Energy Diagrams
Draw initial/final energy levels and a correctly signed change; a level diagram alone gives no reaction time.
AP Chemistry Topic 6.2 · CED printed p. 113 / PDF p. 120. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- System and surroundings as a chosen boundary
- Moles, balanced equations and limiting reactants
- Intermolecular versus intramolecular interactions
- Activation energy versus overall energy change
An energy-level diagram places initial and final configurations on the same vertical scale. Products above reactants give a positive energy/enthalpy change: energy input is needed. Products below reactants give a negative change: energy is transferred out. For melting at a fixed pressure, place liquid above solid by ΔHfus; reversing the process reverses the arrow and sign. Label the vertical axis with energy units when values are supplied and the horizontal direction with process progress. The energy zero is a chosen reference, so relative differences are what matter. A simple initial/final diagram does not need a transition-state peak: it expresses thermochemistry, while a pathway peak adds kinetic information. Do not infer that a negative ΔH alone makes a transformation fast or thermodynamically favored under every condition.
Let's work through it
For a process with initial level 12 and final level 47 kJ mol−1, construct a two-level diagram.
- Draw final above initial on an energy axis labeled kJ mol−1.
- The arrow from 12 to 47 has ΔH =47 −12 =+35 kJ mol−1, so the forward process is endothermic.
- Reverse the arrow for the backward process: ΔH =−35 kJ mol−1. Shifting both levels by the same reference amount leaves the difference unchanged.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
If final particles have a lower enthalpy than initial particles, what sign is ΔH?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
If final particles have a lower enthalpy than initial particles, what sign is ΔH?
Worked answer and reason
Negative, using ΔH =Hfinal −Hinitial.
The endpoint difference describes net energy transferred at constant pressure.
Retrieval 2
Must a thermochemical level diagram include an activation-energy peak?
Worked answer and reason
No. A level diagram can show the endpoint difference alone; a peak adds pathway/barrier information.
Net enthalpy and activation energy are distinct.
Catch the tempting mistake
- The arbitrary zero of the diagram determines ΔH.
- Only final-minus-initial differences matter.
- A bigger downward step automatically means faster reaction.
- Speed depends on pathway barriers and kinetic conditions, not only endpoint levels.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 6 problem 2 · ap application · multiple
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For the teacher
- Ask students to label energy units and sign on arrows, including a reverse arrow.
- Keep reaction speed and thermochemical direction separate.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 6.2.A
- Represent a chemical or physical transformation with an energy diagram.
- 6.2.A.1
- A physical or chemical process can be described with an energy diagram that shows the endothermic or exothermic nature of that process.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Energy Basics — Scientific check: system boundaries, energy conservation, thermal transfer and heat capacities.
- OpenStax Chemistry 2e: Enthalpy — Scientific check: stoichiometric enthalpy, formation values, sign reversal/scaling and Hess cycles.
Glossary and deeper reading · 5 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Mole
A mole is an amount of substance containing exactly 6.02214076 × 10²³ specified entities. The entities can be atoms, molecules, ions, or formula units, so the counted entity must be named. For example, one mole of oxygen molecules contains two moles of oxygen atoms.
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Limiting reactant
The limiting reactant is consumed first under the specified complete-reaction model and limits the product amount. Identifying it requires comparing available amounts with the balanced equation's ratios. For example, abundant hydrogen cannot produce more water after the available oxygen has been consumed.
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Activation energy
Activation energy is the energy barrier associated with a reaction pathway in an introductory energy-profile model. It concerns the pathway and is distinct from the reactant-to-product energy change. For example, a thermodynamically favored reaction can remain slow when its activation barrier is large.
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Enthalpy
Enthalpy is a state function defined from internal energy, pressure, and volume. At constant pressure with only pressure–volume work, an enthalpy change equals the heat transferred to the system. For example, a negative reaction enthalpy indicates heat release for the written reaction under those conditions.
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Thermodynamic favorability
Thermodynamic favorability describes whether a process has a driving force in the stated direction under specified conditions. It does not determine how quickly the process occurs. For example, a reaction with a favorable free-energy change can persist without visible progress when a large kinetic barrier blocks it.