Module 5: Section 2

Question 1

Define standard lattice enthalpy

Answer 1

Enthalpy change when 1 mol of an ionic lattice is formed from its gaseous ions, under standard conditions.

Question 2

What does standard lattice enthalpy measure?

What does a negative standard lattice enthalpy show?

Answer 2

It is a measure of ionic bond length.

The more negative the standard lattice enthalpy, the stronger the bonding.

Question 3

Give 2 factors affeceting lattice enthalpy.

Answer 3

1. Ionic charge

2. Size of the ions.

Question 4

Describe how charge on ions affects lattice enthalpy.

Answer 4

The higher the charge on the ions, the more energy is released when an ionic lattice forms. This is due to the stronger electrostatic forces between the ions.

Question 5

Describe how ion size affects lattice enthalpy.

Answer 5

The smaller the ionic radii of the ions involved, the more exothermic (negative) the lattice enthalpy. Smaller ions have a higher charge density and ions can sit closer together in the lattice Attractions between the ions are stronger.

Question 6

State Hess’s Law

Answer 6

Total enthalpy change of a reaction is always the same, no matter which route is taken.

Question 7

Define the enthalpy change of hydration.

Answer 7

Enthalpy change when 1 mol of gaseous ions dissolves in water.

Question 8

Define the enthalpy change of solution.

Answer 8

Enthalpy change when 1 mol of a solute dissolves in water.

Question 9

Is bond breaking exothermic or endothermic?

Answer 9

Endothermic.

Energy is absorbed by the bonds, to break them.

Question 10

Is bond making exothermic or endothermic?

Answer 10

Exothermic.

Energy is released as bonds form.

Question 11

Give 2 factors affecting enthalpy of hydration.

Answer 11

1. Size of the ions.

2. Charge on the ions.

Question 12

How does ionic charge affect enthalpy of hydration?

Answer 12

Ions with a greater charge have a greater enthalpy of hydration.
Ions with a higher charge can attract water molecules better. Electrostatic attraction formed is stronger. More energy is released when bonds are made giving them a more exothermic enthalpy of hydration.

Question 13

How does ion size affect enthalpy of hydration?

Answer 13

Smaller ions have a higher charge density than bigger ions. They attract the water molecules better and have a more exothermic enthalpy of hydration.

Question 14

What does entropy tell you?

Answer 14

Entropy tells you how much disorder there is.

The more disorder there is, the greater the value of entropy.

Question 15

What does thermodynamically stable mean?

Answer 15

When a substance reaches its maximum entropy state . This means it wont react any further without the input of energy.

Question 16

How do you calculate entropy change of a reaction?

Give the units

Answer 16

ΔS = S products - S reactants

Units of entropy are JK-1mol-1

Question 17

When is a reaction feasible?

Answer 17

When ΔG < 0 OR ΔG = 0

Question 18

Give the formula for ΔG and the units for each component.

Answer 18

ΔG = ΔH - TΔS

ΔG = Jmol-1
ΔH = enthalpy change (J mol-1)
T = temperature (K)
ΔS = entropy change (J K-1 mol-1)

Question 19

What does an oxidising agent do?

Answer 19

Accepts electrons and gets reduced.

Question 20

What does a reducing agent do?

Answer 20

Donates electrons and gets oxidised.

Question 21

What do electrochemical cells do?

Answer 21

Electrochemical cells make electricity.

Question 22

Why is an electrochemical cell a redox process?

Answer 22

There are always 2 reactions within an electrochemical cell - ones an oxidation and ones a reduction.

Question 23

Why is a platinum electrode sometimes used in half-cells?

Why is the metal platinum used, and not another one?

Answer 23

Half-cells involving solutions of 2 aqueous ions of the same element, such as Fe2+ (aq) and Fe3+ (aq) use a platinum electrode. The conversion from Fe2+ to Fe3+ occurs on the surface of the pt electrode.
Platinum is used because it is inert and doesn’t conduct electricity

Question 24

What do electrode potentials measure?

Answer 24

How easily a metal is oxidised.

The more negative electrode potential = more easily oxidised.

Question 25

Define standard electrode potential of a half-cell.

Answer 25

The SEP of a half-cell is the voltage measured, under standard conditions, when the half-cell is connected to a standard hydrogen electrode.

Question 26

What are the standard conditions for electrode potentials?

Answer 26

1. Solutions have a concentration of 1.00moldm-3
2. Temperature must be 298K
3. Pressure must be 100kPa

Question 27

How do conditions affect the value of electrode potential.

Answer 27

The equilibrium position is affected by changes in temperature, pressure and concentration, which changes the cell potential.

Question 28

How do fuel cells generate electricity?

Answer 28

A fuel cell produces electricity by reacting a fuel (hydrogen) with an oxidant, (usually O2).

Question 29

Give 2 benefits of using electrochemical cells.

Answer 29

1. They are more efficient at producing energy than combustion, as combustion loses energy as heat.
2. They produce less pollution, as the only waste product is water.

Question 30

Give 2 drawbacks of using electrochemical cells.

Answer 30

1. Production of cells involves the use of toxic chemicals, which need to be disposed of.
2. The chemicals used to make the cells are often very flammable.