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AQA A Level Physical Chemistry > 8. Thermodynamics > Flashcards

8. Thermodynamics Flashcards

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1
Q

What is enthalpy change?

A

The heat energy transferred in a reaction at constant pressure

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2
Q

Symbol for enthalpy change?

A

ΔH

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3
Q

What is standard enthalpy change?

A

The heat energy transferred in a reaction at 100kPa and 298K

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4
Q

What does the sign on an enthalpy change value depend on?

A

Whether the reaction is exothermic or endothermic

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5
Q

What does a negative enthalpy change value indicate?

A

The reaction is exothermic

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6
Q

What does a positive enthalpy change value indicate?

A

The reaction is endothermic

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7
Q

What is standard enthalpy change of formation?

A

Enthalpy change when 1 mole of substance is formed from its constituent elements with all reactants and products in standard states under standard conditions

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8
Q

What is standard enthalpy change of combustion?

A

Enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions

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9
Q

How is enthalpy change calculated in terms of the sum of bonds broken and formed?

A

sum of bonds broken - sum of bonds formed

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10
Q

What is the equation for enthalpy of formation of sodium oxide?

A

2Na (s) + 1/2 O₂ (g) → Na₂O (s)

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11
Q

What is the ΔH sign for enthalpy of formation?

A

Positive or negative

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12
Q

What is the ΔH sign for enthalpy of combustion?

A

Negative

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13
Q

What is the equation for the enthalpy of combustion of hydrogen?

A

H₂ (g) + 1/2 O₂ (g) → H₂O (l)

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14
Q

What is ionisation enthalpy also known as?

A

First/second etc. ionisation energy

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15
Q

What is first ionisation energy?

A

Enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ions

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16
Q

What is second ionisation energy?

A

Enthalpy change when each ion in one mole of gaseous 1+ ions loses one electron to form one mole of gaseous 2+ ions

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17
Q

What is first electron affinity?

A

Enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions

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18
Q

What is second electron affinity?

A

Enthalpy change when each ion in one mole of gaseous 1- gains one electron to form one mole of gaseous 2- ions

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19
Q

What is the ΔH sign for ionisation enthalpy?

A

Positive

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20
Q

What is the ΔH sign for first electron affinity?

A

Negative

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21
Q

What is the ΔH sign for second electron affinity?

A

Positive

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22
Q

What is the equation for enthalpy of first ionisation energy of magnesium?

A

Mg (g) → Mg⁺ (g) + e⁻

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23
Q

What is the equation for enthalpy of second ionisation energy of magnesium?

A

Mg⁺ (g) → Mg²⁺ (g) + e⁻

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24
Q

What is the equation for first electron affinity of oxygen?

A

O (g) + e⁻ → O⁻(g)

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25
Q

What is the equation for second electron affinity of oxygen?

A

O⁻ (g) + e⁻ → O²⁻ (g)

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26
Q

What is bond dissociation?

A

Enthalpy change when one mole of covalent bonds is broken in the gaseous state

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27
Q

What is enthalpy of atomisation of an element?

A

Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state

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28
Q

What is enthalpy of atomisation of a compound?

A

Enthalpy change when gaseous atoms are produced from one mole of a compound in its standard state

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29
Q

What is lattice enthalpy of formation?

A

Enthalpy change when one mole of a solid ionic compound is formed from into its constituent ions in the gas phase

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30
Q

What is lattice enthalpy of dissociation?

A

Enthalpy change when one mole of a solid ionic compound is broken up into its constituent ions in the gas phase

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31
Q

What is hydration enthalpy?

A

Enthalpy change when one mole of gaseous ions become hydrated (dissolved in water)

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32
Q

What is enthalpy of solution?

A

Enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other

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33
Q

What is the ΔH sign for bond dissociation?

A

Positive

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34
Q

What is the ΔH sign for enthalpy of atomisation of an element?

A

Positive

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35
Q

What is the ΔH sign for enthalpy of atomisation of a compound?

A

Positive

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36
Q

What is the ΔH sign for lattice enthalpy of formation?

A

Positive

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37
Q

What is the ΔH sign for lattice enthalpy of dissociation?

A

Negative

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38
Q

What is the ΔH sign for hydration enthalpy?

A

Negative

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39
Q

What is the ΔH sign for enthalpy of solution?

A

Positive or negative

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40
Q

What is the equation for the bond dissociation of iodine?

A

I₂ (g) → 2I (g)

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41
Q

What is the equation for the enthalpy of atomisation of iodine?

A

1/2 I₂ (g) → I (g)

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42
Q

What is the equation for the enthalpy of atomisation of methane?

