P2: 17 Thermodynamics Flashcards
What can lattice enthalpy be defined as?
Either enthalpy of lattice dissociation
or enthalpy of lattic formation.
Define enthalpy change of formation.
Enthalpy change when one mole of a substance is formed from its constituent elements, all substances in their standard states under standard conditions.
Define bond dissociation enthalpy.
The enthalpy change when one mole of covalent bonds is broken in the gaseous state.
Define enthalpy of atomisation of an element.
The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state
Define enthalpy change of atomisation of a compound.
The enthalpy change when one mole of a compound in its standard state is converted to gaseous atoms.
Define first ionisation energy.
The enthalpy change when one mole of gaseous 1+ ions is formed from one mole of gaseous atoms.
Define second ionisation energy.
The enthalpy change when one mole of gaseous 2+ ions is formed from one mole of gaseous 1+ ions.
Define first electron affinity.
The enthalpy change when one mole of gaseous 1- ions is formed from one mole of gaseous atoms.
Define second electron affinity.
The enthalpy change when one mole of gaseous 2- ions is formed from one mole of gaseous 1- ions.
Define enthalpy change of hydration.
The enthalpy change when one mole of aqueous ions is formed from one mole of gaseous ions.
Define enthalpy change of solution.
The enthalpy change when one mole of a solid ionic compound dissolves in enough water so that the dissolved ions are well separated and do not interact with each other.
Define lattice enthalpy of formation.
The enthalpy change when one mole of a solid ionic compound is formed from its constituent gaseous ions under standard conditions.
Define lattic enthalpy of dissociation.
The enthalpy change when one mole of a solid ionic compound is completely dissociated into its gaseous ions under standard conditions.
Why are theoretical lattic enthalpies often different from experimental values?
Theoretical lattic enthalpy is worked out by doing calculations based on the perfect ionic model of a lattice which assumes all ions are spherical, and have their charge evenly distributed around them.
Experimental lattic enthalpies are worked out from Born-Haber cycles. They are different due to the covalent character of most ionic compounds.
The ions in a lattice aren’t usually exactly spherical. Positive ions polarise neighbouring negative ions, and the more polarisation there is, the more covalent character the lattice will have.
When does entropy increase?
When disorder increases.
