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1 Atomic Structure and Properties 2 Compound Structure and Properties 3 Properties of Substances and Mixtures 4 Chemical Reactions 5 Kinetics 6 Thermochemistry 7 Equilibrium 8 Acids and Bases 9 Thermodynamics and Electrochemistry

6 Thermochemistry

6.1 Endothermic and Exothermic Processes 6.2 Energy Diagrams 6.3 Heat Transfer and Thermal Equilibrium 6.4 Heat Capacity and Calorimetry 6.5 Energy of Phase Changes 6.6 Introduction to Enthalpy of Reaction 6.7 Bond Enthalpies 6.8 Enthalpy of Formation 6.9 Hess’s Law

Enthalpy of Formation

Learning Objective 6.8.A Calculate the enthalpy change for a chemical or physical process based on the standard enthalpies of formation.

Quick Notes

  • Standard enthalpy of formation (ΔHf°) is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions.
  • Use the following equation to calculate the enthalpy change of a reaction:
    ΔH°reaction = ΣΔHf°(products) − ΣΔHf°(reactants)

Full Notes

What Is Enthalpy of Formation?

The standard enthalpy of formation (ΔHf°) is the energy change when 1 mole of a compound forms from its constituent elements in their standard states at 298 K (25°C) and 1 atm.

For elements in their standard states, ΔHf° = 0.

Examples:

Calculating ΔH° for a Reaction

We can calculate the enthalpy change (ΔH) of a reaction by using the standard enthalpies of formation of the reactants and products.

Use the formula:

AP Chemistry diagram showing ΔH°reaction equals the sum of standard enthalpies of formation of products minus the sum for reactants.

Steps

Worked Example

Calculate ΔH° for this reaction: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

Standard enthalpies of formation (in kJ/mol):
CH4(g) = −74.8; O2(g) = 0; CO2(g) = −393.5; H2O(l) = −285.8

  1. Products
    1 mol CO2: 1 × (−393.5) = −393.5
    2 mol H2O: 2 × (−285.8) = −571.6
    Total: −965.1 kJ
  2. Reactants
    1 mol CH4: 1 × (−74.8) = −74.8
    2 mol O2: 2 × 0 = 0
    Total: −74.8 kJ
  3. ΔH°reaction
    ΔH° = (−965.1) − (−74.8) = −890.3 kJ

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Matt’s exam tip

Always base your calculations on a balanced chemical equation and account for the mole ratios of all substances involved. For example, if the balanced equation shows 2 moles of H2O are formed, you must multiply the enthalpy of formation of H2O by 2 when calculating the total enthalpy change.

Summary