6.1.4 Relation Between Enthalpy Change and Heat of Reaction

This topic explains the relationship between enthalpy change (ΔH) and heat of reaction. It shows how heat changes at constant pressure can be equated to changes in enthalpy.

Definition

Enthalpy Change (ΔH): The difference between the enthalpies of the products and the enthalpies of the reactants.

Heat of Reaction: The heat evolved or absorbed in a chemical reaction at constant pressure.

Main Idea

Because most chemical reactions occur at constant pressure, we can equate the heat change in these reactions to the change in enthalpy. This means we can define the heat of a reaction as the change in enthalpy (ΔH) , which is the difference between the enthalpies of the products (final state) and the enthalpies of the reactants (initial state).

Core Concept

Δ H = H_products - H_reactants

• Exothermic process: Heat is released by the system to the surroundings → ΔH is negative.
• Endothermic process: Heat is absorbed by the system from the surroundings → ΔH is positive.
• Combustion: Exothermic process → ΔH is negative.

Detailed Explanation

Enthalpy Change and Heat of Reaction

• Because most chemical reactions occur at constant pressure, we can equate the heat change in these reactions to the change in enthalpy.
• This means we can define heat of a reaction as the change in enthalpy (ΔH) .
• It is the difference between the enthalpies of the products (final state) and the enthalpies of the reactants (initial state).

Δ H = H_products - H_reactants

Sign of ΔH

- The enthalpy change of a reaction can be positive or negative depending upon the process.

• Heat is released by the system to the surroundings.
• ΔH is negative.
• Example: Combustion of fuels.

• Heat is absorbed by the system from the surroundings.
• ΔH is positive.
• Example: Melting of ice.

Combustion as an Example

- Since combustion is an exothermic process in which heat is released by the system to its surroundings, ΔH is negative.

C_(s) + O_2(g) → CO_2(g) Δ H^° = -393.5 kJ

Intermediate Level

Relation Between ΔH and Heat of Reaction

Formula

Δ H = H_products - H_reactants

Examples

Exothermic: C_(s) + O_2(g) → CO_2(g) Δ H^° = -393.5 kJ

Endothermic: H_2(g) + I_2(g) → 2HI_(g) Δ H^° = +53.8 kJ

Key Point

• Most reactions occur at constant pressure.
• Heat change at constant pressure = ΔH.

Important Points

• 1. Heat of reaction at constant pressure = ΔH.
• 2. ΔH = H_products − H_reactants.
• 3. Exothermic: ΔH negative.
• 4. Endothermic: ΔH positive.
• 5. Combustion: Exothermic, ΔH negative.
• 6. Examples: C + O₂ → CO₂, ΔH° = −393.5 kJ.

Remember

• - Heat of reaction at constant pressure = ΔH
• - **ΔH = H_products − H_reactants**
• - Exothermic: ΔH negative
• - Endothermic: ΔH positive
• - Combustion: Exothermic, ΔH negative

Common Mistakes

• 1. Forgetting that ΔH is at constant pressure: Most reactions occur at constant pressure.
• 2. Confusing exothermic and endothermic: Exo = releases; Endo = absorbs.
• 3. Mixing up the sign of ΔH: Exothermic = negative; Endothermic = positive.
• 4. Ignoring the formula: ΔH = Hproducts − Hreactants.

Quick Revision

• Heat of reaction at constant pressure = ΔH.
• ΔH = H_products − H_reactants.
• Exothermic: ΔH negative.
• Endothermic: ΔH positive.
• Combustion: Exothermic, ΔH negative.
• Examples: C + O₂ → CO₂, ΔH° = −393.5 kJ.

60-Second Revision

• Heat of reaction at constant pressure = ΔH
• **ΔH = H_products − H_reactants**
• Exothermic: ΔH negative
• Endothermic: ΔH positive
• Combustion: Exothermic, ΔH negative

QUALITY VALIDATION

• Topic: 6.1.4 Relation Between Enthalpy Change and Heat of Reaction
• Chapter: 6 (Energetics)

• Source re-read: PASS
• Source scope followed: PASS
• Knowledge level matched: PASS
• Important concepts covered: PASS
• Definitions checked: PASS
• Equations checked: PASS
• Derivations checked: N/A
• Examples checked: PASS
• Unsupported knowledge added: NO
• Unnecessary repetition: NO
• Notes format maintained: PASS
• Important source content omitted: NO

• Source limitations:
• NONE

NEXT TOPIC