17.1 Enthalpy change

• Standard molar enthalpy of formation is the enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, all reactants and products in their standard states
• The standard enthalpy of formation of an element is zero
• Standard molar enthalpy change of combustion is the enthalpy change when one mole of substance is completely burnt in oxygen
• Standard enthalpy of atomisation is the enthalpy change which accompanies the formation of one mole of gaseous atoms from the element in its standard state under standard conditions
• First ionisation energy is the standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single positive charge
• Second ionisation energy refers to the loss of a mole of electrons from a mole of singly positively charged ions
• Ionisation enthalpies are always positive because energy has to be put in to pull an electron away from the attraction of the positively charged nucleus of the atom - therefore they are endothermic
• First electron affinity is the standard enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous ions, each with a single negative charge
• Second electron affinity is the enthalpy change when a mole of electrons is added to a mole of gaseous ions each with a single negative charge to form ions each with two negative charges
• Lattice enthalpy of formation is the standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions
• • First electron affinities refer to the single atoms and not the whole molecule
  • They are always negative because energy is given out when an electron is attracted to the positively charged nucleus of an atom
• Second electron affinities are always positive because energy must be put in to overcome the repulsion between an electron and a negatively charged ion
• Lattice enthalpy of dissociation is the standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions