1.6.3Physical Chemistry
Enthalpy Changes — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 33 of 102
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 5Standard Conditions
Pressure
- Pressure Standard pressure is 100 kPa, and any gases involved are taken at this pressure.
Temperature
- Temperature Standard enthalpy changes are quoted for a stated temperature, normally 298 K.
Standard state
- Standard state Each substance is in its physical state under standard conditions, such as carbon as graphite and oxygen as O₂(g).
Symbol
- Symbol The standard sign ° is added to ΔH, as in ΔfH°, to show the standard conditions apply.
Units
- Units Enthalpy changes are given in kJ mol⁻¹ and refer to the amounts shown in the balanced equation.
Part 2 of 5Definitions
Definitions
Reaction ΔrH°
- Reaction ΔrH° The enthalpy change when the amounts of reactants shown in the equation react to give the products, under standard conditions.
Formation ΔfH°
- Formation ΔfH° The enthalpy change when one mole of a compound forms from its elements in their standard states.
Combustion ΔcH°
- Combustion ΔcH° The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions.
Neutralisation
- Neutralisation The enthalpy change when an acid and alkali react to form one mole of water, which is about −57 kJ mol⁻¹ for strong acid and alkali.
| Enthalpy change | Equation example |
|---|---|
| Formation of methane | C(s) + 2H₂(g) → CH₄(g) |
| Combustion of hydrogen | H₂(g) + ½O₂(g) → H₂O(l) |
| Neutralisation | H⁺(aq) + OH⁻(aq) → H₂O(l) |
Part 3 of 5Writing Equations for Definitions
1
Read the definition to see how many moles of the named substance must appear in the equation.
2
For formation, write the elements in standard states on the left and exactly one mole of compound on the right.
3
For combustion, write one mole of fuel reacting with oxygen, and give products in their standard states.
4
Use fractions in front of formulae if necessary to keep the number of moles of the key substance at one.
5
Include state symbols for every species, because the enthalpy change depends on the physical states.
Part 4 of 5Worked Example
Question
- The standard enthalpy of formation of water, H₂O(l), is −286 kJ mol⁻¹.
- Calculate ΔH for 2H₂(g) + O₂(g) → 2H₂O(l).
- The equation forms 2 mol of H₂O(l), which is twice the amount in the formation definition.
- Enthalpy change is proportional to the number of moles, so ΔH is doubled.
- ΔH = 2 × (−286) = −572 kJ.
Answer ΔH = −572 kJ for the equation as written (−572 kJ per two moles of water).
Part 5 of 5Exam Checklist & Tips
1.6.3 – Key Points for the Exam
- Standard conditions 100 kPa pressure and a stated temperature, usually 298 K.
- Standard state The physical state of a substance at 100 kPa and the stated temperature.
- ΔfH° The standard enthalpy change when one mole of a compound is formed from its elements.
- ΔcH° The standard enthalpy change when one mole of a substance burns completely in oxygen.
Common Mistakes
- Writing two moles of product in an equation for an enthalpy of formation.
- Forgetting state symbols, which are needed because enthalpy depends on the state.
- Using a standard enthalpy of formation of an element, which is zero by definition.
- Giving combustion products as H₂O(g) when the standard state at 298 K is liquid.
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