8.3 Enthalpy of Reaction

This topic explains enthalpy of reaction — the heat energy evolved or absorbed in a chemical reaction — and introduces standard enthalpy change.

Definition

The amount of heat or thermal energy evolved or absorbed in a chemical reaction is called enthalpy of reaction.

Simple explanation: Enthalpy of reaction is the heat change that occurs during a chemical reaction.

Main Idea

• Its sign is negative for exothermic reactions (heat is released).
• Its sign is positive for endothermic reactions (heat is absorbed).

Enthalpy of reaction measured at 25 °C (298 K) and one atmospheric pressure is known as standard enthalpy change. It is denoted by ΔH°.

Simple explanation: ΔH° is the heat change measured under standard conditions (25 °C, 1 atm).

Core Concept

Standard Conditions

Simple explanation: The "°" symbol means standard conditions.

Detailed Explanation

What Is Enthalpy?

Enthalpy (H) is the total heat content of a system at constant pressure. In chemical reactions, we measure the change in enthalpy (ΔH), not the absolute enthalpy.

• **ΔH = H(products) − H(reactants)**
• If products have less enthalpy than reactants → heat is released → ΔH negative → exothermic.
• If products have more enthalpy than reactants → heat is absorbed → ΔH positive → endothermic.

Simple explanation: ΔH tells you the difference in heat content between products and reactants.

Exothermic Reactions — Negative ΔH

• The system transfers energy to the surroundings.
• The products have less enthalpy than the reactants.
• ΔH is negative.

Example from the book:

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

Simple explanation: Carbon and oxygen react to form carbon dioxide, releasing 393.5 kJ of heat. The negative sign means heat is given out.

Endothermic Reactions — Positive ΔH

• The system absorbs energy from the surroundings.
• The products have more enthalpy than the reactants.
• ΔH is positive.

Example from the book:

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

Simple explanation: Hydrogen and iodine react to form hydrogen iodide, absorbing 53.8 kJ of heat. The positive sign means heat is taken in.

Book Question

The book asks:

> "Which of the above reactions is endothermic?"

Given: C_(s) + O_2(g) → CO_2(g) Δ H^° = -393.5 kJ H_2(g) + I_2(g) → 2HI_(g) Δ H^° = +53.8 kJ

Answer: The second reaction (H₂ + I₂ → 2HI) is endothermic because ΔH° is positive (+53.8 kJ).

Simple explanation: Positive ΔH = endothermic; negative ΔH = exothermic.

Standard Enthalpy Change (ΔH°)

• 25 °C (298 K)
• 1 atmosphere pressure

It is denoted by ΔH° (the "°" means standard).

• Enthalpy changes depend on temperature and pressure.
• Standard conditions allow comparison between different reactions.

Simple explanation: Standard conditions are like a "common reference point" for comparing heat changes.

Interpreting Thermochemical Equations

• Reactants and products (balanced).
• Physical states (s, l, g, aq).
• Enthalpy change (ΔH°).

Example:

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

• 1 mole of solid carbon reacts with 1 mole of oxygen gas.
• 1 mole of carbon dioxide gas is produced.
• 393.5 kJ of heat is released (negative ΔH).

Intermediate Level

Enthalpy Change and Bond Energies

The enthalpy change of a reaction is related to the energy absorbed in breaking bonds and the energy released in forming bonds:

Δ H = Energy absorbed (bond breaking) - Energy released (bond forming)

• If energy released > energy absorbed → ΔH negative → exothermic.
• If energy absorbed > energy released → ΔH positive → endothermic.

Simple explanation: The net heat change depends on the balance between bond breaking and bond making.

Standard Enthalpy Change

Standard enthalpy change (ΔH°) is measured under standard conditions:

• To allow comparison between different reactions.
• To have a consistent reference point.

Units of Enthalpy

• kJ (kilojoules) — for the reaction as written.
• kJ/mol — per mole of a specified reactant or product.

Example:

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

This means 393.5 kJ is released per mole of carbon reacted.

Common Enthalpy Values

Important Points

• - Enthalpy of reaction: amount of heat or thermal energy evolved or absorbed in a chemical reaction.
• - Sign: negative for exothermic; positive for endothermic.
• - Standard enthalpy change (ΔH°): measured at 25 °C (298 K) and 1 atm pressure.
• - Exothermic: heat released; ΔH negative; products have less enthalpy.
• - Endothermic: heat absorbed; ΔH positive; products have more enthalpy.
• - Examples:
• - C + O₂ → CO₂: ΔH° = −393.5 kJ (exothermic)
• - H₂ + I₂ → 2HI: ΔH° = +53.8 kJ (endothermic)
• - Formula: ΔH = H(products) − H(reactants).
• - Units: kJ or kJ/mol.

Remember

• - Enthalpy of reaction = heat evolved or absorbed.
• - ΔH negative → exothermic (heat out).
• - ΔH positive → endothermic (heat in).
• - Standard conditions: 25 °C (298 K), 1 atm.
• - ΔH° = standard enthalpy change.
• - Examples:
• - C + O₂ → CO₂ ΔH° = −393.5 kJ
• - H₂ + I₂ → 2HI ΔH° = +53.8 kJ

Common Mistakes

• 1. Forgetting the sign convention. Negative ΔH = exothermic; positive ΔH = endothermic.
• 2. Thinking ΔH is always negative. It depends on whether the reaction is exothermic or endothermic.
• 3. Forgetting standard conditions. ΔH° is measured at 25 °C and 1 atm.
• 4. Confusing enthalpy with temperature. Enthalpy is heat content; temperature is a measure of hotness.
• 5. Forgetting to include physical states in thermochemical equations. States affect the enthalpy value.
• 6. Thinking ΔH is the same for all reactions. Each reaction has its own ΔH value.

Quick Revision

• Enthalpy of reaction: the amount of heat or thermal energy evolved or absorbed in a chemical reaction.
• Sign: negative for exothermic; positive for endothermic.
• Standard enthalpy change (ΔH°): enthalpy of reaction measured at 25 °C (298 K) and one atmospheric pressure.
• Examples:
• C(s) + O₂(g) → CO₂(g) ΔH° = −393.5 kJ (exothermic)
• H₂(g) + I₂(g) → 2HI(g) ΔH° = +53.8 kJ (endothermic)
• **ΔH = H(products) − H(reactants)**
• Exothermic: products have less enthalpy than reactants.
• Endothermic: products have more enthalpy than reactants.
• Units: kJ or kJ/mol.

60-Second Revision

• Enthalpy of reaction = heat evolved or absorbed.
• ΔH negative → exothermic.
• ΔH positive → endothermic.
• Standard conditions: 25 °C, 1 atm.
• **Examples:** C + O₂ → CO₂ (ΔH° = −393.5 kJ); H₂ + I₂ → 2HI (ΔH° = +53.8 kJ).
• Warning: ΔH is negative for exothermic, positive for endothermic.