8.2.2 Endothermic Reactions

Definition: A chemical reaction that proceeds with the absorption of heat is called an endothermic reaction.

• In these reactions, heat is transferred from surroundings to the system.
• The temperature of the surroundings decreases.
• ΔH is positive (+).

Simple explanation: Endothermic reactions take in heat — they feel cold.

Examples from the Book

Exothermic reactions:

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

Endothermic reactions:

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

C_(s) + H₂O_(g) → CO_(g) + H_2(g) Δ H^° = +131.4 kJ

N_2(g) + O_2(g) → 2NO_(g) Δ H^° = +180.5 kJ

Simple explanation: Reactions with negative ΔH are exothermic; reactions with positive ΔH are endothermic.

Concept Assessment Exercise 8.1 (from the book)

Classify the following processes as exothermic or endothermic:

• Water → Ice
• Heat is released → Exothermic

• Methane + O₂ → CO₂ + H₂O + heat
• Heat is released → Exothermic

• Solid CO₂ → CO₂ gas
• Heat is absorbed → Endothermic

• Steam → Liquid water
• Heat is released → Exothermic

• CaCO₃ → CaO + CO₂
• Heat is absorbed → Endothermic

Intermediate Level

Thermochemical Equations

• The balanced chemical equation
• The physical states of reactants and products
• The heat 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 sign).

Simple explanation: The thermochemical equation tells you both what happens chemically and how much heat is involved.

ΔH — Enthalpy Change

ΔH (delta H) represents the enthalpy change of the reaction.

• ΔH negative → exothermic (heat released)
• ΔH positive → endothermic (heat absorbed)
• ΔH° → standard enthalpy change (measured at standard conditions)

Simple explanation: ΔH is the "heat tag" of the reaction.

Why Physical States Matter

• Different states have different energy contents.
• For example, H₂O(l) and H₂O(g) have different enthalpies.

Example:

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

The (g) means gaseous state. If the state changes, ΔH would change.

Exothermic vs Endothermic — Comparison

Important Points

• - Thermochemical reaction: a chemical reaction that proceeds with the evolution or absorption of heat.
• - Thermochemical equation: a balanced chemical equation that also shows the heat change of a chemical reaction.
• - Thermochemistry: the branch of chemistry dealing with heat changes in chemical reactions.
• - Exothermic reaction: releases heat; ΔH is negative.
• - Endothermic reaction: absorbs heat; ΔH is positive.
• - Examples:
• - Exothermic: C + O₂ → CO₂ (ΔH = −393.5 kJ)
• - Endothermic: H₂ + I₂ → 2HI (ΔH = +53.8 kJ)
• - Endothermic: N₂ + O₂ → 2NO (ΔH = +180.5 kJ)
• - Physical states must be included in thermochemical equations.
• - ΔH° = standard enthalpy change (measured at standard conditions).

Remember

• - Thermochemical reaction = reaction with heat change.
• - Thermochemical equation = balanced equation + ΔH.
• - Thermochemistry = study of heat changes in reactions.
• - Exothermic → releases heat → ΔH negative.
• - Endothermic → absorbs heat → ΔH positive.
• - Examples:
• - C + O₂ → CO₂ (ΔH = −393.5 kJ) → exothermic
• - H₂ + I₂ → 2HI (ΔH = +53.8 kJ) → endothermic
• - N₂ + O₂ → 2NO (ΔH = +180.5 kJ) → endothermic

Common Mistakes

• 1. Forgetting the sign of ΔH. Negative = exothermic; positive = endothermic.
• 2. Thinking exothermic reactions feel cold. They release heat — they feel hot.
• 3. Thinking endothermic reactions feel hot. They absorb heat — they feel cold.
• 4. Forgetting physical states in thermochemical equations. States are important because they affect the energy content.
• 5. Confusing ΔH with temperature. ΔH is the heat change of the reaction, not the temperature.
• 6. Thinking all reactions are either exothermic or endothermic. Yes — every reaction has an energy change.

Quick Revision

• Thermochemical reaction: a chemical reaction that proceeds with the evolution or absorption of heat.
• Thermochemical equation: a balanced chemical equation that also shows the heat change of a chemical reaction.
• Thermochemistry: the branch of chemistry dealing with heat or thermal energy changes in chemical reactions.
• Exothermic reaction: a reaction that proceeds with the evolution of heat; ΔH is negative.
• Endothermic reaction: a reaction that proceeds with the absorption of heat; ΔH is positive.
• Examples:
• C(s) + O₂(g) → CO₂(g) ΔH° = −393.5 kJ (exothermic)
• H₂(g) + I₂(g) → 2HI(g) ΔH° = +53.8 kJ (endothermic)
• C(s) + H₂O(g) → CO(g) + H₂(g) ΔH° = +131.4 kJ (endothermic)
• N₂(g) + O₂(g) → 2NO(g) ΔH° = +180.5 kJ (endothermic)

60-Second Revision

• Thermochemical reaction = heat change occurs.
• Exothermic → heat released → ΔH negative.
• Endothermic → heat absorbed → ΔH positive.
• Examples:
• Combustion of methane → exothermic.
• Decomposition of limestone → endothermic.
• Warning: ΔH negative = exothermic; ΔH positive = endothermic.