2.2 Energy

Question 0

What does Hess’ law state?

Answer 0

If a reaction can take place by more than 1 route and the initial and final conditions are the same, the total enthalpy change is the same for each route

Question 1

Define lattice enthalpy

Answer 1

The enthalpy change that accompanies the formation of 1 mole of an ionic compound from its gaseous ions under standard conditions

Question 2

Why is the melting point of an ionic compound so high?

Answer 2

• Ionic bonds are electrostatic forces of attraction between oppositely charged ions
• Ionic bonds are strong

Question 3

Describe some features of lattice enthalpy

Answer 3

• It is an exothermic change
• It indicates the strength of the ionic lattice and is a measure of the ionic bond strength
• Cannot be measured directly because it is impossible to form 1 mole of an ionic lattice from gaseous ions

Question 4

What are the key enthalpy changes used for working out lattice enthalpy?

Answer 4

• The standard enthalpy change of formation
• The standard enthalpy change of atomisation
• The first ionisation energy
• The second ionisation energy (for 2+ ions)
• The first electron affinity
• The second electron affinity (for 2- ions)

Question 5

Define ‘the standard enthalpy change of atomisation’

Answer 5

The enthalpy change that takes place when 1 mole of gaseous atoms forms from the element in its standard state

Question 6

Define the first ionisation energy

Answer 6

The enthalpy change accompanying the removal of 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions

Question 7

Define the first electron affinity

Answer 7

The enthalpy change accompanying the addition of 1 electron to each atom in 1 mole of gaseous ions to form 1 mole of gaseous 1- ions

Question 8

What is the datum line on a Born-Haber cycle?

Answer 8

• Zero energy

- The elements in their standard states

Question 9

In the Born-Haber cycle, how can you tell which values are exothermic and which are endothermic?

Answer 9

• ΔH values pointing upwards are endothermic

- ΔH values pointing downwards are exothermic

Question 10

What is found in the lowest energy level of the Born-Haber cycle?

Answer 10

The ionic solid

Question 11

Define the standard enthalpy change of formation

Answer 11

The enthalpy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions

Question 12

Define the standard enthalpy change of solution

Answer 12

The enthalpy change that takes place when 1 mole of a compound is completely dissolved in water under standard conditions

Question 13

What are the 2 processes that take place when an ionic solid dissolves in water?

Answer 13

• Breakdown of the ionic lattice into gaseous ions

- Hydration of the ions

Question 14

Describe the breakdown of the ionic lattice

Answer 14

The process is exact opposite of that producing the lattice enthalpy

• The crystal lattice is broken down
• The ions are separated
• Overcoming the attractive forces between the oppositely charged ions requires energy
• The energy involved has the same magnitude as lattice enthalpy but the sign is opposite

Question 15

Describe hydration

Answer 15

Gaseous ions are hydrated

• The gaseous ions bond with the water molecules
• The positively charged ions will be attracted to the slightly negative oxygen atoms in the water
• The negatively charged ions will be attracted to the slightly positive hydrogen atoms in water

Question 16

Define the standard enthalpy change of hydration

Answer 16

The enthalpy change that takes place when 1 mole of isolated gaseous ions is dissolved in water forming 1 mole of aqueous ions under standard conditions

Question 17

What are the key enthalpy changes for working out the lattice enthalpy of an ionic solid?

Answer 17

• The enthalpy changes of hydration of the constituent gaseous ions
• The enthalpy change of solution of the ionic solid

Question 18

What 2 factors affect the size of lattice enthalpy?

Answer 18

Ionic size

Ionic charge

Question 19

How does ionic size affect lattice enthalpy?

Answer 19

As the ionic radius increases:

• The attraction between the ions decreases
• The lattice enthalpy becomes less negative (less exothermic)

Question 20

How does ionic charge affect lattice enthalpy?

Answer 20

• The compounds with the most negative lattice enthalpies are those which have small, highly charged ions
• When 2 highly charged ions of opposite charge are present in the lattice they attract each other strongly
• A very exothermic reaction will take place when a lattice is formed from 2 highly charged ions

Question 21

What factors affect the size of hydration enthalpy?

Answer 21

Ionic size

Ionic charge

Question 22

How does ionic size affect the size of hydration enthalpy?

Answer 22

As the ionic radius decreases

• The value of the enthalpy change of hydration becomes more negative (more exothermic)
• Hydration depends on the ability of an ion to attract and bond with water molecules. Small ions exert more attraction on water molecules and more energy is released

Question 23

How does ionic charge affect the size of hydration enthalpy?

Answer 23

As the charge on the ion increases

• There is a greater attraction for water molecules
• So the hydration enthalpy is more negative

Question 24

What is entropy?

Answer 24

The quantitative measure of the degree of disorder in a system

Question 25

Why is entropy, S, always a positive number?

Answer 25

Because all substances possess some degree of disorder because particles are always in constant motion

Question 26

How does entropy increase?

Answer 26

It increases during the changes in state that give more randomness (e.g. Liquid to gas)

Question 27

How do you calculate entropy changes?

Answer 27

Sum of products - sum of reactants

Question 28

If a change makes a system more random, is the entropy change positive or negative?

Answer 28

Positive

Question 29

What is the free energy change, ΔG?

Answer 29

The balance between enthalpy, entropy and temperature for a process: ΔG = ΔH - TΔS (T is measured in Kelvin)

Question 30

What is a spontaneous process?

Answer 30

A process that can proceed on its own, once it has begun

Question 31

When is a process spontaneous?

