Energy

Question 1

Define lattice enthalpy

Answer 1

The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions. The more negative the lattice enthalpy, the stronger the ionic bonds. It is exothermic, so deltaH is negative.

Question 2

Define enthalpy change of formation

Answer 2

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

Question 3

Define enthalpy change of atomisation

Answer 3

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

Question 4

Define first ionisation energy

Answer 4

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

Question 5

Define first electron affinity

Answer 5

The enthalpy change accompanying the addition of one electron to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions. An exothermic process

Question 6

Define second electron affinity

Answer 6

The enthalpy change accompanying the addition of one electron to each ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions. Often an endothermic process

Question 7

Define standard enthalpy change of solution

Answer 7

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

Question 8

Define standard enthalpy change of hydration

Answer 8

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

Question 9

What physical properties affects lattice enthalpy and enthalpy of hydration?

Answer 9

Ionic size and ionic charge (which together make charge density)

Question 10

How does ionic size effect lattice enthalpy?

Answer 10

The smaller the ion, the tighter the ions can pack in the lattice and so the stronger the attractive forces, resulting in a more exothermic lattice enthalpy

Question 11

How does ionic charge effect lattice enthalpy?

Answer 11

The higher the charge on an ion, the stronger the attractive forces are, resulting in a more exothermic lattice enthalpy

Question 12

How does ionic size effect enthalpy change of hydration?

Answer 12

Hydration depends on the ability of an ion to attract and bond with water molecules. Small ions exert greater attraction on water molecules, thus forming stronger bonds and a more exothermic enthalpy of hydration is observed.

Question 13

How does ionic charge effect enthalpy change of hydration?

Answer 13

As the charge on an ion increases, it has a greater attraction for water molecules, thus forming stronger bonds and a more exothermic enthalpy of hydration is observed.

Question 14

Define entropy

Answer 14

The quantitative measure of the degree of disorder in a system. It is always positive (as there is always some disorder). As it is related to energy, it can also be described as the dispersal of energy. Systems become more energetically stable at higher entropies (more disorder). Has symbol S; measured in J/K/mol

Question 15

How does entropy change through the phases of matter?

Answer 15

Solids have least entropy; gases have most. This is because the energy is being dispersed more, and so hte entropy increases.

Question 16

How does entropy change when a solid lattice dissolves?

Answer 16

Entropy increases as the energy is becoming more dispersed

Question 17

How does entropy change when a reaction produces a gaseous species?

Answer 17

Entropy increases as gases have a higher entropy than solids or liquids as the energy is more dispersed.

Question 18

How do you calculate the entropy change for a reaction?

Answer 18

deltaS=(sum of S products) - (sum of S reactants)

Question 19

What does the tendency for a process to take place depend upon?

Answer 19

Temperature, T; entropy change, deltaS; enthalpy change, deltaH

Question 20

What is the free energy change?

Answer 20

The balance between entropy, enthalpy and temperature for a process. Given symbol deltaG and units kJ/mol; calculated by:
deltaG = deltaH - TdeltaS

Question 21

When is a reaction feasible?

Answer 21

When deltaG is less than 0
For -ve enthalpy and +ve entropy; always feasible
For +ve enthalpy and -ve entropy; never feasible
For -ve enthalpy and -ve entropy; feasible at low temperatures (maximum temperature)
For +ve enthalpy and +ve entropy; feasible at high temperatures (minimum temperature)
Can calculate max and min temperatures by setting deltaG=deltaH-TdeltaS=0

Question 22

How can endothermic reactions take place spontaneously?

Answer 22

Change in entropy, deltaS, must be positive and the temperature must be above the minimum temperature so that TdeltaS>deltaH

Question 23

Define redox

Answer 23

A reaction is described as ‘redox’ when both reduction (gain of electrons/decrease in ON) and oxidation (loss of electrons/increase in ON) take place

Question 24

Define oxidation number

Answer 24

A measure of the number of electrons that an atom uses to bond with atoms of another element.

Question 25

Define oxidising agent

Answer 25

A reagent that oxidises (takes electrons from/increases the ON of) another species and is reduced itself

Question 26

Define reducing agent

Answer 26

A reagent that reduces (adds electrons to/decreases the ON of) another species and is oxidised itself

Question 27

What are half equations?

Answer 27

Equations that either show the oxidation or the reduction half of a redox reaction, and shows the electrons being gained/lost

Question 28

How can redox equations be constructed?

Answer 28

Adding together relevant half equations, ensuring electrons cancel out
Ensuring the changes in oxidation number total to 0, then adding any extra species, e.g. H+; OH-; H2O

Question 29

Define standard electrode potential

Answer 29

The standard electrode potential of a half cell, E(o), is the e.m.f. of a half cell compared with a standard hydrogen half cell, measured at 298K with solution concentrations of 1mol/dm^3 and gas pressure of 100kPa

Question 30

Describe how to measure the standard electrode potential of a metal/metal ion half cell

Answer 30

Set up the metal half cell: metal strip makes the electrode; electrode in a solution of 1mol/dm^3 of metal ions, solution connected to the solution in a hydrogen half cell via a salt bridge (filter paper soaked in KNO(3) or NH(4)NO(3)); electrode connected by wire with a voltmeter in it to measure e.m.f.

