2.1.2Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Enthalpy Change – Definitions — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 11 of 53 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Standard Enthalpy Definitions
Each definition has a precise equation, and marks are lost if the amounts are wrong.
Reaction
- Reaction ΔrH° is the enthalpy change when the amounts shown in the balanced equation react under standard conditions.
Formation
- Formation ΔfH° is the enthalpy change when one mole of a compound forms from its elements in their standard states under standard conditions.
Combustion
- Combustion ΔcH° is the enthalpy change when one mole of a substance burns completely in oxygen under standard conditions.
Neutralisation
- Neutralisation ΔneutH° is the enthalpy change when an acid and an alkali react to form one mole of water under standard conditions.
Part 2 of 6Writing the Matching Equations
Writing the Matching Equations
Formation of CO₂
- Formation of CO₂ C(s, graphite) + O₂(g) → CO₂(g) has ΔfH° of about −394 kJ mol⁻¹.
Formation of ethanol
- Formation of ethanol 2C(s) + 3H₂(g) + ½O₂(g) → C₂H₅OH(l) uses fractions where needed to make exactly one mole of product.
Combustion of methane
- Combustion of methane CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) has ΔcH° of about −890 kJ mol⁻¹.
Neutralisation
- Neutralisation H⁺(aq) + OH⁻(aq) → H₂O(l) has a value of about −57 kJ mol⁻¹ for strong acids and alkalis.
| Enthalpy change | Equation | Approx. ΔH / kJ mol⁻¹ |
|---|---|---|
| Formation of CO₂ | C(s) + O₂(g) → CO₂(g) | −394 |
| Formation of H₂O(l) | H₂(g) + ½O₂(g) → H₂O(l) | −286 |
| Combustion of CH₄ | CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) | −890 |
Part 3 of 6Key Details
1
Standard state is the most stable physical form of the element at 100 kPa and the stated temperature, such as O₂(g) and C(graphite).
2
The enthalpy of formation of any element in its standard state is zero by definition.
3
Combustion always gives CO₂(g) and H₂O(l) as products, so state symbols matter.
4
Formation and combustion are always defined per mole of the compound named, so fractional coefficients are allowed.
5
Neutralisation is exothermic and is about the same for any strong acid and strong alkali because the reaction is just H⁺ + OH⁻.
Part 4 of 6Other Enthalpy Terms
Other Enthalpy Terms
Atomisation
- Atomisation ΔatH° is the enthalpy change when one mole of gaseous atoms forms from the element in its standard state.
Bond dissociation
- Bond dissociation The enthalpy change when one mole of a covalent bond in gaseous molecules is broken.
First ionisation energy
- First ionisation energy The enthalpy change when one mole of electrons is removed from one mole of gaseous atoms.
Why precise
- Why precise Each definition ties the value to a specific equation, so values can be compared and combined in cycles.
Part 5 of 6Worked Example
Question Write the equation that defines the standard enthalpy of formation of propane, C₃H₈(g).
- The product must be one mole of propane, formed from carbon and hydrogen.
- Use the standard statescarbon as graphite, C(s), and hydrogen as H₂(g).
- Balance so that three carbons and eight hydrogens are present: four H₂ give eight hydrogens.
Answer 3C(s) + 4H₂(g) → C₃H₈(g)
Part 6 of 6Exam Checklist & Tips
2.1.2 – Key Points for the Exam
- Standard enthalpy of formation The enthalpy change when one mole of a compound forms from its elements in their standard states.
- Standard enthalpy of combustion The enthalpy change when one mole of a substance is completely burned in oxygen.
- Standard enthalpy of neutralisation The enthalpy change when an acid and alkali form one mole of water.
- Standard state The physical state of a substance at 100 kPa and a stated temperature.
- Atomisation Formation of one mole of gaseous atoms from an element.
Common Mistakes
- Writing an equation for formation that forms two moles of product.
- Using H₂O(g) instead of H₂O(l) as the product in a combustion equation.
- Forgetting that the formation enthalpy of an element is zero.
- Giving the enthalpy of formation of CO₂ using carbon in the form of diamond.
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