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A2 Chemistry - Unit 5 > Unit 3.5.1 - Thermodynamics > Flashcards

Unit 3.5.1 - Thermodynamics Flashcards

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1
Q

What does ΔH stand for?

A

Enthalpy change

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2
Q

What is enthalpy change?

A

The heat energy transferred at a constant pressure.

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3
Q

Why is enthalpy often written as ΔHɵ?

A

To show the substances are in their standard states and the measurements were made under standard conditions.

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4
Q

What are the standard conditions for enthalpy change?

A

100 kPa pressure and 298 K

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5
Q

What value does exothermic reactions have for ΔH and why?

A

Negative because heat energy is given out so the system has lost energy.

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6
Q

What does value endothermic reactions have for ΔH and why?

A

Positive because heat energy is absorbed so the system has gained energy.

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7
Q

What is the lattice formation enthalpy?

A

The enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions under standard conditions.

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8
Q

What is lattice disassociation enthalpy?

A

The enthalpy change when one mole of a solid ionic compound is completely disassociated into gaseous ions under standard conditons.

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9
Q

What is enthalpy change of formation?

A

The enthalpy change when one mole of compound is formed form it’s elements in their standard states under standard conditions.

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10
Q

What is bond disassociation enthalpy

A

The enthalpy change when all the bonds of the same type in one mole of gaseous molecules are broken.

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11
Q

What is enthalpy change of atomisation of an element?

A

The enthalpy change when one mole of gaseous atoms is formed from an element in it’s standard state.

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12
Q

What is enthalpy change of atomisation of a compound?

A

The enthalpy change when one mole of a compound in it’s standard state is converted into gaseous atoms.

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13
Q

What is first ionisation enthalpy?

A

The enthalpy change when one mole of gaseous +1 ions is formed from one mole of gaseous atoms.

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14
Q

What is second ionisation enthalpy?

A

The enthalpy change when one mole of gaseous +2 ions is formed from one mole of gaseous +1 ions.

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15
Q

What is first electron affinity?

A

The enthalpy change when one mole of gaseous -1 ions is made from one mole of gaseous atoms.

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16
Q

What is second electron affinity?

A

The enthalpy change when one mole of gaseous -2 ions is made from one mole of gaseous -1 ions.

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17
Q

What is enthalpy of hydration?

A

The enthalpy change when one mole of aqueous ions is formed from gaseous ions.

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18
Q

What is enthalpy of solution?

A

The enthalpy change when one mole of a solute is dissolved in sufficient solvent that no further enthalpy change occurs on further dilution.

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19
Q

What type of bonds hoid ionic bond lattices together?

A

Ionic bonds.

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20
Q

Is energy required to break or form bonds?

A

Break

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21
Q

Is energy given out to break or form bonds?

A

Form

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22
Q

What are lattice enthalpies measures off?

A

Strength of ionic bond and how much energy they give out or take in.

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23
Q

Is lattice disassociation or lattice formation enthalpy negative and why?

A

Lattice formation enthalpy is negative because energy is given out when bonds are formed.

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24
Q

As lattice enthalpy can’t be measured directly, what do you have to use instead?

A

Born Haber cycles

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25
Q

What is Hess’s law?

A

The total enthalpy change of reaction is always the same no matter which route is taken.

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26
Q

What indirect route do you take to find lattice enthalpy of formation?

A
  • electron affinity of - ion - ionisation enthalpy for + ion - atomisation enthalpies + enthalpy of formation.
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27
Q

How do Born Haber cycles for Group 2 elements differ from Group 1 elements?

A

You get a second ionisation enthalpy because of the +2 charge.

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28
Q

What model do you base theoretical lattice enthalpies on?

A

The purely ionic model of a lattice.

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29
Q

What does the purely ionic model assume?

A

All the ions are spherical and have their charge evenly distributed around them.

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30
Q

Are the experimental lattice enthalpies from doing a Born Haber cycle the same or different as the theoretical lattice enthalpies based on the purely ionic model?

A

Different

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31
Q

Why aren’t the theoretical values correct?

A

Ionic compounds usually have some covalent charactiers.

32
Q

What is meant by covalent characters?

A

The positive and negative ions in a lattice aren’t usally spherical. Positive ions polarise neighbouring negative ions to different extents. The more polarisation there is the more covalent the bond will be.

33
Q

If the experimental and theoretical lattice enthalpies for a compound are very different what does this show you?

A

The compound has a lot of covalent character.

34
Q

If experimental and theoretical lattice enthalpies for a compound are very similar what does this show you?

A

The compound has very little covalent character.

35
Q

What two things happen when a solid lattice dissolves in water?

A
  1. The bonds between the ions break. 2. Then the bonds between the ions and water are made.
36
Q

Why can water form bonds with ions?