Question 31

Describe how to measure the standard electrode potential of a non metal/non metal ion half cell

Answer 31

Set up the non metal half cell: as often a gas, have solution containing non metal ions with a platinum electrode and wire. Around electrode, set up glass tube with holes at bottom (in bit with solution) to insert gas through. Connect to a standard hydrogen half cell. Connect solutions by a salt bridge (filter paper soaked in KNO(3) or NH(4)NO(3)) and electrodes by a wire with a voltmeter in. `

Question 32

Describe how to measure the standard electrode potential of a metal ion/metal ion half cell

Answer 32

Set up metal ion half cell: solution containing equimolar concentrations of each metal ion; platinum electrode. Connect to a standard hydrogen half cell. Connect solutions with a salt bridge (filter paper soaked in KNO(3) or NH(4)NO(3)) and electrodes with wires with a voltmeter in.

Question 33

How do you calculate the standard cell potential?

Answer 33

E(cell) = E(positive terminal) - E(negative terminal)

Question 34

How do you know which way a reaction ‘wants’ to go?

Answer 34

Depends what other half equation it is combined with. The most positive one wants to be reduced and go forward; the least positive one wants to be oxidised and go backwards.

Question 35

How can you predict the feasibility of a reaction?

Answer 35

See what the standard electrode potentials are for each reaction and deduce which way the reactions want to go. Now look at your starting materials. Are these the reactants or the products, according to which way the reactions will go? So, will there be a reaction?

Question 36

What are the limitations of predictions about the feasibility of a reaction from standard cell potentials?

Answer 36

Non-standard conditions such as changes in concentration result in equilibria shifting and the electrode potential changing, thus affecting the feasibility of a reaction
Predictions are about the possibility of a reaction, and don’t take into account very high activation energies that may need to be overcome in order for a reaction to occur
Standard electrode potentials apply to aqueous equilibria; many reactions take place under very different conditions

Question 37

How can electrode potentials be applied to modern application?

Answer 37

Can use them to make storage cells such as batteries. Rechargable cells work by reproducing the reactants from the products whilst recharging.

Question 38

What are fuel cells?

Answer 38

Fuel cells use the energy from a reaction of an external fuel supply with oxygen to create a voltage

Question 39

What happens in a hydrogen-oxygen fuel cell?

Answer 39

Hydrogen is the fuel that oxygen reacts with. The half equations are:
2H(2)O(l) + 2e- H(2)(g) + 2OH-(aq) negative terminal
0.5O(2)(g) + H(2)O(l) + 2e- 2OH-(aq) positive terminal
Hydrogen is oxidised as oxygen is reduced.
Overall reaction: H(2)(g) + 0.5O(2)(g) -> H(2)O(l)

Question 40

Outline some of the new developments in fuel cell vehicles

Answer 40

Using hydrogen fuel cells to power vehicles. Hydrogen can be produced by an onboard reformer from a hydrogen-rich fuel such as methanol in the following reaction at around 300*C:
CH(3)OH + H(2)O -> 3H(2) + CO(2)
Alternatively, fuel cells are being developed that use the hydrogen-rich fuel directly. This is an advantage because a liquid fuel is easier to store, and methanol can be generated from biomass (so theoretically, carbon neutral).

Question 41

What are the advantages of fuel cell vehicles?

Answer 41

Hydrogen-rich fuels only produce small amounts of CO(2) and air pollutants compared to hydrocarbon fuels
Incomplete combustion of hydrocarbon fuels results in carbon monoxide which has to be removed in a catalytic converter
A petrol engine has an efficiency of less than 20% (much is wasted as heat), whilst hydrogen fuel cell vehicles are 40-60% efficient, so fuel consumption drops by more than half

Question 42

How can hydrogen be stored in fuel cell vehicles?

Answer 42

Liquid under pressure: still need a very low temperature; stored in a giant ‘thermos flask’ so no boil
Adsorbed onto the surface of a solid material
Absorbed within some solid material: dissociate into H atoms which are incorporated as hydrides within a solid lattice

Question 43

What are the limitations of hydrogen fuel cells?

Answer 43

Large scale storage and transportation of hydrogen is difficult in terms of safety, feasibility of a pressurised liquid and a limited life cycle of a solid adsorber/absorber. Need cost/energy-efficient solution to transport
Fuel cells have a limited lifetime, requiring regular replacement and disposal following high production costs
Use toxic chemicals in production

Question 44

What are the limitations of the hydrogen economy?

Answer 44

A hydrogen economy may contribute largely to future energy needs, but limitations include:
Public and political acceptance of hydrogen as a fuel
Logistical problems in handling and maintenance of hydrogen systems
Initial manufacture of hydrogen: it is an energy carrier, not an energy source. Must be produced from electrolysis of water or by reacting methane (finite fuel) with steam. More energy may be used in making hydrogen than is saved by its use. Maybe use renewable forms of energy to generate hydrogen?