Unit 1: Section 6 - Thermodynamics CDS * Flashcards
enthalpy definitions lattice enthalpy and born-haber cycles enthalpies of solution entropy free energy change
what is enthalpy change?
the heat energy transferred in a reaction at constant pressure
what are standard conditions?
298 K
100 kPa
what is the enthalpy change for exothermic and endothermic reactions?
exothermic - negative ΔH, heat is given out
endothermic - positive ΔH, heat is taken in
what is the enthalpy change of formation?
the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
what is bond dissociation enthalpy?
the enthalpy change when all the bonds of the same type in 1 mole of gaseous molecule are broken
what is the enthalpy change of atomisation?
the enthalpy change when 1 mole of monatomic gaseous atoms is formed from an element/compound in its standard state
what is the first ionisation energy?
the enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms
what is the second ionisation energy?
the enthalpy change when 1 mole of gaseous 2+ ions is formed from 1 mole of gaseous 1+ ions
what is the first electron affinity?
the enthalpy change when 1 mole of gaseous 1- ions is formed from 1 mole of gaseous atoms
what is the second electron affinity?
the enthalpy change when 1 mole of gaseous 2- ions is formed from 1 mole of gaseous 1- ions
what is the enthalpy change of hydration?
the enthalpy change when 1 mole of aqueous ions is formed from 1 mole of gaseous ions
what is the enthalpy change of solution?
the enthalpy change when 1 mole of an ionic compound dissolves into its aqueous ions
what is the lattice enthalpy of formation?
the enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions
what is lattice enthalpy of dissociation?
the enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions under standard conditions
what does Hess’s law state?
the total enthalpy change of a reaction is always the same, no matter which route is taken
what is a born-haber cycle?
start with the enthalpy of formation (elements in standard states –> ionic lattice)
then use enthalpy of atomisation and ionisation ( turn into gas, and turn diatomic molecules into monatomic e.g. Cl2 –> 2Cl then remove electrons)
use electron affinity (electron from ionisation added to other atom)
then lattice enthalpy of formation (turns gaseous ions into ionic lattice)
how to work out theoretical lattice enthalpy?
doing some calculations on the purely ionic model of a lattice.
what is the purely ionic model of a lattice?
it assumes that all ions are spherical, and have their charge evenly distributed around them
what is experimental lattice enthalpy?
finding lattice enthalpy from experiments, the value is often different to theoretical because most ionic compounds have some covalent character
what is covalent character?
the positive and negative ions in a lattice aren’t usually exactly spherical.
positive ions polarise neighbouring negative ions to different extents, the more polarisation, the more covalent the bonding
what does it mean if a ionic compound has covalent character?
the bonding in the lattice is stronger than expected, because covalent bonds are stronger than ionic.
experimental enthalpy will be a lot higher than theoretical
what happens when a solid ionic lattice dissolves?
bonds between ions break to give free ions - endothermic
bonds between ions and water are made - exothermic
enthalpy change of solution is the overall effect on the enthalpy of these
why can water molecules bond to ions?
oxygen is more electronegative than hydrogen, so it draws electrons towards itself, creating a dipole.
positive hydrogen can form bonds with negative ions
negative oxygen form bonds with positive ions
when do substances dissolve?
substances generally only dissolve if the energy released is roughly the same, or greater than the energy taken in.
so soluble substances tend to have exothermic enthalpies in solution