A

CH₄ (g) → C (g) + 4H (g)

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43
Q

What is the equation for the lattice enthalpy of formation of magnesium chloride?

A

Mg²⁺ (g) + 2Cl⁻ (g) → MgCl₂ (s)

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44
Q

What is the equation for the lattice enthalpy of dissociation of magnesium chloride?

A

MgCl₂ (s) → Mg²⁺ (g) + 2Cl⁻ (g)

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45
Q

What is the equation for the hydration enthalpy of magnesium ions?

A

Mg²⁺ (g) → Mg²⁺ (aq)

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46
Q

What is the equation for the enthalpy of solution of sodium chloride?

A

NaCl (s) → Na⁺ (aq) + Cl⁻ (aq)

47
Q

What can 3 stages can reactions involving the formation of ionic compounds be broken into?

A
  1. Formation of free gaseous atoms from the elements in their standard states (enthalpy of atomisation)
  2. Addition or removal of electrons to form ions (enthalpy of ionisation/electron affinity)
  3. Attraction of ions to form the ionic compound (enthalpy of lattice formation)
48
Q

What can be used to calculate the enthalpy change in the formation of ionic compounds?

A

Born-Haber cycles

49
Q

What principles are applied in a Born-Haber cycle?

A

Hess’s Law

50
Q

In a Born-Haber cycle, which way do the arrows for endo and exothermic changes go?

A
  • endothermic changes go up

* exothermic changes go down

51
Q

In Born-Haber cycles, how can the enthalpy changes required to form free gaseous atoms be obtained?

A

From the atomisation enthalpies or bond dissociation enthalpies

52
Q

Why is first electron affinity always exothermic?

A

The electron is attracted by the nuclear charge

53
Q

Why are second and third electron affinities always endothermic?

A

An electron is being added to an already negative ion (which repel)

54
Q

In Born-Haber cycles, which way are lattice enthalpies of formation drawn?

A

Downwards as they are exothermic

55
Q

In Born-Haber cycles, which way are lattice enthalpies of dissociation drawn?

A

Upwards as they are endothermic

56
Q

In a Born-Haber cycle, what should be done if the enthalpy of lattice dissociation is given instead of lattice formation?

A

Turn the arrow around and this value will be subtracted from the rest of the alternative route

57
Q

In a Born-Haber cycle, what should be done if the bond dissociation of Cl2 was given instead of enthalpy of atomisation of Cl?

A

Divide the value by 2 as in bond dissociation 2 moles of Cl are formed

58
Q

In a Born-Haber cycle, why is the lattice enthalpy usually shown as lattice enthalpy of formation (with a negative value)?

A

So that an equation can be written where the enthalpy of formation of the ionic compound equals the sum of all the other enthalpy changes

59
Q

If there is stronger attraction in an ionic compound, what will happen to the lattice formation enthalpy?

A

It will increase

60
Q

What is the lattice formation enthalpy dependent on?

A
  • charge on ions

* size of ions

61
Q

What effect does a higher charge on ions have on lattice formation?

A

Increase

62
Q

What effect does smaller ions have on lattice formation?

A

Increase

63
Q

What is lattice enthalpy found in Born-Haber cycles often called?

A

The experimental value

64
Q

What is the perfect ionic model?

A

Both ions are spheres with no polarisation

65
Q

What does a difference between theoretical and experimental values of lattice enthalpy show?

A

The compounds are not perfectly ionic

66
Q

Why is the perfect ionic model not realistic?

A

In reality the positive ion will attract the outer electrons of the negative ion - this leads to polarisation

67
Q

What does a bigger difference between theoretical and experimental values of lattice enthalpy indicate?

A

The bigger difference, the more covalent character

68
Q

For lattice enthalpy values, which is the experimental and which is the theoretical?

A

The experimental value is the one calculated by Born-Haber cycles

69
Q

What does it mean, that the values for theoretical and experimental values of lattice enthalpy for NaCl are very close?

A

It is an ionic compound

70
Q

What does it mean, that the values for theoretical and experimental values of lattice enthalpy for AgBr are very different?

A

It would be classed as ionic with covalent character

71
Q

What does the magnitude of the lattice enthalpy indicate?

A

The overall strength of the ionic bonding

72
Q

What does the difference between theoretical and experimental values of lattice enthalpy indicate?

A

The amount of covalent character

73
Q

Why is water a polar molecule?

A

Because of the difference in electronegativity of the oxygen and hydrogen in the molecule

74
Q

How do ions become hydrated?

