Chemical Energetics – Endothermic & Exothermic Reactions

Vocabulary flashcards summarising the essential terms, definitions, and examples from the lecture on energy changes in chemical reactions.

Thermochemistry

The study of heat energy changes that occur during chemical reactions.

Exothermic Reaction

A reaction that releases thermal energy to the surroundings, causing the temperature of the surroundings to rise.

Endothermic Reaction

A reaction that absorbs thermal energy from the surroundings, causing the temperature of the surroundings to fall.

Enthalpy (H)

The chemical energy stored in a substance, measured in joules.

Enthalpy Change (ΔH)

The transfer of thermal energy during a reaction; negative for exothermic processes and positive for endothermic processes.

Sign of ΔH for Exothermic

ΔH < 0 (negative), indicating energy is released.

Sign of ΔH for Endothermic

ΔH > 0 (positive), indicating energy is absorbed.

Calorie

An older energy unit; 1 calorie equals 4.2 joules.

Joule (J)

The SI unit of energy used to measure enthalpy and enthalpy changes.

Energy-Level Diagram

A graph showing relative energies of reactants and products, illustrating ΔH and activation energy.

Activation Energy (Ea)

The minimum energy required to start a chemical reaction by breaking necessary bonds.

Bond Energy

The energy needed to break one mole of a specific covalent bond in the gas phase.

Bond Breaking

An endothermic process that absorbs energy from the surroundings.

Bond Making

An exothermic process that releases energy to the surroundings.

Bond-Energy Rule for Reaction Type

A reaction is exothermic when ΣE(bonds broken) < ΣE(bonds formed) and endothermic when ΣE(bonds broken) > ΣE(bonds formed).

Combustion of Carbon

C(s) + O2(g) → CO2(g); ΔH = –349 kJ mol⁻¹ (exothermic).

Photosynthesis

6CO2 + 6H2O → C6H12O6 + 6O2; ΔH = +2800 kJ mol⁻¹ (endothermic, driven by light energy).

Hand Warmer

A practical device that produces heat through an exothermic reaction, often reusable via a reverse endothermic process.

Cold Pack

A sports-injury pack that cools rapidly using an endothermic reaction (e.g., dissolving NH4NO3).

Self-Heating Can

Packaging in which an exothermic reaction heats food or drink when a seal is broken.

Hot Pack

A disposable heat source that releases heat through exothermic dissolution of CaCl2 or MgSO4.

Reusable Heat Pack

A sodium-acetate system that crystallises exothermically and is reset by endothermic melting.

Steps to Draw an Energy-Level Diagram

1 Identify reaction type; 2 draw energy axis; 3 add reactant and product energy levels; 4 arrow from reactants to products; 5 label species; 6 mark ΔH sign.

Bond-Energy Calculation Formula

ΔH = ΣE(bonds broken) – ΣE(bonds formed).

Energy In

Total bond energies of reactants that must be supplied to break bonds.

Energy Out

Total bond energies released when new bonds form in the products.

Neutralisation Reaction

Combination of acid and base that is typically exothermic (ΔH negative).

Thermal Decomposition

A reaction in which heat is absorbed to break compounds, making it endothermic (ΔH positive).

Activation-Energy Source Example

A lighter provides Ea to initiate combustion by supplying heat.

Energy-Change Sign Rule

ΔH negative → reaction gives out heat; ΔH positive → reaction absorbs heat.