8. Thermodynamics Flashcards
(114 cards)
1
Q
What is enthalpy change?
A
The heat energy transferred in a reaction at constant pressure
2
Q
Symbol for enthalpy change?
A
ΔH
3
Q
What is standard enthalpy change?
A
The heat energy transferred in a reaction at 100kPa and 298K
4
Q
What does the sign on an enthalpy change value depend on?
A
Whether the reaction is exothermic or endothermic
5
Q
What does a negative enthalpy change value indicate?
A
The reaction is exothermic
6
Q
What does a positive enthalpy change value indicate?
A
The reaction is endothermic
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
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
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
10
Q
What is the equation for enthalpy of formation of sodium oxide?
A
2Na (s) + 1/2 O₂ (g) → Na₂O (s)
11
Q
What is the ΔH sign for enthalpy of formation?
A
Positive or negative
12
Q
What is the ΔH sign for enthalpy of combustion?
A
Negative
13
Q
What is the equation for the enthalpy of combustion of hydrogen?
A
H₂ (g) + 1/2 O₂ (g) → H₂O (l)
14
Q
What is ionisation enthalpy also known as?
A
First/second etc. ionisation energy
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
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
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
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
19
Q
What is the ΔH sign for ionisation enthalpy?
A
Positive
20
Q
What is the ΔH sign for first electron affinity?
A
Negative
21
Q
What is the ΔH sign for second electron affinity?
A
Positive
22
Q
What is the equation for enthalpy of first ionisation energy of magnesium?
A
Mg (g) → Mg⁺ (g) + e⁻
23
Q
What is the equation for enthalpy of second ionisation energy of magnesium?
A
Mg⁺ (g) → Mg²⁺ (g) + e⁻
24
Q
What is the equation for first electron affinity of oxygen?
A
O (g) + e⁻ → O⁻(g)
25
What is the equation for second electron affinity of oxygen?
O⁻ (g) + e⁻ → O²⁻ (g)
26
What is bond dissociation?
Enthalpy change when one mole of covalent bonds is broken in the gaseous state
27
What is enthalpy of atomisation of an element?
Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state
28
What is enthalpy of atomisation of a compound?
Enthalpy change when gaseous atoms are produced from one mole of a compound in its standard state
29
What is lattice enthalpy of formation?
Enthalpy change when one mole of a solid ionic compound is formed from into its constituent ions in the gas phase
30
What is lattice enthalpy of dissociation?
Enthalpy change when one mole of a solid ionic compound is broken up into its constituent ions in the gas phase
31
What is hydration enthalpy?
Enthalpy change when one mole of gaseous ions become hydrated (dissolved in water)
32
What is enthalpy of solution?
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
33
What is the ΔH sign for bond dissociation?
Positive
34
What is the ΔH sign for enthalpy of atomisation of an element?
Positive
35
What is the ΔH sign for enthalpy of atomisation of a compound?
Positive
36
What is the ΔH sign for lattice enthalpy of formation?
Positive
37
What is the ΔH sign for lattice enthalpy of dissociation?
Negative
38
What is the ΔH sign for hydration enthalpy?
Negative
39
What is the ΔH sign for enthalpy of solution?
Positive or negative
40
What is the equation for the bond dissociation of iodine?
I₂ (g) → 2I (g)
41
What is the equation for the enthalpy of atomisation of iodine?
1/2 I₂ (g) → I (g)
42
What is the equation for the enthalpy of atomisation of methane?
CH₄ (g) → C (g) + 4H (g)
43
What is the equation for the lattice enthalpy of formation of magnesium chloride?
Mg²⁺ (g) + 2Cl⁻ (g) → MgCl₂ (s)
44
What is the equation for the lattice enthalpy of dissociation of magnesium chloride?
MgCl₂ (s) → Mg²⁺ (g) + 2Cl⁻ (g)
45
What is the equation for the hydration enthalpy of magnesium ions?
Mg²⁺ (g) → Mg²⁺ (aq)
46
What is the equation for the enthalpy of solution of sodium chloride?
NaCl (s) → Na⁺ (aq) + Cl⁻ (aq)
47
What can 3 stages can reactions involving the formation of ionic compounds be broken into?
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
What can be used to calculate the enthalpy change in the formation of ionic compounds?
Born-Haber cycles
49
What principles are applied in a Born-Haber cycle?
Hess's Law
50
In a Born-Haber cycle, which way do the arrows for endo and exothermic changes go?
* endothermic changes go up
| * exothermic changes go down
51
In Born-Haber cycles, how can the enthalpy changes required to form free gaseous atoms be obtained?
From the atomisation enthalpies or bond dissociation enthalpies
52
Why is first electron affinity always exothermic?
The electron is attracted by the nuclear charge
53
Why are second and third electron affinities always endothermic?
An electron is being added to an already negative ion (which repel)
54
In Born-Haber cycles, which way are lattice enthalpies of formation drawn?
Downwards as they are exothermic
55
In Born-Haber cycles, which way are lattice enthalpies of dissociation drawn?
Upwards as they are endothermic
56
In a Born-Haber cycle, what should be done if the enthalpy of lattice dissociation is given instead of lattice formation?
Turn the arrow around and this value will be subtracted from the rest of the alternative route
57
In a Born-Haber cycle, what should be done if the bond dissociation of Cl2 was given instead of enthalpy of atomisation of Cl?
