Thermochemistry & Thermodynamics

Adiabatic Process

A process in which no heat is exchanged between the system and its surroundings.

q = 0, ΔU = w J

Bomb Calorimeter

A rigid sealed vessel used to measure the heat released by combustion at constant volume.

q_V = ΔU J

Thermochemistry & Thermodynamics

Bond Dissociation Enthalpy

The enthalpy change when one mole of a specific bond is broken homolytically in the gas phase.

A–B(g) → A·(g) + B·(g) kJ/mol

Thermochemistry & Thermodynamics

Calorific Value

The heat released by the complete combustion of unit mass of a fuel.

CV = ΔH_c / molar mass kJ/g

Thermochemistry & Thermodynamics

Calorimetry

The experimental measurement of heat exchanged during a physical or chemical change.

q = mcΔT J

Thermochemistry & Thermodynamics

Carnot Cycle

An idealised reversible cycle of two isothermal and two adiabatic steps that defines the maximum efficiency of a heat engine.

η = 1 − T_cold / T_hot Dimensionless

Thermochemistry & Thermodynamics

Closed System

A system that can exchange energy but not matter with its surroundings.

Dimensionless

Thermochemistry & Thermodynamics

Coupled Reactions

Two reactions linked through a shared intermediate so that a favourable one drives an unfavourable one.

ΔG_total = ΔG₁ + ΔG₂ < 0 kJ/mol

Thermochemistry & Thermodynamics

Cyclic Process

A process in which a system undergoes a series of changes and returns to its exact initial state.

ΔU = 0, q = −w J

Thermochemistry & Thermodynamics

Endergonic Reaction

A reaction with a positive Gibbs free energy change, which requires an input of energy to proceed.

ΔG > 0 kJ/mol

Thermochemistry & Thermodynamics

Enthalpy

A thermodynamic state function equal to internal energy plus the product of pressure and volume.

H = U + pV kJ/mol

Thermochemistry & Thermodynamics

Enthalpy Level Diagram

A diagram plotting the enthalpies of reactants and products to show the direction and size of the enthalpy change.

kJ/mol

Thermochemistry & Thermodynamics

Enthalpy of Atomisation

The enthalpy change when one mole of gaseous atoms is produced from an element in its standard state.

ΔH_at° kJ/mol

Thermochemistry & Thermodynamics

Enthalpy of Dilution

The enthalpy change when a solution is diluted from one concentration to another by adding solvent.

ΔH_dil kJ/mol

Thermochemistry & Thermodynamics

Enthalpy of Fusion

The enthalpy change when one mole of a solid melts to a liquid at its melting point.

ΔH_fus kJ/mol

Thermochemistry & Thermodynamics

Enthalpy of Neutralisation

The enthalpy change when an acid and a base react to form one mole of water under standard conditions.

H⁺ + OH⁻ → H₂O, ΔH = −57.1 kJ kJ/mol

Thermochemistry & Thermodynamics

Enthalpy of Reaction

The heat absorbed or released when a reaction occurs at constant pressure in the molar amounts of the balanced equation.

ΔH_rxn = ΣΔH_f(products) − ΣΔH_f(reactants) kJ/mol

Thermochemistry & Thermodynamics

Enthalpy of Sublimation

The enthalpy change when one mole of a solid converts directly into vapour.

ΔH_sub = ΔH_fus + ΔH_vap kJ/mol

Thermochemistry & Thermodynamics

Enthalpy of Vaporisation

The enthalpy change when one mole of a liquid becomes vapour at its boiling point.

ΔH_vap kJ/mol

Thermochemistry & Thermodynamics

Entropy

A measure of the dispersal of energy and the number of microscopic arrangements available to a system.

S = k ln W J/(mol·K)

Thermochemistry & Thermodynamics

Entropy Change of the Surroundings

The entropy change experienced by the surroundings when they absorb or release heat from a system.

ΔS_surr = −ΔH_sys / T J/K

Thermochemistry & Thermodynamics

Exergonic Reaction

A reaction with a negative Gibbs free energy change, which therefore releases usable energy and proceeds spontaneously.

ΔG < 0 kJ/mol

Thermochemistry & Thermodynamics

First Law of Thermodynamics

The energy of an isolated system is constant; energy can be converted between forms but never created or destroyed.

ΔU = q + w J

Thermochemistry & Thermodynamics

Free Energy and Equilibrium Constant

The relation connecting the standard Gibbs energy change of a reaction to its equilibrium constant.

ΔG° = −RT ln K kJ/mol