13: Energetics II Flashcards
Lattice energy definition
The energy change when one mole of an ionic solid is formed from its gaseous ions
Enthalpy change of atomisation definition
The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state
Electron affinity enthalpy
The enthalpy change when one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous ions with a -1 charge
Is electron affinity endothermic or exothermic?
1st is exothermic, subsequent are endothermic
Process for constructing Born Haber cycles
- Atomise the metal
- Ionise the metal however many time needed
- Atomise the non-metal
- Add the electrons to the non-metal (e.a.)
- The remaining difference to the formation is lattice enthalpy
Effect of bond strength of lattice enthalpy
Lattice enthalpy is higher for compounds with stronger bonds
Assumptions for lattice enthalpies
Perfect ionic model
How to tell degree of covalency from lattice enthalpy
The experimental model will be different from the theoretical - the greater the difference the greater the covalency
How do cations polarise anions
They pull the negative charge cloud towards the cation
Factors affecting the polarisation of anions?
- Cation size (smaller polarises more as closer to electrons)
- Anion size (bigger allows for easier distortion)
- Charge difference (greater attraction between cation and negative charge field)
Enthalpy change of solution
The enthalpy change when one mole of ionic substance dissolves in water
Enthalpy change of hydration
The enthalpy change when one mole of gaseous ions dissolve in water
Is enthalpy of hydration endothermic or exothermic?
Exothermic
Factors affecting lattice enthalpy?
- Size of ions (as radius increases, the attractive force decreases so lattice enthalpy values increase as the bonds are weaker)
- Charge difference (greater the difference in charges , the greater the attraction so lattice enthalpy decreases)
Factors affecting hydration enthalpy?
-Charge density (as charge density increases, the hydration enthalpy decreases as the ions attract water molecules more strongly)
Is hydration enthalpy endothermic/exothermic?
Exothermic
What are spontaneous reactions?
Reactions that occur without external influence
What is entropy?
A measure of disorder
What is the overall trend of entropy?
Entropy always increases
Entropy of different states
Solid (ordered structure) < Liquid < Gas
What happens to entropy when a solute dissolves?
Entropy increases as the regular structure of the solid is broken down
What happens to entropy when the number of moles increase?
Entropy increases
Equation for change in enthalpy
ΔS(total) = ΔS(system) + ΔS(surrounding)
Units for entropy
J/(K+mol)
Equation for change in enthalpy surroundings
ΔS(surroundings) = -ΔH(reaction) / T
Equation for Gibbs Free Energy
ΔG = ΔH - TΔS(system)
Requirement for a reaction to take place
ΔG needs to be negative
Equation for Gibbs Free Energy (in terms of equilibria)
ΔG = -RTln(K)
Reason why a reaction might not occur even if ΔG is negative
A reaction has a high activation energy (limited by kinetic factors)
