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chemistry a2 rafa > 5.2 Energy > Flashcards

5.2 Energy Flashcards

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1
Q

Define lattice enthalpy

A

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

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2
Q

Define standard enthalpy change of formation

A

The enthalpy change the accompanies the formation of 1 mole of a compound from its constituent elements in their standard states under standard conditions

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3
Q

Define first ionisation energy

A

The energy change that accompanies the removal of 1 mole of electrons from one mole of gaseous atoms

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4
Q

Define the standard enthalpy change of solution

A

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

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5
Q

Define the standard enthalpy of hydration

A

the enthalpy change that takes place when dissolving one mole of gaseous ions in water

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6
Q

What is the standard electrode potential of a half cell?

A

the e.m.f. of a half cell compared with a standard hydrogen half cell, measured at 298 K with solution concentrations of 1 moldm-3 and gas pressure of 100kPa

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7
Q

What does a half cell consist of?

A

An element in two oxidation states

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8
Q

What is the simplest half cell?

A

metal placed in an aqueous solution of its ions

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9
Q

What is the simplest half cell containing non-metal and non-metal ions?

A

hydrogen half cell, comprised of H2 and H+ ions with a coated platinum electrode

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10
Q

What is the purpose of platinum in a hydrogen half cell?

A

to be in contact with both the H2 and the H+ ions a

to allow the transfer of electrons into and out of the half cell via a connecting wire

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11
Q

What is the platinum electrode in a hydrogen half cell coated in?

A

platinum black, a spongy coat which can transfer electrons

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12
Q

What is a standard hydrogen half cell composed of?

A
  • HCl(aq) of conc. 1 mol dm-3, as the source of H+
  • hydrogen gas, at 100kPa pressure
  • inert platinum electrode
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13
Q

What acts as an electrode in metal half cells and why?

A

platinum as there is no other solid piece of metal that could act as an electrode

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14
Q

What is the e.m.f of a hydrogen half cell?

A

0V

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15
Q

What are salt bridges usually made from?

A

a piece of filter paper soaked in an aqueous solution of ionic substance, usually KNO3 or NH4NO3

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16
Q

What does a more negative Estandard value mean?

A

greater tendency for the half cell to undergo oxidation

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17
Q

What does a more positive Estandard value mean?

A

greater tendency for the half cell to undergo reduction

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18
Q

What does a positive Estandard mean?

A

half cell accept electrons from the hydrogen half cell

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19
Q

What does the voltmeter that joins the half cells measure?

A

potential difference of the cell - the difference between the electrode potentials of the cells

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20
Q

Equation for standard electrode potential

A

Estandardcell = Estandard (positive terminal)- E standard (negative terminal)

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21
Q

Limitations of standard conditions of electrode cells

A
  • predictions can be made about the equilibrium position but not the reaction rate, which may be extremely slow due to a high activation energy
  • may not be standard conditions
  • standard electrode potentials apply to aqueous equilibria - may not be aqueous
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22
Q

What does a large difference between Estandard values indicate?

A

reaction is more likely to take place

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23
Q

What happens when the difference in Estandard values is less than 0.4?

A

the reaction is unlikely to take place

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24
Q

What three types can cells be divided into?

A
  • non-rechargeable cells
  • rechargeable cells
  • fuel cells
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25
Q

Describe non-rechargeable cells

A

provide electricity until chemicals have reacted to such an extent that the voltage falls

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26
Q

Describe rechargeable cells?

A

the cell reaction can be reversed during charging

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27
Q

Common examples of rechargeable cells

A
  • nickel and cadmium, used in rechargeable batteries

- lithium ion and lithium polymer batteries, used in laptops

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28
Q

Describe fuel cells

A

cell reaction uses external supplies of a fuel and an oxidant
-consumed and need to be continuously supplied

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29
Q

What does a fuel cell use?

A

energy of the reaction of a fuel with oxygen to create a voltage

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30
Q

What happens in an alkaline hydrogen fuel cell?

