Topic 13 Energetics 2

Question 1

What’s the symbol for lattice enthalpy and what is its definition

Answer 1

Delta lattice H - enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions

Question 2

What’s the symbol for enthalpy change of atomisation and what is it

Answer 2

Delta at H - the enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state

Question 3

What’s the symbol for 1st electron affinity and what is it

Answer 3

Delta ea1 H - The enthalpy change when 1 mole of gaseous 1- ions are made from 1 mole of gaseous atoms.

Question 4

Why are born Haber cycles used to calculate lattice enthalpy

Answer 4

It can’t be calculated directly from an experiment

Question 5

List the exothermic reactions in a born haber cycle (3)

Answer 5

1. Lattice enthalpy
2. Enthalpy of formation
3. 1st electron affinity

Question 6

List the endothermic reactions in a born have cycle (3)

Answer 6

1. 2x enthalpy of atomisation
2. 2x ionisation energies
3. 2nd electron affinity

Question 7

Why is 1st Ea exothermic

Answer 7

Because the negative electron is attracted to the positive nucleus

Question 8

Why is 2nd Ea endothermic

Answer 8

The negative electron is repelled by the negative 1- ion

Question 9

What leads to a difference in experimental and theoretical lattice enthalpys

Answer 9

Covalent character in ionic bonds

Question 10

What do we assume to calculate theoretical lattice enthalpys

Answer 10

A purely ionic model :
1. Ions are perfectly spherical
2. Charge is evenly distributed in the sphere

Question 11

Why do most ionic compounds not follow the purely ionic model

Answer 11

The positive ion distorts the charge distribution in the negative ion

Question 12

Do compounds with covalent character have weaker or stronger ionic bonds . And how does this affect theoretical lattice enthalpy in comparison to experimental

Answer 12

Stronger - lattice enthalpy is more negative in experimental values

Question 13

Define enthalpy change of solution

Answer 13

Enthalpy change when 1 mole of an ionic substance is dissolved in the minimum amount of solvent to ensure no further enthalpy change is observed upon further dilution

Question 14

What is required for a substance to dissolve

Answer 14

1. Substance bonds must break
2. Bonds form between solvent and solute

Question 15

Why do ionic compounds dissolve in water

Answer 15

1. Water is polar
2. Delta + H attracted to anion delta - O is attracted to carrion
3. The structure starts to break down

Question 16

What must be true about bond strength in solution for a substance to dissolve

Answer 16

New bonds must be the same strength or greater than those broken

Question 17

Define enthalpy of hydration

Answer 17

Enthalpy change when 1 mole of aqueous ions is made from 1 mole of gaseous ions

Question 18

What is breaking a solid lattice into gaseous ions called

Answer 18

Lattice dissociation

Question 19

How do you calculate enthalpy of solution

Answer 19

Make a Hess cycle with lattice dissociation and Hydration

Question 20

What two things affect enthalpy of hydration and how

Answer 20

Charge and size of ion
Larger charge - greater enthalpy
Smaller ions - greater enthalpy

Question 21

What is entropy

Answer 21

The measure of disorder

Question 22

Whats another way of describing entropy

Answer 22

The number of ways energy can be shared out between particles

Question 23

What two things effect entropy

Answer 23

Number of particles and state of particles

Question 24

Formula for entropy change

Answer 24

Entropy products - entropy reactants

Question 25

Units of entropy

Answer 25

JK-1mol-1

Question 26

When is a reaction entropically feasible

Answer 26

Entropy change is positive

Question 27

Formula for total entropy change

Answer 27

Entropy change in the system + entropy change in the surroundings

Question 28

Formula for entropy change in surroundings

Answer 28

Formula is - enthalpy change/ T (kelvins) Remember enthalpy is normally in kJ/mol so we must convert

Question 29

What does Gibbs free energy tell us

Answer 29

Wether a reaction is feasible or not

Question 30

Formula for gibbs free energy

Answer 30

Delta G = enthalpy change - T x entropy change (system)

Question 31

Units of free energy change

Answer 31

Jmol^-1

Question 32

What’s the basic rule for gibbs free energy

Answer 32

If gibbs free energy is less than or equal to 0 the reaction is feasible

Question 33

When would a reaction not occur even if it’s feasible

Answer 33

Activation energy too high or rate of reaction too slow

Question 34

In the gibbs free energy equation what happens when enthalpy is negative and entropy change is negative

Answer 34

The reaction is feasible at low temperatures

Question 35

In the gibbs free energy equation what happens when enthalpy is positive and entropy change is negative

Answer 35

The reaction is never feasible

Question 36

How do you find the lowest temp a reaction is feasible at

Answer 36

Delta G = 0 so 0 = enthalpy change - T x entropy change Therefore T = enthalpy / entropy

Question 37

In an equilibrium what is the relationship between the equilibrium constant and entropy change

Answer 37

Entropy change is positive then Kc is less that 1 Entropy change is negative then Kc is greater than 1

Question 38

What formula links gibbs free energy, R, T and K (equilibrium constant)

Answer 38

Delta G = -RT lnK

Question 39

What equation do you use to find if a reaction is feasible in equilibrium

Answer 39

Delta G = -RT lnK

Question 40

What does lattice enthalpy provide a measure of

Answer 40

Ionic Bond strength