Enthalpy of Formation
Use coefficient-weighted formation values with the correct phase and reference-state convention.
AP Chemistry Topic 6.8 · CED printed p. 120 / PDF p. 127. 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
A standard enthalpy of formation ΔHf° refers to forming one mole of a substance in its stated phase from elements in their reference states at the specified temperature. The standard pressure is 1 bar; many tables use 298 K, but the standard symbol alone does not force that temperature. Elements in their reference states have ΔHf° =0 by convention: O2(g) does, but isolated O(g) does not; graphite is carbon’s reference state in ordinary tables, not every carbon allotrope. For a reaction, . Each ν is the balanced coefficient. Include negative tabulated values with their signs; the subtraction of a negative reactant value increases the result. Match phases and conditions. A liquid-water value cannot silently replace vapor-water data, because condensation changes enthalpy.
Let's work through it
Use supplied 298 K formation values: ΔHf°[CO2(g)] =−393.5, ΔHf°[CO(g)] =−110.5, and O2(g) =0 kJ mol−1. Find CO(g) +½O2(g) → CO2(g).
- Products sum =−393.5 kJ. Reactants sum =−110.5+½(0) =−110.5 kJ.
- Subtract reactants: −393.5−(−110.5) =−283.0 kJ per reaction as written.
- The half coefficient on oxygen is legitimate in a molar thermochemical equation. If the equation is doubled, ΔH doubles to −566.0 kJ.
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
Is ΔHf° zero for every elemental form?
See every recall question with its answer
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Retrieval 1
Is ΔHf° zero for every elemental form?
Worked answer and reason
No. It is zero for the element’s reference state at the specified temperature; atoms or other allotropes can have nonzero values.
The zero is a defined reference convention.
Retrieval 2
Why must liquid and vapor product labels be matched to the data table?
Worked answer and reason
Their enthalpies differ by the phase-change enthalpy, so changing a phase changes the reaction result.
Formation data include physical state.
Catch the tempting mistake
- Subtract reactant formation values as positive magnitudes.
- Retain each table sign and then perform products minus reactants.
- The ° symbol means 25 °C in every context.
- It marks standard states; temperature must be specified separately.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 6 problem 13 · ap synthesis · data
- Unit 6 problem 14 · ap application · symbolic
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For the teacher
- Require a coefficient-weighted product and reactant sum before subtraction.
- Use a liquid/vapor comparison to test phase awareness.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 6.8.A
- Calculate the enthalpy change for a chemical or physical process based on the standard enthalpies of formation.
- 6.8.A.1
- Tables of standard enthalpies of formation can be used to calculate the standard enthalpies of reactions. EQN: ΔH° reaction = ΣΔHf ° products − ΣΔHf ° reactants
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: 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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Standard enthalpy of formation
Standard enthalpy of formation is the enthalpy change for forming one mole of a substance from its elements in their standard reference forms. The substance's phase and specified temperature matter. For example, an element in its standard reference form has zero standard formation enthalpy by convention.
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Condensation
Condensation is a phase change from gas to liquid. It releases energy to the surroundings as particles form more closely associated liquid arrangements. For example, water vapor condensing on a cooler surface forms liquid droplets.