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A2 Chemistry Unit 1 > 2.2 Energy > Flashcards

2.2 Energy Flashcards

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0
Q

What does Hess’ law state?

A

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

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1
Q

Define lattice enthalpy

A

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

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2
Q

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

A
  • Ionic bonds are electrostatic forces of attraction between oppositely charged ions
  • Ionic bonds are strong
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3
Q

Describe some features of lattice enthalpy

A
  • 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
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4
Q

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

A
  • 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)
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5
Q

Define ‘the standard enthalpy change of atomisation’

A

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

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6
Q

Define the first ionisation energy

A

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

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7
Q

Define the first electron affinity

A

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

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8
Q

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

A
  • Zero energy

- The elements in their standard states

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9
Q

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

A
  • ΔH values pointing upwards are endothermic

- ΔH values pointing downwards are exothermic

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10
Q

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

A

The ionic solid

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11
Q

Define the standard enthalpy change of formation

A

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

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12
Q

Define the standard enthalpy change of solution

A

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

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13
Q

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

A
  • Breakdown of the ionic lattice into gaseous ions

- Hydration of the ions

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14
Q

Describe the breakdown of the ionic lattice

A

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
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15
Q

Describe hydration

A

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
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16
Q

Define the standard enthalpy change of hydration

A

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

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17
Q

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

A
  • The enthalpy changes of hydration of the constituent gaseous ions
  • The enthalpy change of solution of the ionic solid
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18
Q

What 2 factors affect the size of lattice enthalpy?

A

Ionic size

Ionic charge

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19
Q

How does ionic size affect lattice enthalpy?

A

As the ionic radius increases:

  • The attraction between the ions decreases
  • The lattice enthalpy becomes less negative (less exothermic)
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20
Q

How does ionic charge affect lattice enthalpy?

A
  • 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
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21
Q

What factors affect the size of hydration enthalpy?

A

Ionic size

Ionic charge

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22
Q

How does ionic size affect the size of hydration enthalpy?

A

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
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23
Q

How does ionic charge affect the size of hydration enthalpy?

A

As the charge on the ion increases

  • There is a greater attraction for water molecules
  • So the hydration enthalpy is more negative
24
Q

What is entropy?

A

The quantitative measure of the degree of disorder in a system

25
Q

Why is entropy, S, always a positive number?

A

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

26
Q

How does entropy increase?

A

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

27
Q

How do you calculate entropy changes?

A

Sum of products - sum of reactants

28
Q

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

A

Positive

29
Q

What is the free energy change, ΔG?

A

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

30
Q

What is a spontaneous process?

A

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

31
Q

When is a process spontaneous?

A

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

32
Q

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

A

One that is less than 0

33
Q

What is an oxidising agent?

A

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

34
Q

What is a reducing agent?

A

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

35
Q

What does a half cell comprise of?

A

An element in 2 oxidation states

- A metal placed in an aqueous solution of its ions

36
Q

What is the electrode potential of a half cell?

A

Its tendency to gain or lose electrons in equilibrium

37
Q

How is a simple electrochemical cell made?

A

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)
38
Q

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

A

Lowest (most negative)

39
Q

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

A

More negative

40
Q

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

A

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
41
Q

Define ‘standard electrode potential of a half cell’

A

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

42
Q

What are the standard conditions?

A

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

43
Q

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

A

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

44
Q

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

A

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

45
Q

What is the standard cell potential?

A

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

46
Q

What is an electromotive force?

A

The voltage produced by a cell when no current flows

47
Q

Define ‘cell reaction’

A

The overall chemical reaction taking place in the cell

- The sum of the oxidation and reduction half equations taking place in each half cell

48
Q

What are the 3 main types of electrochemical cells?

A
  • Non-rechargeable cells
  • Rechargeable cells
  • Fuel cells
49
Q

Explain how a non-rechargeable cell works

A
  • Provide electrical energy until the chemicals have reacted to such an extent that the voltage falls
  • The cell is then ‘flat’ and is discarded
50
Q

Explain how a rechargeable cell works

A
  • 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
51
Q

Explain how a fuel cell works

A
  • 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
52
Q

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

A
  • A liquid fuel is easier to store than hydrogen gas

- Methanol can be generated from biomass

53
Q

What are the advantages of fuel cell vehicles?

A

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

54
Q

What different strategies are there for storing hydrogen?

A
  • 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
55
Q

What is the main problem in storing hydrogen?

A

It is a gas with a very low boiling point

56
Q

What are the limitations of hydrogen fuel cells?

A
  • 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
57
Q

What are the limitations of the ‘hydrogen economy’?

A
  • 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.

A2 Chemistry Unit 1 (3 decks)

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