6.1.2 Thermochemical Reactions

This topic explains thermochemical reactions — chemical reactions that involve the evolution or absorption of heat. It covers exothermic and endothermic reactions, thermochemical equations, and examples.

Definition

Thermochemical Reaction: A chemical reaction which proceeds with the evolution or absorption of heat.

Thermochemical Equation: A balanced chemical equation which also shows the heat change of a chemical reaction.

Thermochemistry: The branch of chemistry which deals with the heat or thermal energy changes in chemical reactions.

Main Idea

When a chemical change takes place, energy is exchanged between the system and its surroundings. Energy has many forms such as heat, light, and work. A chemical reaction that proceeds with the evolution or absorption of heat is called a thermochemical reaction. A balanced chemical equation that also shows the heat change is called a thermochemical equation.

Core Concept

• Thermochemical reaction: Reaction with heat change.
• Thermochemical equation: Balanced equation with heat change.
• Thermochemistry: Study of heat changes in reactions.
• Exothermic: Heat is evolved (released).
• Endothermic: Heat is absorbed.

Detailed Explanation

What is a Thermochemical Reaction?

• When a chemical change takes place, energy is exchanged between system and its surroundings.
• Energy has many forms such as heat, light, work, etc.
• A chemical reaction which proceeds with the evolution or absorption of heat is called a thermochemical reaction.
• A balanced chemical equation which also shows heat change of a chemical reaction is called a thermochemical equation.
• The branch of chemistry which deals with the heat or thermal energy changes in chemical reactions is called thermochemistry.

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

Types of Thermochemical Reactions

• A chemical reaction that proceeds with the evolution of heat is called an exothermic reaction.
• In an exothermic reaction, the chemical system transfers energy to the surroundings as the reactants are converted to products.
• Example: Burning of fuels is a highly exothermic reaction.
• The energy released can be used to heat a room, or to drive an engine, or to cook food.

Examples: C_(s) + O_2(g) → CO_2(g) Δ H^° = -393.5 kJ 2H_2(g) + O_2(g) → 2H₂O_(l) Δ H^° = -571.6 kJ C_(s) + 12O_2(g) → CO_(g) Δ H^° = -110.5 kJ

• 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.

Examples: H_2(g) + I_2(g) → 2HI_(g) C_(s) + H₂O_(g) → CO_(g) + H_2(g) N_2(g) + O_2(g) → 2NO_(g)

Concept Assessment Exercise 6.1

Classify the following processes as exothermic or endothermic: (a) Freezing of water (b) Combustion of methane (c) Sublimation of dry ice (d) H₂O_(g) → H₂O_(l) (e) Decomposition of limestone

Answers: (a) Freezing of water — Exothermic (heat released) (b) Combustion of methane — Exothermic (heat released) (c) Sublimation of dry ice — Endothermic (heat absorbed) (d) H₂O_(g) → H₂O_(l) — Exothermic (condensation releases heat) (e) Decomposition of limestone — Endothermic (heat absorbed)

Intermediate Level

Thermochemical Reactions

Thermochemical Equation

• Balanced equation with heat change.
• Example:

Thermochemistry

- Branch of chemistry dealing with heat changes in reactions.

Examples

Exothermic: 2H_2(g) + O_2(g) → 2H₂O_(l) Δ H^° = -571.6 kJ

Endothermic: N_2(g) + O_2(g) → 2NO_(g)

Important Points

• 1. Thermochemical reaction: Reaction with heat change.
• 2. Thermochemical equation: Balanced equation with heat change.
• 3. Thermochemistry: Study of heat changes.
• 4. Exothermic: Heat released, ΔH negative.
• 5. Endothermic: Heat absorbed, ΔH positive.
• 6. Examples: Burning fuel (exothermic), melting ice (endothermic).

Remember

• - Thermochemical reaction: Heat change
• - Thermochemical equation: Balanced equation + heat change
• - Thermochemistry: Study of heat changes
• - Exothermic: Releases heat, ΔH negative
• - Endothermic: Absorbs heat, ΔH positive

Common Mistakes

• 1. Confusing exothermic and endothermic: Exo = releases; Endo = absorbs.
• 2. Forgetting the sign of ΔH: Exothermic = negative; Endothermic = positive.
• 3. Thinking all reactions release heat: Some absorb heat.
• 4. Mixing up thermochemical equation with regular equation: Thermochemical equation includes heat change.

Quick Revision

• Thermochemical reaction: Reaction with heat change.
• Thermochemical equation: Balanced equation with heat change.
• Thermochemistry: Study of heat changes.
• Exothermic: Heat released, ΔH negative.
• Endothermic: Heat absorbed, ΔH positive.
• Examples: Burning fuel (exothermic), melting ice (endothermic).

60-Second Revision

• Thermochemical reaction: Heat change
• Thermochemical equation: Balanced equation + heat change
• Thermochemistry: Study of heat changes
• Exothermic: Releases heat, ΔH negative
• Endothermic: Absorbs heat, ΔH positive

QUALITY VALIDATION

• Topic: 6.1.2 Thermochemical Reactions
• Chapter: 6 (Energetics)

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• 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

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