THERMODYNAMICS

Thermodynamics

a branch of physical chemistry which deals with the heat, temperature and physical properties of matter

Ions in lattices

are held together by ionic bonds

lattice formation enthalpy

enthalpy change when 1 mole of a solid ionic compound in formed from its gaseous ions

lattice dissociation enthalpy

the enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions

lattice enthalpy can’t be measured directly

instead you have to combine the enthalpies from a number of other processes to work out the lattice enthalpy

lattice enthalpy=

atomisation+ionisation+electron affinity+formation

Enthalpy change of formation

enthalpy change when 1 mole of a compound is formed from its elements in their standard states

bond dissociation enthalpy

enthalpy change when all bonds of the same type in 1 mole of gaseous molecules are broken

enthalpy change of atomisation of an element

the enthalpy change when 1 mole of gaseous atoms is formed from an element in it standard state

enthalpy change of atomisation of a compound

enthalpy change when 1 mole of a compound in its standard state is converted to gaseous atoms

first ionisation energy

enthalpy change when 1 mole of gaseous 1+ ion is formed from 1 mole of gaseous atoms

second ionisation energy

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

first electron affinity

enthalpy change when 1 mole of gaseous 1- ions is made from 1 mole of gaseous atoms

second electron affinity

enthalpy change when 1 mole of gaseous 2- ions is made from 1 mole of gaseous 1- ions

enthalpy change of hydration

enthalpy change when 1 mole of aqueous ions is formed from gaseous ions

enthalpy change of solution

enthalpy change when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution

born-haber cycle

• enthalpy of Formation

• enthalpies of Atomisation

• enthalpies of Ionisation

• electron Affinity

• lattice enthalpy of ionic compounds

Hess’ law

total enthalpy change of a reaction is always the same no matter which route is taken

born haber cycles for compounds containing group 2 elements have an extra step

purely ionic model of lattice assumptions:

• all ions are spherical

• all ions have evenly distributed charge around them

theoretical enthalpy is always different from experimental enthalpy

this is evidence ionic compounds usually have some covalent character

positive and negative ions in lattice are usually exactly spherical

positive ions polarise neighbouring ions to different extents, the more polarisation there is the more covalent the bonding will be

2 things happen when a solid ionic lattice dissolves in water

• Lattice enthalpy of dissociation: bonds between ions break to make gaseous ions, endothermic

• Enthalpy change of hydration: bonds between ions and water are made, exothermic

water can form bonds with ions because it is polar

oxygen is more electronegative that hydrogen, so it draws the bonding electrons towards itself, creating a dipole

positive ions form weak bonds

with the partial negative charge on the oxygen atom of water

negative ions for weak bonds

with the partial positive charge on the hydrogen atom of water

enthalpy change of solution is the overall effect…

…on the enthalpy of bond breaking and bond making

enthalpy change of solution can be calculated

Enthalpy of hydration of positive and negative ions - lattice dissociation enthalpy

entropy

the measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles

dissolution

dissolving a solid also increases its entropy as dissolved particles can move freely as they are no longer held in place

more particles means more entropy

the more particles there are the more ways they and their energy can be arranged

calculating entropy change

products - reactants

free energy change

is a measure used to predict whether a reaction is feasible

a reaction is feasible when

ΔG is negative or equal to 0

negative ΔG doesn’t guarantee a reaction will occur or tell you about its rate

the reaction may only be theoretically feasible, with an extremely high activation energy or have such a slow rate you wouldn’t be able to notice tis happening at all

exothermic reactions and POSITIVE ENTROPY change

these reactions are feasible at any temperature

endothermic reactions and has a NEGATIVE ENTROPY change

these reactions are not feasible at any temperature

if the reaction is endothermic and entropy is positive

the reaction will only be feasible above a certain temperature

if the reaction is exothermic and enthropy is is positive

reaction is only feasible below a certain temperature

when ΔG is 0

a reaction is JUST feasible

variation in ionisation energy, FROM STRONGEST TO WEAKEST

O²⁺>O> O^- >O^2-

CATION>NEUTRAL ELEMENT>ANION