M5S4: Enthalpy, Entropy, Free Energy

Enthalpy change of formation, Δ_fH

Enthalpy change when 1 mole of a compound is formed from its elements

Enthalpy change of atomisation of an element, Δ_atH

Enthalpy change when 1 mole of gaseous atoms is formed from an element

Enthalpy change of atomisation of an compound, Δ_atH

Enthalpy change when 1 mole of compound is converted to gaseous atoms

First ionisation energy

Energy needed to change 1 mole of gaseous atoms into 1 mole of gaseous 1+ ions

Second ionisation energy

Energy needed to change 1 mole of gaseous 1+ ions into 1 mole of gaseous 2+ ions

First electron affinity

Energy needed to change 1 mole of gaseous atoms into 1 mole of gaseous 1- ions

Second electron affinity

Energy needed to change 1 mole of gaseous 1- ions into 1 mole of gaseous 2- ions

Lattice enthalpy Δ_LEH

Enthalpy change when 1 mole of solid ionic compound is formed form its gaseous ions

Enthalpy change of hydration, Δ_hydH

Enthalpy change when 1 mole of gaseous ions is dissolved in water

Enthalpy change of solution, Δ_solH

Enthalpy change when 1 mole of solute is dissolved in a solvent, such as water

Factors affecting lattice enthalpy

• Ionic charge: Higher charge on ions = stronger electrostatic attraction between ions so more energy released when lattice forms

• Ionic radius: Smaller ionic radius of ions = higher charge density of that ion = greater electrostatic attraction = more exothermic enthalpy

Born-Harber cycle

How to find enthalpy change of solution

Factors affecting enthalpy of hydaration

• Ionic charge: Ions with greater charge have greater Δ_hydH because higher charge = better at attaching water molecules

• Ionic radius: Smaller ions have greater Δ_hydH because greater charge density = attract water molecules better so more exothermic Δ_hydH

Entropy

• Measure of dispersal of energy in system

• Tells how much disorder there is - num ways particles can be arranged & num was energy can be shared out

Factors affecting entropy

• Physical state

• Dissolution - dissolving solid ↑ entropy

• Num particles - more gas particles ↑ entropy

Entropy equation

ΔS = S_products - S_reactants

Free energy

Measure used to predict whether a reaction is feasible

Feasible reaction

Once started, will carry on to completion without energy being supplied to it

If ΔG is negative/= 0, reaction is feasible

Free energy formula

ΔG = ΔH - TΔS