Lattice enthalpy, Born Haber cycles, entropy and free energy Module 5

Question 1

What is the standard lattice enthalpy?

Answer 1

The enthalpy change when one mole of an ionic lattice is formed from it’s gaseous ions under standard conditions (always exothermic)

Question 2

What aspects about the ions will increase the lattice enthalpy value?

Answer 2

The higher the charge on the ions, as stronger electrostatic forces between the ions so more energy released

The smaller ionic radii, the more exothermic reaction will be as higher charge density and smaller ionic radii mean the attractions between ions are stronger

Question 3

What’s the first ionisation energy?

Answer 3

The energy required to remove remove 1 mole of electrons from 1 mole of gaseous ions

Question 4

What’s the enthalpy of formation?

Answer 4

Is the enthalpy change when 1 mole of a compound is formed from it’s elements in their standard states

Question 5

Electron affinity definition?

Answer 5

The amount of energy spent when an electron is added to an atom in the gaseous form to form a negative ion

Question 6

How to draw a born haber cycle?

Answer 6

At the very bottom have the ionic lattice
Then above it have the elements of the ionic lattice in their standard states, with line going downwards showing energy change of formation
Then do the atomisation energy of each of the elements, the solids will go to gases, and any diatomic molecules will be come singular, the lines are going upwards
Do the ionisation energies of what was the metal, gives +1 charge to the metal gaseous ion and produces an electron, lines go up
Do the electron affinitys of the other element, decreasing it’s by 1 and removing an electron until you reach it’s gaseous ions, arrow may go up or down, look to see if value is positive or negative
Lattice enthalpy goes from ions to salt, so to calculate it work out value of all the other arrows going in the correct direction to the salt from the gaseous ions

Question 7

What’s important about moles when doing born haber cycles?

Answer 7

Have to take them into account, so if 2 moles are being affected by electron affinity multiply it by 2

Also for formation if you only need 1 ion of the gas, in natural state will be 1/2 of the diatomic gas

Question 8

When a solid ioninc lattive dissolves in water what are the 2 things that happen?

Answer 8

The bonds between ions break to give gaseous ions, this is endothermic and the opposite to lattice enthalpy

Bonds between the gaseous ions and water are made this is exothermic and is called the enthalpy change of hydration

These 2 factors combined are the enthalpy change of solution

Question 9

Definition of enthalpy change of hydration?

Answer 9

The enthalpy change when 1 mole of gaseous ions dissolves in water

Question 10

What’s the enthalpy change of solution?

Answer 10

The enthalpy change when 1 mole of solute dissolves in water

Question 11

How do you work out enthalpy change of solution if given the enthalpy change of hydration and lattice enthalpy?

Answer 11

Draw an arrow from the salt to it’s aqueous ions, this line is the enthalpy change of solution
Bellow it have the gaseous ions
Draw arrow from the gaseous ions to the salt, this is the lattice enthalpy
Then draw arrow from gaseous ions to aqueous ions,this is enthalpy change of hydration
Add hydration and lattice enthalpy to find the one of solution

Question 12

2 things which affect the enthalpy change of solution and why?

Answer 12

Ions with greater charge have greater enthalpy change of hydration as they are better at attracting water molecules, so electrostatic attraction between water and ion is stronger, more energy is released when bonds are made

Smaller ions have greater enthalpy change of hydration, they have higher charge density, so attract water molecules better, stronger bonds mean more energy released when bonds made

Question 13

What is entropy?

Answer 13

A measure of disorder of particles, the higher it is the more disordered the particles

Question 14

Things that affect entropy?

Answer 14

Physical states, solid < liquid < gas

More particles = more entropy

Question 15

What does the entropy value have to be for a reaction to be feasible?

Answer 15

Positive

Question 16

How do you calculate entropy?

Answer 16

Entropy = Entropy of products - Entropy of reactants

So entropy has to increase during a reaction for it to be feesible

Question 17

Formula for the free energy change ΔG?

Answer 17

ΔG = ΔH - TΔS

ΔH = enthalpy change (J mol^-1)
T= temperature in Kelvin
ΔS= entropy change in (J K^-1 mol^-1)

Question 18

What can ΔG tell us?

Answer 18

If it’s positive reaction isn’t feasible

Question 19

How do you rearrange ΔG to find at what temperature a reaction becomes feasible?

Answer 19

T= ΔH / ΔS

Question 20

Why might a reaction have a negative ΔG and still not be feasible?

Answer 20

Really high activation energy

Question 21

Equation for ΔH?

Answer 21

ΔH = (mass (of solution) x specific heat capacity x change in temperature) / the number of moles of reactant

Have to convert from J to kJ aswell