

A focused answer to AP Chemistry Topic 6.2, covering how an energy diagram represents the relative potential energies of reactants and products, the sign of the enthalpy change for endothermic and exothermic reactions, and how to read the diagram, with full worked examples.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-04

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this topic is asking
What the diagram shows
Reading the sign of the enthalpy change
Reversing a reaction
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What this topic is asking

The College Board (Topic 6.2) wants you to draw and interpret an energy diagram that shows the relative enthalpies of reactants and products and the enthalpy change of the reaction. Where the reaction energy profile of Unit 5 emphasized the activation barrier, here the focus is on the start and end levels and the difference between them.

What the diagram shows

The vertical axis is energy (enthalpy); the horizontal axis is the progress of the reaction. The two flat levels are the reactants (left) and products (right). For this topic the activation barrier may or may not be drawn; the essential information is the relative heights of the two levels.

Reading the sign of the enthalpy change

So a diagram that goes downhill from reactants to products is exothermic, and one that goes uphill is endothermic. The greater the drop or rise, the larger the magnitude of the enthalpy change. This visual reading is the quickest way to classify a reaction and to compare the enthalpy changes of different reactions.

Reversing a reaction

Because enthalpy is a state function (its value depends only on the initial and final states, not the path), reversing a reaction simply swaps the reactant and product levels. The reverse reaction therefore has $\Delta H$ of the same magnitude but the opposite sign: if the forward reaction is exothermic with $\Delta H = -100\ \text{kJ mol}^{-1}$ , the reverse is endothermic with $\Delta H = +100\ \text{kJ mol}^{-1}$ . This state-function property underlies Hess's law (Topic 6.9).

Reading and reversing a diagram

A reaction has reactants at $300\ \text{kJ mol}^{-1}$ and products at $380\ \text{kJ mol}^{-1}$ .

step 1 Calculate the enthalpy change

$\Delta H = E_\text{products} - E_\text{reactants} = 380 - 300 = +80\ \text{kJ mol}^{-1}$ .

step 2 Classify the reaction

$\Delta H > 0$ and products lie above reactants, so the reaction is endothermic.

step 3 Consider the reverse reaction

Swapping the levels gives reactants at $380$ and products at $300\ \text{kJ mol}^{-1}$ , so $\Delta H_\text{reverse} = 300 - 380 = -80\ \text{kJ mol}^{-1}$ .

step 4 Interpret

The reverse reaction is exothermic with the same magnitude of enthalpy change, as required for a state function.

Try this

Q1. A reaction diagram goes downhill by $120\ \text{kJ mol}^{-1}$ from reactants to products. State $\Delta H$ and classify the reaction. [2 points]

Cue. $\Delta H = -120\ \text{kJ mol}^{-1}$ ; exothermic (products below reactants).

Q2. Explain why the forward and reverse reactions have enthalpy changes of equal magnitude but opposite sign. [2 points]

Cue. Enthalpy is a state function; reversing the reaction swaps the start and end states, changing only the sign of the difference.

Exam-style practice questions

Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AP 2023 (style)4 marksSection II (long FRQ, part). On an energy diagram, the reactants of a reaction lie at

250\ \text{kJ mol}^{-1}

and the products at

150\ \text{kJ mol}^{-1}

. (a) Calculate the enthalpy change of the reaction. (b) State whether the reaction is endothermic or exothermic. (c) Sketch how the diagram would look for the reverse reaction and state its

\Delta H

. (d) Justify why the magnitude of

\Delta H

is the same for the forward and reverse reactions.

Show worked answer →

A 4-point conceptual FRQ on energy diagrams.

(a) Enthalpy change (1 point): $\Delta H = E_\text{products} - E_\text{reactants} = 150 - 250 = -100\ \text{kJ mol}^{-1}$ .
(b) Classification (1 point): $\Delta H < 0$ , so the reaction is exothermic (products lie below reactants).
(c) Reverse reaction (1 point): the diagram is flipped so reactants are at $150$ and products at $250\ \text{kJ mol}^{-1}$ ; $\Delta H = 250 - 150 = +100\ \text{kJ mol}^{-1}$ , endothermic.
(d) Justify (1 point): enthalpy is a state function, so reversing the reaction swaps reactants and products and changes only the sign of $\Delta H$ , not its magnitude.

Markers reward the enthalpy change, the exothermic classification, the reversed diagram with the opposite sign, and the state-function reasoning.

AP 2021 (style)1 marksSection I (multiple choice). On an energy diagram, an exothermic reaction is shown by products that are (A) higher in energy than reactants (B) lower in energy than reactants (C) at the same energy as reactants (D) at the highest point of the curve. Justify your choice.

Show worked answer →

A 1-point conceptual MCQ. The answer is (B).

For an exothermic reaction the system loses energy, so the products are lower in energy than the reactants on the diagram and $\Delta H = E_\text{products} - E_\text{reactants} < 0$ . The trap is (A), which describes an endothermic reaction.

Related dot points

Sources & how we know this

AP Chemistry Course and Exam Description — College Board (2020)