A
  • H2 and O2 flow in and react water flows out

- OH- ions flow from O2 end of fuel cell towards H2 end

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31
Q

What do more exothermic lattice enthalpy values indicate?

A
  • stronger ionic bonds

- higher melting and boiling points

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32
Q

What is the standard enthalpy change of atomisation?

A

when one mole of gaseous atoms is formed from its elements in its standard state

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33
Q

Is enthalpy change of atomisation endo or exothermic?

A

ALWAYS endothermic

-bonds have to be broken

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34
Q

Define second ionisation energy

A

one mole of gaseous 2+ ions are formed from one mole of gaseous 1+ ions

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35
Q

Is first electron affinity always endothermic or exothermic?

A

exothermic

-electron is attracted to the outer cell of an atom by its nucleus

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36
Q

Define atomisation

A

The enthalpy change when one mole of gaseous atoms is formed from one element in its standard state

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37
Q

Define first electron affinity

A

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

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38
Q

Define second electron affinity

A

The enthalpy change accompanying the additions of one electron to each ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions

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39
Q

Do we expect lattice enthalpy to be exothermic or endothermic?

A

Exothermic as bonds are being formed

40
Q

What can lattice enthalpy tell us about?

A

Strength of ionic bonds

-more energy released = harder to break up lattice

41
Q

Is the first ionisation energy always exothermic or endothermic and why?

A
  • endothermic

- we must overcome the attraction between the nucleus and the electron

42
Q

Is second electron affinity always exothermic or endothermic?

A
  • endothermic

- combining electron with a 1- ion and must put in energy to overcome repulsion

43
Q

Why can we not measure lattice enthalpy directly?

A

It is not possible to form one mole of ionic solid from gaseous ions in practice

44
Q

Two stages of a solid dissolving

A
  • ionic lattice breaks down

- free ions become part of solution (hydration)

45
Q

What is the enthalpy of breaking down an ionic lattice equal to?

A

-(lattice enthalpy)

46
Q

Is breaking down a lattice always endothermic or exothermic?

A

endothermic

47
Q

What is the magnitude of lattice enthalpy dependent on?

A
  • size of ions involved
  • charge density
  • ionic bond strength
48
Q

Do large ions or small ions form stronger bonds with each other?

A

Smaller ions because they can get closer together

49
Q

What does greater charge density mean?

A
  • attractive force is stronger so bonds are stronger

- lattices of smaller, more highly charged ions will have stronger bonds and give out the most energy when they form

50
Q

Equation for charge density

A

charge/size

51
Q

What is the magnitude of the enthalpy of hydration affected by?

A
  • size of ions

- charge density

52
Q

What is entropy?

A

the quantitative measure of the degree of disorder in a system

53
Q

What is the standard entropy?

A

the entropy content of one mole of the substance under standard conditions

54
Q

What is the equation of entropy change?

A

Change in entropy of a system = (sum of standard entropy of products) - (sum of standard entropy of reactants)

55
Q

Equation for total entropy change of universe?

A

(entropy change of system) + (entropy change of surroundings)

56
Q

What is the standard entropy change of reaction?

A

entropy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states

57
Q

What is the free energy change?

A

The balance between enthalpy, entropy and temperature for a process:
free energy change = change in enthalpy - temperature (in K) x change in entropy

58
Q

What does G stand for?

A

Free energy

59
Q

What does H stand for?

A

Enthalpy

60
Q

What does S stand for?

A

Entropy

61
Q

Why is free energy useful?

A

can tell us if a reaction is feasible

62
Q

What values of delta G show us a reaction is feasible?

A

Negative ones

63
Q

Equation for the entropy of the surroundings?

A

-change in enthalpy/ temperature

64
Q

What things play a role in predicting weather or not a reaction will happen?

A
  • thermodynamics (free energy)

- kinetics

65
Q

What are examples of reaction kinetics that are used to predict feasibility?