A
  • ions in an ionic lattice
  • cations attracted to Oˢ⁻ and anions to Hˢ⁺
  • distorts charge cloud of ions and reduces forces holding them together
  • in solution, ions move from lattice and become surrounded by water molecules
75
Q

When has a solid dissolved?

A

When the ions no longer interact with each other

76
Q

When does enthalpy of hydration increase?

A

For smaller ions with an increasing charge - due to increased attraction from the nucleus

77
Q

What 3 processes can dissolving an ionic compound be broken into?

A
  1. Breaking the ionic solid into gaseous ions → lattice dissociation
  2. Hydrating the positive ions → enthalpy of hydration of cations
  3. Hydrating the negative ions → enthalpy of hydration of anions
78
Q

How can the processes for dissolving an ionic compound be shown?

A

In a Hess cycle or a Born-Haber cycle

79
Q

When is a compound more likely to dissolve, in terms of enthalpy of solution?

A

When the enthalpy of solution is more exothermic

80
Q

What equation can be used to calculate the enthalpy of solution?

A

ΔH solution = ΔH lattice dissociation + ΔH hydration

81
Q

What are feasible/spontaneous reactions?

A

Ones that occur of their own accord - thermodynamically possible

82
Q

What type of reaction are many feasible reactions?

A

Exothermic

83
Q

When is it possible for an endothermic reaction to occur spontaneously?

A

When there is an increase in disorder and involve mixing or spreading out

84
Q

What is entropy?

A

The degree of disorder in a system

85
Q

What is the degree of disorder in a system?

A

Entropy

86
Q

What is the symbol for entropy?

A

S

87
Q

When will the value for entropy of a reaction be positive?

A

When the products are more disordered than the reactants

88
Q

What happens to particles at 0K (absolute 0)?

A

Particles do not move as they have no energy

89
Q

What happens to entropy as a substance melts?

A

Large jump - increasing value of entropy

90
Q

What happens to entropy as a substance boils?

A

Very large jump - increasing value of entropy

91
Q

What effect will dissolving a substance have on entropy?

A

Increase entropy

92
Q

What effect will a reaction that produces a gas have on entropy?

A

Increase entropy

93
Q

What factors can increase the entropy of a reaction?

A
  • dissolving a substance
  • reactions that produce a gas
  • reactions that produce more moles of a product
94
Q

What formula can be used to calculate the entropy change of a reaction?

A

ΔS = ΣS products - ΣS reactants

95
Q

How can you see what entropy values are?

A

By looking them up in tables

96
Q

When are chemical reactions favoured?

A

If there is an increase in entropy

97
Q

What happens in terms of feasibility if ΔS is positive?

A

Likely to be feasible

98
Q

What happens in terms of feasibility if ΔS is negative?

A

Unlikely to be feasible

99
Q

Is entropy the only factor involved in the feasibility of reactions?

A

No

100
Q

What does the chance that a reaction proceeds depend on?

A

A balance between enthalpy, entropy and temperature

101
Q

What is Gibbs free energy a combination of?

A

Enthalpy, entropy and temperature

102
Q

What equation shows the relationship between enthalpy, entropy and temperature?

A

ΔG = ΔH - TΔS

103
Q

In ΔG = ΔH - TΔS, what does G stand for?

A

Gibbs free energy (kJmol⁻¹)

104
Q

In ΔG = ΔH - TΔS, what does H stand for?

A

Enthalpy (kJmol⁻¹)

105
Q

In ΔG = ΔH - TΔS, what does T stand for?

A

Temperature (K)

106
Q

In ΔG = ΔH - TΔS, what does S stand for?

A

Entropy (JK⁻¹mol⁻¹)

107
Q

In ΔG = ΔH - TΔS, what must be remembered for the units of S?

A

Needs to be divided by 1000

108
Q

In ΔG = ΔH - TΔS, what does it mean if ΔG is less than or equal to zero?

A

The reaction is feasible, even if ΔH is positive

109
Q

In ΔG = ΔH - TΔS, what does it mean if ΔG is positive?

A

The reaction is not feasible, even if ΔH is negative

110
Q

Why might many endothermic reactions proceed spontaneously?

A

There is an increase in entropy

also become feasible when temperature is increased

111
Q

Why might some exothermic reactions not proceed spontaneously?

A

Because there is a decrease in entropy

112
Q

When does the effect of entropy become more important, and why?

A

Endothermic reactions because ΔG depends on TΔS

113
Q

What is the critical temperature?

A

The temperature at which ΔG=O, i.e. the point at which the reaction is just feasible

114
Q

What equation is used to calculate the critical temperature of a reaction?

A

T = ΔH/ΔS

AQA A Level Physical Chemistry (12 decks)

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