1 Enthalpy Definitions

Enthalpy change of formation The standard enthalpy change of formation of a compound is the energy transferred when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), all reactants and products being in their standard states Na (s) + ½Cl2 (g)  NaCl (s) [fH = – 411.2 kJ mol-1 ]Enthalpy of atomisation The enthalpy of atomisation of an element is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state Na (s)  Na(g) [atH = +148 kJ mol-1 ] ½ O2 (g)  O (g) [atH = +249 kJ mol-1 ] The enthalpy change for a solid metal turning to gaseous atoms can also be called the Enthalpy of sublimation and will numerically be the same as the enthalpy of atomisation Na (s)  Na(g) [Hsub = +148 kJ mol-1 ] Bond dissociation enthalpy (bond energy) The bond dissociation enthalpy is the standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms (or free radicals) Cl2 (g)  2Cl (g) dissH = +242 kJ mol-1 Or CH4 (g)  CH3 (g) + H(g) dissH = +435 kJ mol-1 For diatomic molecules the dissH of the molcule is the same as 2x atH of the element Cl2 (g)  2Cl (g) Hdiss = +242 kJ mol-1 ½ Cl2 (g)  Cl (g) atH = +121 kJ mol-1 First Ionisation enthalpy The first ionisation enthalpy is the enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a +1 charge Mg (g)  Mg+ (g) + e- [ IE 1H] Second Ionisation enthalpy The second ionisation enthalpy is the enthalpy change to remove 1 mole of electrons from one mole of gaseous 1+ ions to produces one mole of gaseous 2+ ions. Mg+ (g)  Mg 2+ (g) + e- [ IE 2H] First Electron affinity The first electron affinity is the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a –1 charge O (g) + e-  O- (g) [eaH] = -141.1 kJ mol-1 ] The first electron affinity is exothermic for atoms that normally form negative ions because the ion is more stable than the atom and there is an attraction between the nucleus and the electron second electron affinity The second electron affinity is the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions. O – (g) + e-  O2- (g) [eaH = +798 kJ mol-1 ] The second electron affinity for oxygen is endothermic because it take energy to overcome the repulsive force between the negative ion and the electron Enthalpy of lattice formation The Enthalpy of lattice formation is the standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form. Na+ (g) + Cl- (g)  NaCl (s) [H Latt = -787 kJ mol-1 ] Enthalpy of lattice dissociation The Enthalpy of lattice dissociation is the standard enthalpy change when 1 mole of an ionic crystal lattice form is separated into its constituent ions in gaseous form. NaCl (s)  Na+ (g) + Cl- (g) [H Latt = +787 kJ mol-1 ] Note the conflicting definitions and the sign that always accompanies the definitions Enthalpy of Hydration Hhyd Enthalpy change when one mole of gaseous ions become aqueous ions . X+ (g) + aq  X+ (aq) For Li+ hydH = -519 kJ mol-1 or X- (g) + aq  X- (aq) For F- hydH= -506 kJ mol-1 This always gives out energy (exothermic, -ve) because bonds are made between the ions and the water molecules Enthalpy of solution The enthalpy of solution is the standard enthalpy change when one mole of an ionic solid dissolves in an large enough amount of water to ensure that the dissolved ions are well separated and do not interact with one another NaCl (s) + aq  Na+ (aq) + Cl-(aq)