5.2 energy

what are the ∆H units

Kj mol⁻¹

what is meant by ∆_f H ?

Enthalpy change when elements form 1 mole of a compound under standard conditions

what is meant by ∆_at H?

Formation of 1 mol of gaseous atoms

what is meant by first ionisation energy?

energy needed to remove 1 e⁻ from 1 mole of gaseous atoms

what is meant by second ionisation energy

energy needed remove 1 e⁻ from 1 mole of gaseous ion atoms

what is meant by first electron affinity ?

1 mol of gaseous atoms gain 1 mol of e⁻

what is meant by second electron affinity

energy needed to change 1 mole of gaseous 1- ions into gaseous 2- ions ( adding another e⁻ )

what is meant by ∆_LE H (lattice enthalpy) ?

Formation of 1 mol of ionic lattice from gaseous ions

what is meant by ∆_hyd H (Hydration) ?

dissolving of 1 mol of gaseous ions in water

∆H of hydration general equation

X_(g) → X_(aq)

what is meant by ∆_sol H (solution) ?

dissolving 1 mol of solute

∆_sol H general equation

YX_(s) → YX_(aq)

First e⁻ affinity general equation

X_(g) + e⁻ → X⁻_(g)

Second e⁻ affinity general equation

X⁻_(g) + e⁻ → X^²-_(g)

bond strength and lattice enthalpy relationship

More negative the lattice enthalpy, the stronger the ionic bonding

What are the factors affecting lattice enthalpy

ionic charge

ionic radius

How does ionic radius affect lattice enthalpy

the smaller the ionic radius of ions → higher charge density → stronger electrostatic force of attraction between ions → lattice enthalpy is more exothermic

How does ionic charge affect lattice enthalpy

higher ionic charge → stronger the electrostatic force of attraction → lattice enthalpy is more exothermic

Ionic radius relationship

Increases down the group

When to use a Born-Haber cycle

Ionic compounds

Born-Haber cycle where do endothermic reactions arrows go

Arrows point upwards → energy increases

Born-Haber cycle where do exothermic reactions arrows go

Arrows point downwards → energy decreases

Born Harbour cycle General structure ( Image)

What is the direction of born haber cycles

Atoms → gaseous atoms → gaseous ions → ionic lattice

Why are born cycles used to measure lattice enthalpy

lattice enthalpy can't be measured directly due to gaseous ions

Born-Habour cycle what happens when a diatomic element undergoes ∆Hat

forms 1 gaseous atom

Born-Habour cycle why is ∆Hea2 always endothermic

requires energy to get over the e⁻ / anion repulsion

Born-Habour cycle why is ∆Hea1 always exothermic

e⁻ is added to atom → attracted to positive nucleus

releases energy

born Harbour cycle what to do to ∆Hatt and ∆Hea when forming more then one atom

∆Hatt /∆Hea x by number of atoms / ions formed

NaCl ∆H_sol reaction image ( Image)

Why is ∆H_hyd always exothermic

Electronegative O in H₂O is attracted to metal cation

attraction → releases energy

What are the ∆H needed to work out ∆H_sol ?

∆H_hyd

∆H_LE

∆H_sol

What are the factors effecting ∆H_hyd ?

Ionic charge → greater the ionic charge

Ionic radius → smaller the ion→ more exothermic ∆H_hyd + ∆H_LE

Stronger attraction to H₂O

Suggest a reason why ∆H_sol is more exothermic in RbF then KF

K has a smaller ionic radius then Rb

Which ∆H is more significant in group 1 fluorides?

∆H_LE is more significant than ∆H_hyd

Which ∆H is more significant in group 1 chlorides?

∆H_hyd is more significant ∆H_LE

What is entropy (S)

A measure of the dispersal of energy in particles

disorder and entropy relationship

As disorder increases, entropy increases

What are the units of entropy ( S)

S → JK⁻¹mol⁻¹

State of matter and entropy relationship ?

Gas → random → highest disorder

liquid

solid ( fixed regular pattern) → lowest disorder

How to increase entropy?

Thermal decomposition

dissolving a solid

More moles of Gas in reactants

thermal decomposition reaction what is happening to entropy?

Solid → gas

more disorder in reactants

S increases ∆S is positive

Explain the changes in entropy

∆S is negative

S decreases

3 mol of gas to 2 mol of gas

How to calculate ∆S ?

what does a reaction being feasible/ spontaneous mean?

Processes occur without a continuous supply of energy

what does ∆G mean

Gibbs free energy

When are reactions feasible ?

Negative ∆G

∆G is less then 0

reaction is feasible

How to work out ∆G

∆G = ∆H - T∆S

what are the units for T

K

How to convert JK⁻¹mol⁻¹ to KJK⁻¹mol⁻¹

/1000

How to convert KJK⁻¹mol⁻¹ to JK⁻¹mol⁻¹

X 1000

How to work minimum temperature needed for a reaction to be feasible?

T = ∆H / ∆S

∆S / 1000 → get KJK⁻¹mol⁻¹

Limitations of ∆G

Predicts in standard conditions only

Reaction may not occur due to too high EA

Feasibility doesn’t equal rate ( kinetics)

Which ∆H enthalpies always point down ?

∆Hf

∆Hea1

∆HLE ( all exothermic)

Suggest why its difficult to use a born- haber cycle for carbonates / sulfates

Carbonate ions contain more than one type of atom

Cycle would need to include formation of carbonates

Conditions for a reaction to be feasible + effect of increasing temp

Exothermic ( - ∆H)

+∆S

∆G is less than 0 → Reaction is feasible

Increase T → ∆H gets more negative → more feasible

Conditions for a reaction not be feasible + effect of increasing temp

Endothermic ( + ∆H)

-∆S

∆G is more than 0 → not feasible

increase temp → T∆S becomes more positive

Effect of feasibility + Temperature

More feasible as T increases

∆G gets less positive

Why is the ∆H _Hyd of Cl⁻ ions / anions exothermic?

bonds form between the Cl⁻ and H