Unit 6 · Thermodynamics

Endothermic process ● (core concept) — A process in which the system gains energy from its surroundings (e.g., ice melting, water evaporating); the surroundings cool as a result.

Exothermic process ● (core concept) — A process in which the system loses energy to its surroundings (e.g., water condensing, freezing); the surroundings warm as a result.

System and surroundings ● (core concept) — Energy lost by the system is gained by the surroundings, and vice versa; energy is never created or destroyed in the exchange.

Heat of solution ● (core concept) — The enthalpy change when a substance dissolves; dissolving can be endothermic or exothermic depending on the relative strengths of the solute and solvent intermolecular forces.

Energy diagram ● (core concept) — A diagram showing the relative energies of reactants and products, indicating whether a process is endothermic or exothermic.

Heat transfer ● (core concept) — Energy transferred between objects through molecular collisions; the terms 'heat transfer' and 'heat exchange' describe this, not a substance called heat.

Thermal equilibrium ● (core concept) — The state reached when objects in contact no longer exchange net energy; they have the same average kinetic energy and temperature.

Specific heat capacity (c) ● (core concept) — The energy needed to raise 1 gram of a substance by 1°C: q = mcΔT.

Molar heat capacity ● (core concept) — The energy needed to raise 1 mole of a substance by 1°C.

Calorimetry ● (core concept) — The experimental measurement of heat transferred in a process, often using a calorimeter.

First law of thermodynamics ● (core concept) — Energy is conserved: the energy lost by the system in a process equals the energy gained by the surroundings.

Molar enthalpy of fusion and vaporization ● (core concept) — The energy required to melt (ΔH_fus) or vaporize (ΔH_vap) one mole of a substance at constant temperature; freezing and condensation release an equal and opposite amount of energy.

Phase changes at constant temperature ● (core concept) — Melting, freezing, vaporization, and condensation occur at constant temperature while energy is absorbed or released.

Enthalpy change (ΔH) ● (core concept) — The heat absorbed or released by a process at constant pressure: negative ΔH means heat is released (exothermic), positive ΔH means heat is absorbed (endothermic).

Bond enthalpy (bond energy) ● (core concept) — The average energy required to break one mole of a particular bond in the gas phase. Reaction enthalpy is estimated as Σ(bond enthalpies of bonds broken) − Σ(bond enthalpies of bonds formed).

Bond breaking vs. bond forming ● (core concept) — Breaking bonds always requires energy (endothermic); forming bonds always releases energy (exothermic).

Standard enthalpy of formation (ΔH_f°) ● (core concept) — The enthalpy change when one mole of a compound forms from its elements in their standard states. Reaction enthalpy: ΔH°_rxn = ΣΔH_f°(products) − ΣΔH_f°(reactants).

Hess's law ● (core concept) — The enthalpy change of an overall reaction equals the sum of the enthalpy changes of the steps that make it up. Reversing a reaction flips the sign of ΔH; scaling a reaction scales ΔH; adding reactions adds their ΔH values.