Essential Enthalpy Definitions

You'll encounter loads of enthalpy terms in chemistry, but they all follow the same basic principle: energy change per mole. Each definition describes what happens when exactly one mole undergoes a specific process.

Formation enthalpy (ΔHf) tells you the energy change when making a compound from its elements. For example, making sodium oxide from sodium metal and oxygen gas is exothermic - it releases energy. Combustion enthalpy (ΔHc) measures energy released when substances burn completely in oxygen, like methanol burning to produce carbon dioxide and water.

Bond dissociation enthalpy and atomisation enthalpy both involve breaking things apart, so they're endothermic (require energy input). Breaking iodine molecules into separate atoms or turning solid iodine into gaseous atoms both need energy.

Ionisation energies are always endothermic because you're forcing electrons away from atoms. The first ionisation energy removes one electron, whilst the second removes another from an already positive ion - which requires even more energy.

💡 Remember: Formation and combustion are usually exothermic, whilst bond breaking and ionisation are always endothermic.

Advanced Enthalpy Types and Calculations

Electron affinity shows a split personality - the first is usually exothermic (atoms actually want that first extra electron), but the second is endothermic because you're forcing another electron onto an already negative ion.

Lattice energies come in two flavours: formation (exothermic - ions love coming together) and dissociation (endothermic - breaking apart ionic compounds). Hydration enthalpy describes when gaseous ions dissolve in water, releasing energy as they get surrounded by water molecules.

The key calculation formulas work like algebraic equations. For combustion: products minus reactants using standard values. For mean bond enthalpy: energy needed to break bonds minus energy released forming new ones.

Entropy and Gibbs free energy determine if reactions actually happen. When ΔG° equals zero, you've found the temperature where a reaction just balances - neither favouring products nor reactants.

🔥 Pro tip: Solution enthalpy equals lattice dissociation plus hydration enthalpy - it's like a two-step energy journey from solid to dissolved ions.

Enthalpy Change Overview

Enthalpy change (ΔH) represents the heat energy transferred during chemical reactions at constant pressure. This fundamental concept connects all the specific enthalpy types you've learned.

Whether you're calculating formation enthalpies, combustion reactions, or lattice energies, you're essentially measuring how much energy gets absorbed or released when chemical bonds break and form.

⚡ Key insight: All enthalpy calculations follow the same pattern - energy in minus energy out determines whether reactions feel hot or cold to touch.