ENERGETICS TERMS + DEFINITIONS

enthalpy change (ΔH)

heat energy change measured under conditions of constant pressure

enthalpy of formation

enthalpy change when 1 mole of a substance is formed from its constituent elements with all substances in their standard states

enthalpy of combustion

enthalpy change when 1 mole of a substance undergoes complete combustion in oxygen with all substances in standard states

what are the standard conditions?

• 100kPa

• 298K

• (1 mol dm^-3)

Hess’s law definition

enthalpy change for a chemical reaction is independent of the route taken, given that the initial and final conditions are the same

Hess’s law is a version of the first law of thermodynamics, which states what?

energy is always conserved

first ionisation enthalpy

enthalpy change when 1 mole of gaseous atoms loses one electron per atom to produce gaseous 1+ ions

second ionisation energy

enthalpy change when one mole of gaseous 2+ ions is produced from one mole of 1+ ions

first electron affinity

enthalpy change when 1 mole of gaseous atoms gains 1 electron per atom to produce 1- ions

second electron affinity

enthalpy change when 1 mole of gaseous 1- ions gains 1 electron per ion to produce gaseous 2- ions

enthalpy of atomisation

enthalpy change when 1 mole of gaseous atoms is produced from an element in its standard state

hydration enthalpy

enthalpy change when 1 mole of gaseous ions becomes hydrated/dissolved in water

enthalpy of solution

enthalpy change when 1 mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and don’t interact with each other

bond dissociated enthalpy

enthalpy change when 1 mole of covalent bonds is broken in gaseous state

lattice enthalpy of formation

enthalpy change when 1 mole of solid ionic compound is formed from its constituent ions in the gas phase

lattice enthalpy of dissociation

enthalpy change when 1 mole of solid ionic compound is broken up into its constituent ions in the gas phase

enthalpy of formation example with Na₂O

including state symbols

2Na (s) + ½ O₂ (g) → Na₂O (s)

enthalpy of combustion example with hydrogen

including state symbols

H₂ (g) + ½ O₂ (g) → H₂O (g)

1st ionisation enthalpy/energy example with magnesium

including state symbols

Mg (g) → Mg⁺ (g) + e^-

2nd ionisation enthalpy/energy example with magnesium

including state symbols

Mg⁺ (g) → Mg²⁺ (g) + e^-

1st electron affinity example with oxygen

including state symbols

O (g) + e^- → O^- (g)

2nd electron affinity example with oxygen

including state symbols

O^- (g) + e^- → O^2- (g)

enthalpy of atomisation example with iodine

including state symbols

½ I₂ (s) → 2I (g)

hydration enthalpy example with magnesium ions

including state symbols

Mg²⁺ (g) + aq → Mg²⁺ (aq)

enthalpy of solution example with MgCl₂

including state symbols

MgCl₂ (s) + aq → Mg²⁺ (aq) + 2Cl^- (aq)

bond dissociation enthalpy example with I₂

including state symbols

I₂ (g) → 2I (g)

lattice enthalpy of formation example with Mg²⁺

including state symbols

Mg²⁺ (g) + 2Cl^- (g) → MgCl₂ (s)

lattice enthalpy of dissociation example with MgCl₂

including state symbols

MgCl₂ (s) → Mg²⁺ (g) + 2Cl^- (g)