Divide the value by 2 as in bond dissociation 2 moles of Cl are formed
58
In a Born-Haber cycle, why is the lattice enthalpy usually shown as lattice enthalpy of formation (with a negative value)?
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
If there is stronger attraction in an ionic compound, what will happen to the lattice formation enthalpy?
It will increase
60
What is the lattice formation enthalpy dependent on?
* charge on ions
| * size of ions
61
What effect does a higher charge on ions have on lattice formation?
Increase
62
What effect does smaller ions have on lattice formation?
Increase
63
What is lattice enthalpy found in Born-Haber cycles often called?
The experimental value
64
What is the perfect ionic model?
Both ions are spheres with no polarisation
65
What does a difference between theoretical and experimental values of lattice enthalpy show?
The compounds are not perfectly ionic
66
Why is the perfect ionic model not realistic?
In reality the positive ion will attract the outer electrons of the negative ion - this leads to polarisation
67
What does a bigger difference between theoretical and experimental values of lattice enthalpy indicate?
The bigger difference, the more covalent character
68
For lattice enthalpy values, which is the experimental and which is the theoretical?
The experimental value is the one calculated by Born-Haber cycles
69
What does it mean, that the values for theoretical and experimental values of lattice enthalpy for NaCl are very close?
It is an ionic compound
70
What does it mean, that the values for theoretical and experimental values of lattice enthalpy for AgBr are very different?
It would be classed as ionic with covalent character
71
What does the magnitude of the lattice enthalpy indicate?
The overall strength of the ionic bonding
72
What does the difference between theoretical and experimental values of lattice enthalpy indicate?
The amount of covalent character
73
Why is water a polar molecule?
Because of the difference in electronegativity of the oxygen and hydrogen in the molecule
74
How do ions become hydrated?
* 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
When has a solid dissolved?
When the ions no longer interact with each other
76
When does enthalpy of hydration increase?
For smaller ions with an increasing charge - due to increased attraction from the nucleus
77
What 3 processes can dissolving an ionic compound be broken into?
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
How can the processes for dissolving an ionic compound be shown?
In a Hess cycle or a Born-Haber cycle
79
When is a compound more likely to dissolve, in terms of enthalpy of solution?
When the enthalpy of solution is more exothermic
80
What equation can be used to calculate the enthalpy of solution?
ΔH solution = ΔH lattice dissociation + ΔH hydration
81
What are feasible/spontaneous reactions?
Ones that occur of their own accord - thermodynamically possible
82
What type of reaction are many feasible reactions?
Exothermic
83
When is it possible for an endothermic reaction to occur spontaneously?
When there is an increase in disorder and involve mixing or spreading out
84
What is entropy?
The degree of disorder in a system
85
What is the degree of disorder in a system?
Entropy
86
What is the symbol for entropy?
S
87
When will the value for entropy of a reaction be positive?
When the products are more disordered than the reactants
88
What happens to particles at 0K (absolute 0)?
Particles do not move as they have no energy
89
What happens to entropy as a substance melts?
Large jump - increasing value of entropy
90
What happens to entropy as a substance boils?
Very large jump - increasing value of entropy
91
What effect will dissolving a substance have on entropy?
Increase entropy
92
What effect will a reaction that produces a gas have on entropy?
Increase entropy
93
What factors can increase the entropy of a reaction?
* dissolving a substance
* reactions that produce a gas
* reactions that produce more moles of a product
94
What formula can be used to calculate the entropy change of a reaction?
ΔS = ΣS products - ΣS reactants
95
How can you see what entropy values are?
By looking them up in tables
96
When are chemical reactions favoured?
If there is an increase in entropy
97
What happens in terms of feasibility if ΔS is positive?
Likely to be feasible
98
What happens in terms of feasibility if ΔS is negative?
Unlikely to be feasible
99
Is entropy the only factor involved in the feasibility of reactions?
No
100
What does the chance that a reaction proceeds depend on?
A balance between enthalpy, entropy and temperature
101
What is Gibbs free energy a combination of?
Enthalpy, entropy and temperature
102
What equation shows the relationship between enthalpy, entropy and temperature?
ΔG = ΔH - TΔS
103
In ΔG = ΔH - TΔS, what does G stand for?
Gibbs free energy (kJmol⁻¹)
104
In ΔG = ΔH - TΔS, what does H stand for?
Enthalpy (kJmol⁻¹)
105
In ΔG = ΔH - TΔS, what does T stand for?
Temperature (K)
106
In ΔG = ΔH - TΔS, what does S stand for?
Entropy (JK⁻¹mol⁻¹)
107
In ΔG = ΔH - TΔS, what must be remembered for the units of S?
Needs to be divided by 1000
108
In ΔG = ΔH - TΔS, what does it mean if ΔG is less than or equal to zero?
The reaction is feasible, even if ΔH is positive
109
In ΔG = ΔH - TΔS, what does it mean if ΔG is positive?
The reaction is not feasible, even if ΔH is negative
110
Why might many endothermic reactions proceed spontaneously?
There is an increase in entropy
| also become feasible when temperature is increased
111
Why might some exothermic reactions not proceed spontaneously?
Because there is a decrease in entropy
112
When does the effect of entropy become more important, and why?
Endothermic reactions because ΔG depends on TΔS
113
What is the critical temperature?
The temperature at which ΔG=O, i.e. the point at which the reaction is just feasible
114
What equation is used to calculate the critical temperature of a reaction?
T = ΔH/ΔS
