enthalpy changes

enthalpy of formation definition

energy of one mol of compound to be formed from its element at standard conditions

ionisation energy / enthalpy definition

amount of energy required to remove one mol of electrons from one mole of a gaseous compound under standard conditions to form a gaseous ion

is always endothermic (positive enthalpy change)

why is ionisation energy always endothermic

energy must be used to overcome attraction between an electron and nucleus

standard enthalpy change of atomisation

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

is always endothermic (positive enthalpy change)

why is standard enthalpy change of atomisation always endothermic

energy is required to break the bonds between the atoms to break the element into gaseous atoms

Bond enthalpy definition

he amount of energy required to break one mole of a specific covalent bond in the gas phase is called the bond dissociation energy

• Bond dissociation energy (E) is usually just simplified to bond energy or bond enthalpy

Bond enthalpy is usually treated as a bond breaking process, so it is quoted in data tables as an endothermic energy change with positive values

expressed per mole of bonds

Lattice enthalpy definition

• As a formation process, it is the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions (under standard conditions)

• exothermic because ions are combined to form a solid ionic lattice, which releases energy

Larger negative lattice enthalpy means… (4)

ionic compound is more stable than its gaseous ions

due to strong electronegative forces of attraction between atoms

no electrostatic forces of attraction between the ions in the gas phase, the gaseous ions are less stable than the ions in the ionic lattice

The more exothermic the value is, the stronger the ionic bonds within the lattice are

explain how and why the lattice enthalpy of magnesium fluoride differs from that of calcium chloride (6)W

-ions have he same charges in both compounds

F- has a smaller radius than Cl-

Mg2+ has a smaller radius than Ca2+
Mg2+ and F- have higher charge densities than Ca2+ / Cl-

Are more strongly attracted to each other

Stronger ionic bonds, more stable, so lattice enthalpy is more exothermic

why is the first ionisation energy of calcium less positive than the first ionisation

• less positive means more exothermic

• Removing an electron from Ca+ is more difficult than Ca since in Ca+ there are less electrons, in the outer shell, so the electrons are slightly closer together

• due to less repulsion between them

How does number of electrons / electron shells affect lattice enthalpy

• higher ionic charge = higher charge density, so lattice enthalpy is more exothermic

• less electrons / electron shells = smaller ionic radius, higher charge density, so lattice enthalpy is more exothermic

standard enthalpy of solution definition

energy when one mol of ionic substance dissolves in sufficient water to form an infinitely dilute solution

enthalpy of hydration definition

enthalpy change when one mole of gaseous ion are transferred from gas form to water to form aqueous solution

lower enthalpy of hydration means (3)

• higher charge density of ion

• stronger attraction to dipoles of water molecules

• therefore more exothermic

how are enthalpy of solution and hydration enthalpy related

enthalpy of solution = reverse lattice enthalpy + hydration enthalpy