A
  • activation energy (may be too high for a reaction to happen)
  • rate of reaction (may be very slow and not be seen to happen)
66
Q

Oxidation number of hydrogen

A

+1

67
Q

Oxidation number of oxygen

A

-2

68
Q

What is an oxidising agent?

A

the species that is reduced in a reaction and causes another species to be oxidised

69
Q

What is a reducing agent?

A

the species that is oxidised and causes another species to be reduced

70
Q

In a redox titration, what value is known?

A

Either a reducing agent or the oxidising agent

71
Q

What do we use as indicators in redox titrations?

A
  • normal acid base indicator

- self indicate when changing oxidation states

72
Q

Is MnO4- an oxidising agent or a reducing agent?

A

oxidising

73
Q

What is the colour change when manganate (VII) is reduced and what is the change in oxidation states?

A

Deep purple at 7+ to colourless 2+

74
Q

What is the equation for the reduction of manganate (VII) ions?

A

8H+ + MnO4- +5e- = Mn2+ + 4H2O

75
Q

What do we use as a source of manganate (VII) ions?

A

Potassium permanganate

KMnO4

76
Q

In an iron (II) and potassium permanganate titration, what ion is being oxidised?

A

Fe2+

77
Q

Equation for the titration reaction between iron (II) and potassium permanganate

A

8H+ + MnO4- + 5Fe2+ = Mn2+ +4H2O + 5Fe3+

78
Q

Where is the end point of a potassium permanganate and iron (II) titration?

A

-goes purple as MnO4- goes to Mn2+

79
Q

What are the uses of a iron (II) and potassium permanganate titration?

A
  • working out the percentage composition of iron in a sample

- assuming all of the iron has formed 2+ ions in solution

80
Q

Aside from potassium permanganate, what else can be used to oxidise Fe2+ ions?

A 
Potassium dichromate (VI)
Cr2O72- ions
81
Q

What is the overall equation for the titration between Fe2+ and chromium (VI)?

A

14H+ + Cr2O72- + 6Fe2+ = 2Cr3+ + 7H2O + 6Fe3+

82
Q

How do you determine the end point of an Fe2+ and chromium (VI) titration?

A
  • NO CLEAR COLOUR CHANGE

- indicator needed

83
Q

What is an indicator used to determine the end point of De2+ and chromium (VI) titrations and what is its colour change?

A

diphenylamine sulfonate

gives a violet-blue end-point

84
Q

How is the end point in an iodine titration determined and why?

A
  • iodine has a blue-black colour in presence of starch

- iodide is colourless in starch

85
Q

What titration uses the reduction of iodine?

A

iodine and sodium thiosulfate

86
Q

What does sodium thiosulfate do to iodine?

A

reduces it

87
Q

What is the titration equation for the reaction between iodine and sodium thiosulfate?

A

2S2O3- + I2 = S4O62- + 2I-

88
Q

What is the formula for a thiosulfate ion?

A

S2O3-

89
Q

What is the formula for a tetrathionate ion?

A

S4O62-

90
Q

Uses of an iodine - sodium thiosulfate titration?

A
  • work out the concentration of oxidising agent by letting it oxide excess I-
  • use titration to find the amount of I2 present after oxidation
  • work backwards to find the conc of the oxidising agent
91
Q

First thing to do when working with unknown titrations?

A

find the full reaction equation by using two half equations of oxidation and reduction

92
Q

Advantages of fuel cells over fossil fuels

A
  • only H2O formed (non polluting)

- greater efficiency

93
Q

Disadvantages of fuel cells over fossil fuels

A
  • H2 difficult to store
  • H2 difficult to initially manufacture
  • limited life cycle of H2 absorber
94
Q

What mass do you use when doing netball change of solution?

A

Mass of solution, not mass of water

95
Q

How concordat does a titre have to be?

A

0.1cm3

chemistry a2 rafa (8 decks)

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