CHEMICAL ENERGETICS + EQUILIBRIA DEFINITIONS

STANDARD ENTHALPY CHANGE OF REACTION

The enthalpy change when the reactants in the stoichiometric equation react to form the products under standard conditions.

STANDARD ENTHALPY CHANGE OF COMBUSTION

The enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions.

STANDARD ENTHALPY CHANGE OF FORMATION

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

STANDARD ENTHALPY CHANGE OF NEUTRALISATION

The enthalpy change when one mole of water is formed by reacting an acid and an alkali under standard conditions.

Hydration enthalpy

The hydration enthalpy is the enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form a very dilute solution under standard conditions. Hydration enthalpies are always negative.

Enthalpy change of solution

The enthalpy change of solution is the enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form a very dilute solution under standard conditions.

Enthalpy of atomisation

The enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions.

Lattice energy

The energy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions.

1st Electron afiinity

The enthalpy change when 1 mole of electrons is added to 1 mole gaseous atoms to form 1 mole of gaseous ions with a single negative charge under standard conditions.

Entropy Change ΔS

ΣS(products) - ΣS(reactants)

Gibbs Equation

ΔG = ΔH -TΔS

Hess’s law

The enthalpy change of a reaction is independent of the reaction route it takes.

What factors affect the electron affinities of an element?

Nuclear charge, Atomic radius, Shielding, Attraction between the incoming electron and nucleus.

If there is a stronger attraction between the incoming electron and the nucleus of the atom, is the electron affinity more or less exothermic.

More exothermic (more negative).

If there is a weaker attraction between the incoming electron and the nucleus of the atom, is the electron affinity more or less exothermic.

Less exothermic (more positive).

How does electron affinity change from group 11 to group 17

Electron affinities of non-metals become more exothermic across a period, with a maximum at Group 17.

How does electron affinity change going down the group.

The electron affinities generally become less exothermic for each successive element going down both Groups, apart from the first member of each Group (oxygen and fluorine respectively).

Why does fluorine and oxygen have more positive or less exothermic electron affinities than the next element in their respective groups

Oxygen and fluorine have very small atomic radii, resulting in a high electron density around their nuclei.
This leads to strong repulsion between the incoming electron and the electrons already present in the compact 2p orbitals.
The increased repulsion reduces the attraction between the nucleus and the incoming electron.
As a result, the first electron affinities of oxygen and fluorine are less exothermic or more positive than expected compared to the elements below them in their respective groups.

Gibbs free energy Definition

The energy change that takes into account both the entropy change of a reaction and the enthalpy change.

Partition coefficient

The ratio of the concentrations of a solute in two different immiscible solvents in contact with each other when equilibrium has been established.

Buffer solution

A solution that minimises change in pH when moderate amounts of acid or base are added.

Solubility product

The product of the concentrations of each ion in a saturated solution of a sparingly soluble salt at 298K raised to the power of their relative concentrations.

What factors affect lattice energy?

Charge and Ionic radius (charge density overall)

Does lattice energy decrease down the group?

Yes

Why does lattice energy decrease down the group

Higher charge density down the group, strong electrostatic force of attraction between oppositely charged ions, so more energy is released when lattice energy is formed, therefore the lattice energy becomes more negative and more exothermic.

Why are there often differences between experimental and theoretical lattice energies?

The theoretical lattice energy assumes completely ionic bonds between the ions when in realtiy they have some covalent character due to ion polarisation which alters the strength of the bonds.

Ion polarisation

The distortion of the electron cloud of an anion by a neighbouring cation.

Polarising power

The ability of a cation to attract the electron cloud of an anion.

Why does the solubility of group 2 hydroxides increase down the group

The hydration enthalpy of the cation will decrease down the group and the energy requried to break up the lattice will become more positive down the group. The decrease in lattice energy is greater than the decrease in hydration, therefore the enthalpy of solution becomes more exothermic.

Why does the solubility of group 2 sulphates decrease down the group

The charge density of the metal ion decreases down the group, so the hydration enthalpy of the cation will become more positive down the group. The energy requried to break up the lattice will decrease down the group. However the decrease in lattice energy is smaller than the decrease in hydration enthalpy therefore the enthalpy of solution becomes less exothermic.