A

Oxygen is a lot more electronegative than hydrogen so it draws the bonding pair of electrons towards itself creating a dipole. Positive ions can form weak bonds with the partial negative charge on the oxygen. Negative ions can form weak bonds with the partial positive charge on the hydrogen ions.

37
Q

What type of diagram can you use to find the enthalpy change of solution?

A

An enthalpy cycle

38
Q

What indirect route can you do to find the enthalpy change of solution?

A

Lattice disassociation enthalpy + enthalpy hydration of + ion + enthalphy hydration of - ion.

39
Q

What formula relates energy and enthalpy change?

A

Enthalpy change reaction = total energy absorbed - total energy released.

40
Q

What is another way to write Enthalpy change reaction = total energy absorbed - total energy released.

A

Enthalpy change reaction - total enthalpies or bonds broken - total enthalpies of bonds formed.

41
Q

What is mean bond enthalpy?

A

The energy required to break 1 mol of a given covalent bond averaged over a range of compounds.

42
Q

Why are mean bond enthalpies never perfectly accurate?

A

it is only an average for a given type of bond in many different compounds and in various places within the compound.

43
Q

What does entropy show you?

A

How much disorder there is.

44
Q

What two things is entropy a measure of?

A

The number of ways that particles can be arranged and the number of ways that energy can be shared out between the particles.

45
Q

What symbol represents entropy?

A

S

46
Q

What are the three main things that affect entropy?

A

Physical state, dissolution and the number of particles.

47
Q

How does physical state affect entropy?

A

Gas particles whizz around anywhere they like so they have the most random arrangement of particles so they have the highest entropy, solid particles just wobble about a fixed point so there isn’t much randomness so they have the lowest entropy.

48
Q

What does dissolution involve?

A

Dissolving a solid.

49
Q

How does dissolution affect entropy?

A

Dissolving a solid increases it’s entropy as the dissolved particles can move more freely increasing the number of ways the particles and their energy can be arranged.

50
Q

How does the number of particles affect entropy?

A

More particles mean more entropy, the more particles you have the more ways they and their energy can be arranged.

51
Q

What is a spontaneous change?

A

What that happens by itself, you don’t need to give it energy.

52
Q

What is another name for a spontaneous change?

A

A feasible change.

53
Q

Even though you have to supply energy for an endothermic reaction why are they sometimes spontaneous?

A

In some reactions the entropy increases such a lot that the reaction will happen by itself without you supplying energy.

54
Q

Name two spontaneous endothermic reactions.

A

1, Evaporation of water. 2. The reaction of sodium hydrogen carbonate with hydrochloric acid.

55
Q

Why is evaporation of water spontaneous?

A

You are going from a liquid to a gas so the entropy increases.

56
Q

Why is the reaction between sodium hydrogen carbonate with hydrochloric acid spontaneous?

A

There are more particles in the products and CO2 is evolved which is a gas further increasing entropy.

57
Q

For reactions to happen does entropy change have to be negative of positive?

A

Positive.

58
Q

What two entropy changes are there during a reaction?

A

An entropy change between the reactants and products and an entropy change of the surroundings.

59
Q

What is the entropy change of the system?

A

Entropy change between the reactants and the products.

60
Q

ΔStotal =?

A

ΔSsystem + ΔSsurroundings

61
Q

ΔSsystem = ?

A

Sproducts - Sreactants

62
Q

ΔSsurroundings = ?

A
63
Q

What are the units for ΔS?

A

JK-1mole -1

64
Q

What is the symbol for free energy change?

A

ΔG

65
Q

What is ΔG used to measure?

A

To predict whether a reaction is feasible.

66
Q

What does ΔG have to be for a reaction to be feasible?

A

Less than or equal to 0

67
Q

What does ΔG take account of?

A

The changes in enthalpy and entropy in a system

68
Q

ΔG = ?

A

ΔH-TΔS

69
Q

What are the units for ΔG and ΔH?

A

Jmole-1

70
Q

Why might a reaction not always happen even though ΔG is -?

A

The reaction might have a really high activation energy or be so slow that you wouldn’t notice it happening at all.

71
Q

What values of ΔH and ΔS will always be feasible at any temperature and why?

A

-ΔH and +ΔS, because ΔG will always be -

72
Q

What values of ΔH and ΔS will not be feasible at any temperature and why?

A

+ΔH and -ΔS, because ΔG will always be +

73
Q

What are the two values ΔH and ΔS which means a reaction is sometimes feasible and why?

A
  1. +ΔH and +ΔS then ΔG will be - for higher temperatures and so the reaction will be feasible at higher temperatures. 2. -ΔH and -ΔS then ΔG will be - for lower temperatures and so the reaction will be feasible for lower temperatures.
74
Q

To find at what temperature a reaction become feasible what do you set ΔG = to?

A

0

75
Q

What equation can you use to find temperature at which a reaction just becomes feasible?

A

A2 Chemistry - Unit 5 (5 decks)

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