Chemistry: Enthalpy, Reactions, and Ionic Lattice Concepts

Exothermic reaction

A reaction that releases energy to the surroundings (ΔH is negative). This usually happens when bonds are formed, releasing energy.

Endothermic reaction

A reaction that absorbs energy from the surroundings (ΔH is positive). This usually happens when bonds are broken, requiring energy.

Enthalpy of formation (ΔHf°)

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states (298 K and 100 kPa).

Ionization energy

The energy required to remove an electron from a gaseous atom to form a positive ion. Always endothermic.

First ionization energy

The energy required to remove the first electron from 1 mole of gaseous atoms to form 1 mole of +1 ions.

Second ionization energy

The energy required to remove a second electron from a +1 ion to form a +2 ion. Always endothermic and larger than the first.

Electron affinity

The energy change when an electron is added to a gaseous atom to form a negative ion.

First electron affinity

The enthalpy change when 1 mole of gaseous atoms gains 1 electron to form 1 mole of negative ions. Usually exothermic.

Second electron affinity

The enthalpy change when an electron is added to a negatively charged ion. Always endothermic due to repulsion.

Solute

The substance that is dissolved in a solution.

Solvent

The liquid that dissolves the solute.

Solution

A homogeneous mixture formed when a solute dissolves in a solvent.

Reversible reaction

A reaction that can proceed in both forward and reverse directions, often involving both exothermic and endothermic steps.

Enthalpy of atomization (ΔHat°)

The enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state. Always endothermic.

Bond dissociation enthalpy

The energy required to break 1 mole of a covalent bond in gaseous molecules into separate atoms. Always endothermic.

Enthalpy of hydration (ΔHhyd°)

The enthalpy change when 1 mole of gaseous ions dissolves in water to form aqueous ions. Always exothermic.

Enthalpy of solution (ΔHsol°)

The enthalpy change when 1 mole of a substance dissolves in water to form a solution. Can be exothermic or endothermic.

Lattice enthalpy (formation)

The enthalpy change when 1 mole of an ionic solid is formed from its gaseous ions. Exothermic.

Lattice enthalpy (dissociation)

The enthalpy change when 1 mole of an ionic solid is separated into its gaseous ions. Endothermic.

Lattice enthalpy strength

A measure of the strength of attraction between ions in an ionic lattice. Greater lattice enthalpy means stronger ionic bonding.

Born-Haber cycle

A thermochemical cycle that uses Hess's Law to calculate lattice enthalpy of ionic compounds.

Purpose of Born-Haber cycle

To determine lattice enthalpy using known enthalpy changes such as ionization energy, electron affinity, and atomization.

Hess's Law

The total enthalpy change of a reaction is the same regardless of the path taken.

Key rule of Born-Haber cycles

The sum of all enthalpy changes in the cycle equals zero (energy conservation).

Steps in Born-Haber cycle

1. Atomization of elements → 2. Ionization of metal → 3. Electron affinity of non-metal → 4. Lattice formation.

Sign reversal rule

If you go against the direction of an arrow in a Born-Haber cycle, you must reverse the sign of the enthalpy value.

Lattice enthalpy in dissolution

The energy required to break the ionic lattice into gaseous ions (endothermic).

Hydration enthalpy in dissolution

The energy released when gaseous ions interact with water to form aqueous ions (exothermic).

Relationship in dissolution

ΔHsol = ΔHlatt + ΔHhyd

Dissolution process

Dissolving involves two steps: breaking ionic bonds (endothermic) and forming ion-dipole interactions with water (exothermic).

Charge of ions

Higher charges increase electrostatic attraction, leading to higher lattice enthalpy.

Ionic radius

Smaller ions allow stronger attraction, increasing lattice enthalpy.

Overall effect on lattice enthalpy

Higher charge and smaller size = stronger attraction = more exothermic lattice formation.

Polarization in ionic compounds

Distortion of an anion's electron cloud by a cation, leading to partial covalent character.

Factors increasing polarization

Small, highly charged cations and large, highly charged anions.

Effect of polarization

Increases covalent character and can make experimental lattice enthalpy differ from theoretical values.