4.1.4Reactivity 3 — What are the mechanisms of chemical change?
Standard Enthalpy Change — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 12 of 13 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.
Section 1 of 5Standard Conditions
Pressure
- Pressure Standard pressure is 100 kPa for reactions involving gases.
Temperature
- Temperature Enthalpy values are usually quoted at 298 K (25 °C), although temperature is not part of the definition of standard state.
Concentration
- Concentration Solutions are taken at 1 mol dm⁻³, and each substance is in its most stable physical state under these conditions.
Symbol
- Symbol The superscript ° in ΔH° indicates standard conditions, and the unit is kJ mol⁻¹.
Why it matters
- Why it matters Fixed conditions allow enthalpy values from different experiments to be compared and combined.
Section 2 of 5Key Definitions
1
Standard enthalpy of combustion, ΔHc°, is the enthalpy change when one mole of a substance burns completely in oxygen under standard conditions.
2
Standard enthalpy of formation, ΔHf°, is the enthalpy change when one mole of a compound forms from its elements in their standard states.
3
The enthalpy of formation of an element in its standard state is zero by definition.
4
Combustion is always exothermic, while formation can be exothermic or endothermic.
5
For a reaction, ΔH° = ΣΔHf°(products) − ΣΔHf°(reactants), with each term multiplied by its coefficient.
| Substance | ΔHf° / kJ mol⁻¹ |
|---|---|
| CO₂(g) | −393.5 |
| H₂O(l) | −285.8 |
| O₂(g) | 0 |
Section 3 of 5Using the Definitions
Using the Definitions
Equation writing
- Equation writing The equation for formation has one mole of the compound as the only product, with fractions allowed for the elements.
State symbols
- State symbols State symbols are essential, as ΔHf° differs between H₂O(l) and H₂O(g).
Combustion products
- Combustion products Complete combustion of an organic compound gives CO₂(g) and H₂O(l) under standard conditions.
Units
- Units Values are per mole of the substance named in the definition, not per mole of reaction in general.
Section 4 of 5Worked Example
Question Calculate ΔH° for C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l), given ΔHf°(C₃H₈) = −104.7, ΔHf°(CO₂) = −393.5, ΔHf°(H₂O(l)) = −285.8 kJ mol⁻¹.
- ΣΔHf°(products) = 3(−393.5) + 4(−285.8) = −1180.5 − 1143.2 = −2323.7.
- ΣΔHf°(reactants) = −104.7 + 5(0) = −104.7.
- ΔH° = −2323.7 − (−104.7).
- ΔH° = −2219.0 kJ mol⁻¹.
Answer ΔH° = −2219 kJ mol⁻¹
Section 5 of 5Exam Checklist & Tips
4.1.4 – Key Points for the Exam
- Standard conditions Pressure of 100 kPa, solutions at 1 mol dm⁻³, usually 298 K.
- Standard enthalpy of formation Enthalpy change when one mole of a compound forms from its elements in their standard states.
- Standard enthalpy of combustion Enthalpy change when one mole of a substance burns completely in oxygen under standard conditions.
- Standard state The most stable form of a substance at 100 kPa and the stated temperature.
Common Mistakes
- Writing the formation equation with more than one mole of the compound.
- Omitting state symbols or using the wrong state for water.
- Giving elements such as O₂(g) a non-zero formation enthalpy.
- Forgetting that formation of carbon uses graphite as the standard state.
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