Answer 31

If a chemical system becomes more stable and its overall energy decreases

Question 32

For a reaction to be feasible, what ΔG value must it have?

Answer 32

One that is less than 0

Question 33

What is an oxidising agent?

Answer 33

The substance that is reduced (takes electrons from the substance that is oxidised)

Question 34

What is a reducing agent?

Answer 34

A substance that is oxidised (gives electrons to the substance that is reduced)

Question 35

What does a half cell comprise of?

Answer 35

An element in 2 oxidation states | - A metal placed in an aqueous solution of its ions

Question 36

What is the electrode potential of a half cell?

Answer 36

Its tendency to gain or lose electrons in equilibrium

Question 37

How is a simple electrochemical cell made?

Answer 37

2 half cells with different electrode potentials are connected together by a wire and a salt bridge - 1 half cell releases electrons - the other half cell gains electrons - The difference in electrode potential is measured with a voltmeter - The wire connects the 2 metals, allowing electrons to be transferred between the 2 half cells - The salt bridge connects the 2 solutions, allowing ions to be transferred between the half cells. Can be made out of a strip of filter paper soaked in an aqueous solution of an ionic compound that does not react with either of the half-cell solutions (e.g. KNO3)

Question 38

Which is the most reactive (so the greatest oxidising agent): the metal with the highest or lowest electrode potential?

Answer 38

Lowest (most negative)

Question 39

Is the more negative, or more positive, metal at the negative terminal?

Answer 39

More negative

Question 40

What is the problem with non-metal/non-metal ion half cells? (E.g. Hydrogen half cells)

Answer 40

There is no electrode to connect the wire to - A platinum electrode is placed in the solution so that it is in contact with both H2 and H^+ ions - The platinum is inert and does not react at all - Its surface is coated with platinum black, a spongy coating in which electrons can be transferred between the non-metal and it's ions

Question 41

Define 'standard electrode potential of a half cell'

Answer 41

The electromotive force of a half cell compared with a standard hydrogen half cell, measured at 298K with solution concs of 1 mol dm^-3 and a gas pressure of 1 atm

Question 42

What are the standard conditions?

Answer 42

298K (25°C) 1 atm pressure (gas) 1 mol dm^-3

Question 43

Which half cell is used as the reference for the measurement of voltages in electrochemical cells?

Answer 43

A hydrogen half cell (given an electrode potential of 0)

Question 44

What is the equation for working out the standard cell potential?

Answer 44

Standard cell potential = Electrode potential of positive terminal - electrode potential of negative terminal

Question 45

What is the standard cell potential?

Answer 45

The electromotive force between the 2 half cells making up the cell under standard conditions - The difference between the standard electrode potentials of each half cell

Question 46

What is an electromotive force?

Answer 46

The voltage produced by a cell when no current flows

Question 47

Define 'cell reaction'

Answer 47

The overall chemical reaction taking place in the cell | - The sum of the oxidation and reduction half equations taking place in each half cell

Question 48

What are the 3 main types of electrochemical cells?

Answer 48

- Non-rechargeable cells - Rechargeable cells - Fuel cells

Question 49

Explain how a non-rechargeable cell works

Answer 49

- Provide electrical energy until the chemicals have reacted to such an extent that the voltage falls - The cell is then 'flat' and is discarded

Question 50

Explain how a rechargeable cell works

Answer 50

- The chemicals in the cell react, providing electrical energy - The cell reaction can be reversed during recharging - The chemicals in the cell are regenerated and the cell can be used again

Question 51

Explain how a fuel cell works

Answer 51

- The cell reaction uses external supplies and an oxidant, which are consumed and needs to be provided continuously - The cell will continue to provide electrical energy so long as there is a supply of fuel and oxidant

Question 52

What are the advantages of a methanol fuel cell compared with use of hydrogen gas?

Answer 52

- A liquid fuel is easier to store than hydrogen gas | - Methanol can be generated from biomass

Question 53

What are the advantages of fuel cell vehicles?

Answer 53

Less pollution and less CO2 - Hydrogen-rich fuels produce only small amounts of CO2 and air pollutants Greater efficiency - A petrol engine is less than 20% efficient in converting chemical energy by the combustion of petrol, so much of the chemical energy is wasted as heat - Hydrogen fuel cell vehicles are 40-60% efficient in converting the fuel's energy. This means that fuel consumption drops by more than half compares with a petrol- or diesel-powered car

Question 54

What different strategies are there for storing hydrogen?

Answer 54

- Hydrogen can be stored as a liquid under pressure (but this still requires a very low temp) - Hydrogen can be adsorbed onto the surface of a solid material in a similar way that a catalyst is able to hold gases on its surface - Hydrogen can be absorbed within some solid materials

Question 55

What is the main problem in storing hydrogen?

Answer 55

It is a gas with a very low boiling point

Question 56

What are the limitations of hydrogen fuel cells?

Answer 56

- The large-scale storage and transportation of hydrogen poses problems. A cost-effective and energy-efficient infrastructure needs to be in place to deliver large quantities of hydrogen fuel over long distance - The feasibility of storing a pressurised liquid - Current 'adsorbers' and 'absorbers' of hydrogen have a limited lifetime - Current fuel cells have a limited lifetime, requiring regular replacement and disposal following high production costs - Fuel cells use toxic chemicals in their production

Question 57

What are the limitations of the 'hydrogen economy'?

Answer 57

- The use of H as a fuel has to be accepted politically and by the general public - There are logistical problems in the handling and maintenance of H systems - H is an 'energy carrier' and not an 'energy source'. It must first be manufactured, which could end up using more energy than is saved.