R1.2.3 Standard enthalpy changes of combustion and formation (Higher Level Only)

Standard Enthalpy of Combustion ( $Δ H_{c}^{\circ}$ ) and Formation ( $Δ H_{f}^{\circ}$ )

Standard Enthalpy of Combustion ( $Δ H_{c}^{\circ}$ )

Definition

Standard enthalpy of combustion

The standard enthalpy of combustion ( $Δ H_{c}^{\circ}$ ) is the energy change when one mole of a substance is completely burned in oxygen under standard conditions (298 K and 1 atm).

The products of combustion must also be in their standard states (e.g., CO₂ as a gas and H₂O as a liquid).

Example

The combustion of methane (CH₄) is represented as:

${CH}_{4} (g) + 2 O_{2} (g) \to {CO}_{2} (g) + 2 H_{2} O (l) Δ H_{c}^{\circ} = - 890 {kJ mol}^{- 1}$

This equation tells us that burning 1 mole of methane releases 890 kJ of energy.

Key Features of $Δ H_{c}^{\circ}$

Exothermic Nature: Combustion reactions are always exothermic, meaning $Δ H_{c}^{\circ}$ values are negative because energy is released to the surroundings.
Standard States: All reactants and products must be in their standard states.

Example

Oxygen is a gas (O₂(g)), and water is a liquid (H₂O(l)) at standard conditions.

Applications: $Δ H_{c}^{\circ}$ values are essential for comparing the energy efficiency of different fuels, such as coal, gasoline, and hydrogen.

Example

Combustion of Butane

The combustion of butane (C₄H₁₀), a fuel commonly used in lighters, is represented as:

$C 4 H 10 (g) + \frac{13}{2} O_{2} (g) \to 4 {CO}_{2} (g) + 5 H_{2} O (l) Δ H_{c}^{\circ} = - 2878 {kJ mol}^{- 1}$

This means burning 1 mole of butane releases 2878 kJ of energy.

Tip

$Δ H_{c}$ values for common substances are listed in Section 14 of the IB Chemistry Data Booklet. Use these values for accurate calculations.

Standard Enthalpy of Formation ( $Δ H_{f}^{\circ}$ )

Definition

Standard enthalpy of formation

The standard enthalpy of formation ( $Δ H_{f}^{\circ}$ ) is the energy change when one mole of a compound is formed from its elements in their standard states under standard conditions.

Example

The formation of water (H₂O) from hydrogen and oxygen is:

$H_{2} (g) + \frac{1}{2} O_{2} (g) \to H_{2} O (l) Δ H_{f}^{\circ} = - 286 {kJ mol}^{- 1}$

This means that forming 1 mole of liquid water from hydrogen gas and oxygen gas releases 286 kJ of energy.

Key Features of $Δ H_{f}^{\circ}$

Reference Point: The $Δ H_{f}^{\circ}$ of any element in its standard state is zero.

Example

$Δ H_{f}^{\circ}$ of O₂(g) = 0
$Δ H_{f}^{\circ}$ of C(s, graphite) = 0

Exothermic or Endothermic: Formation reactions can be either exothermic (negative $Δ H_{f}$ ) or endothermic (positive $Δ H_{f}^{\circ}$ ), depending on the compound.
Applications: $Δ H_{f}^{\circ}$ values are used to calculate reaction enthalpy changes using Hess’s law.

Example

Formation of Methane

The formation of methane (CH₄) from carbon and hydrogen is represented as:

$C (s) + 2 H_{2} (g) \to {CH}_{4} (g) Δ H_{f}^{\circ} = - 74.8 {kJ mol}^{- 1}$

This means forming 1 mole of methane releases 74.8 kJ of energy.

Note

$Δ H_{f}^{\circ}$ values for many compounds are listed in Section 13 of the IB Chemistry Data Booklet.

Reflection

Theory of Knowledge

How does the use of models (like $Δ H_{c}^{\circ}$ and $Δ H_{f}^{\circ}$ ) in chemistry help us understand energy changes? What are the limitations of these models in predicting real-world behavior?

Self review

Can you calculate the enthalpy change for a reaction using ΔHf values? What about using ΔHc values? Practice solving a problem from your data booklet to reinforce your understanding.

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R1.2.3 Standard enthalpy changes of combustion and formation (Higher Level Only)

Standard Enthalpy of Combustion (ΔHc∘) and Formation (ΔHf∘)

Standard Enthalpy of Combustion (ΔHc∘)

Key Features of ΔHc∘

Combustion of Butane

Standard Enthalpy of Formation (ΔHf∘)

Key Features of ΔHf∘

Formation of Methane

Reflection

You've read 2/2 free chapters this week.

Introduction to Enthalpy Change

Standard Enthalpy of Combustion ( $Δ H_{c}^{\circ}$ ) and Formation ( $Δ H_{f}^{\circ}$ )

Standard Enthalpy of Combustion ( $Δ H_{c}^{\circ}$ )

Key Features of $Δ H_{c}^{\circ}$

Standard Enthalpy of Formation ( $Δ H_{f}^{\circ}$ )

Key Features of $Δ H_{f}^{\circ}$