Unit 5: Chemical Energetics and Atomic Structure Definitions

A collection of key definitions and enthalpy change concepts from Chemistry Unit 5, covering atomic structure, bonding, and chemical energetics.

Isotope

Atoms of the same element which have the same proton number, but a different nucleon number.

Atomic orbital

A region of space around the nucleus of an atom that can be occupied by one or two electrons.

First ionisation energy ($IE_1$)

The energy needed to remove one electron from each atom in one mole of atoms of the element in the gaseous state to form one mole of gaseous $1+$ ions.

Electronegativity

The power of an atom to attract electrons to itself.

Ionic bonding

The electrostatic attraction between oppositely charged ions.

Metallic bonding

The electrostatic attraction between positive metal ions and delocalised electrons.

Covalent bonding

Electrostatic attraction between the nuclei of two atoms and a shared pair of electrons.

Bond energy

The energy required to break one mole of a particular covalent bond in the gaseous state.

Standard enthalpy change of reaction ($\Delta H_r^{\theta}$)

The enthalpy change when the amounts of reactants shown in the equation react to give products under standard conditions, where reactants and products must be in their standard states.

Standard enthalpy change of formation ($\Delta H_f^{\theta}$)

The enthalpy change when one mole of a compound is formed from its elements under standard conditions, where reactants and products must be in their standard states. For the formation of $1$ mol of $CS_2$: $C(\text{graphite}) + 2S(s) \rightarrow CS_2(l)$, $\Delta H = +98.7\,kJ\,mol^{-1}$.

Standard enthalpy change of formation of an element

The standard enthalpy change of formation of any element in its standard state is $0$. There will be an energy change only with compounds.

Standard enthalpy change of combustion ($\Delta H_c^{\theta}$)

The enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions, where reactants and products must be in their standard states.

Standard enthalpy change of neutralisation

The enthalpy change when one mole of water is formed by the reaction of an acid with an alkali under standard conditions.

Standard enthalpy change of solution ($\Delta H_{sol}^{\theta}$)

The enthalpy change when one mole of solute is dissolved in solvent to form an infinitely dilute solution under standard conditions.

Standard enthalpy change of atomisation ($\Delta H_{at}^{\theta}$)

The enthalpy change when one mole of gaseous atoms is formed from its elements under standard conditions.

Standard enthalpy change of hydration ($\Delta H_{hyd}^{\theta}$)

The enthalpy change when one mole of a hydrated salt is formed from one mole of the anhydrous salt under standard conditions. Example: $Na_2S_2O_3(s) + 5H_2O(l) \rightarrow Na_2S_2O_3.5H_2O(s)$, $\Delta H = -55.0\,kJ\,mol^{-1}$.