5D - Lattice enthalpy & Free energy

Question 1

Define Lattice enthalpy (Δ_LEH^⦵)

Answer 1

Lattice enthalpy (Δ_LEH^⦵) = The enthalpy change where 1 mole of an ionic compound is formed from its gaseous ions under standard conditions

Question 2

Standard enthalpy change of formation (Δ_fH^⦵)

Answer 2

Standard enthalpy change of formation (Δ_fH^⦵) = Enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions

Question 3

First ionisation energy (Δ_IE1H^⦵)

Answer 3

First ionisation energy (Δ_IE1H^⦵) = The enthalpy change when 1 mole of a electrons are removed from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions

NOTE:

The 1 = 1st ionisation energy, it would 2 if it’s 2nd ionisation etc…

Question 4

Standard enthalpy change of atomisation (Δ_atH^⦵)​

Answer 4

Standard enthalpy change of atomisation (Δ_atH^⦵)​ = Enthalpy change when 1 mole of gaseous atoms are formed from their elements in their standard states under standard conditions

Question 5

1st electron affinity (Δ_EA1H^⦵)

Answer 5

1st electron affinity (Δ_EA1H^⦵) = Enthalpy change when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous 1- ions

NOTE:

The 1 will change on which electron affinity it is e.g. Δ_EA2H^⦵ would be 2nd electron affinity

Question 6

Define standard enthalpy change of hydration (Δ_hydH^⦵)

Answer 6

Standard enthalpy change of hydration (Δ_hydH^⦵) = The enthalpy change when 1 mole of gaseous ions are added to water to form 1 mole of aqueous ions under standard conditions

Question 7

The way you construct a Born-Haber cycle and the hess cycle that goes with it

Answer 7

Question 8

Hess cycle between Δ_rH^⦵, Δ_fH^⦵ and Δ_LE​H^⦵

Answer 8

Question 9

Is the value of lattice enthaply likely to be positive or negative?

Answer 9

Negative

Question 10

Are electron affinities exothermic or endothermic?

Answer 10

Δ_EA1H^⦵ = Exothermic (negative) - electron is bound to nucleus

Subsequent electron affinities = Endothermic - forcing electrons onto something that is already negative so they’d attact

Question 11

Question:

Answer 11

Question 12

Question:

Answer 12

Question 13

Question:

Construct the hess cycle and a horn haber type diagram to represent the hess cycle afterwards

Answer 13

Question 14

What will the signs be? Why?

Δ_LEH^⦵

Δ_hydH^⦵

Δ_solH^⦵

Answer 14

Δ_LEH^⦵ = negative

Δ_hydH^⦵ = negative

Negative as forming bonds

Δ_solH^⦵ = Positive or negative (relative difference between energy you give out to break lattive enthalpy and energy you get back after putting water in)

Question 15

What are the 2 main properties of an ion that will affect its attraction to other ions and to solvent molecules

Answer 15

• Charge (ionic charge)
• Size (ionic radius)

Question 16

Name the 4 things that need to be considered when thinking about the radius of an ion

Answer 16

• Charge on ion
• Nuclear charge
• No’ of electron shells
• sheilding electrons

Question 17

Effect of ionic size on:

• Lattice enthalpy
• Melting point

(down a group)

Answer 17

As you do down group 1:

• Ionic radius increases
• Attraction between ions decrease (weaker electrostatic forces of attraction)
• Lattice enthalpy less negative
• Melting point decreaes

Question 18

Effect of ionic charge on lattice enthalpy

(Across a period)

Answer 18

Going across a period:

• Ionic charge increases
• Stronger electrostatic forces of attraction
• Lattice enthalpy becomes more negative
• Melting point increases

Question 19

Effect of ionic size on enthalpy change of hydration

• Down a group
• Across a period

Answer 19

DOWN A GROUP:

• Ionic radius increases
• Attraction between ion and water molecules decreases
• Enthalpy change of hydration is less negative

ACROSS A PERIOD:

• Ionic charge increases
• Attraction with water molecules increases
• Enthalpy change of hydration becomes more negative

Question 20

Describe an experiement to determine the enthalpy change of solution (5)

Answer 20

Experiment to determine the Δ_solH^⦵ of anhydrous sodium thiosulphate, Na₂S₂O₃

Coffee cup calorimeter:

1. Measure 25cm³ of distilled water into a plastic cup inside a polystyrene cup. Support polystyrene cup with a 250cm³ beaker
2. Add known mass of anhydrous Na₂S₂O_3(s)
3. Measure temperature of water to closest 0.5º
4. Stir mixture with thermometer until all Na₂S₂O_3(s) has dissolved and the temperature no longer changes
5. Record final temperature

Question 21

Steps for calulating the Δ_solH^⦵ from experimental data?

Use this as an example

Answer 21

NOTE: It’s basically the same as a ΔH calculation from AS, just use the mass of solution instead of mass of solid.

1. Find Q
   
   • Q = mcΔT (KJ)
   • Use the mass of solution formed = mass of water + mass of solid
2. Moles of solid that has dissolved
   
   • n = M / Mr
3. Calculate Δ_solH^⦵ for the reaction
   
   • Δ_solH^⦵ = Q / n

Question 22

Define Entropy

Symbol, units and what must the value be?

Answer 22

Entropy = The degree of disorder in a system (must be increasing)

Symbol = S

Units = JK^-1mol^-1

Question 23

For the following physical states put them in order of lowest → highest entropy

Liquid

Gas

Plasma

Solid

Answer 23

Solid = Lowest entropy value

Liquid

Gas

Plasma = Highest entropy value

Question 24

Equation for entropy change = ?

Units = ?

Answer 24

Entropy change = ∑entropy change of products - ∑entropy change of reactants

ΔS = ∑S_products - ∑S_reactants

Units = JK^-1mol^-1

Question 25

Define Free energy

Equation for free energy change and units

Answer 25

Free energy change (ΔG) = Measure of feasibility of a reaction

• ΔG < 0 (feasible)
• ΔG > 0 (not feasible)
• ΔG = 0 (boundary)

Units for Free energy change (ΔG) = KJmol^-1

• ΔH = KJmol^-1 and ΔS = JK^-1mol^-1 ∴ remember to convert KJ → J by x1000!
• Temp must be in K

Question 26

Why might a feasible process not actually appear to occur?

Answer 26

It has a very large E_a so rate of reaction is too slow to be percieved