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Energetics II Flashcards

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1
Q

How does an ionic compound form

A

Metallic atoms transfer electrons to nonmetallic atoms to form positive and negative ions which attract to form a lattice

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2
Q

Melting point and boiling point of ionic substances?

A

High

Lots of energy to overcome strong electrostatic attraction between oppositely charged ions

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3
Q

Standard enthalpy change of formation definition

A

Enthalpy change when one mole of a compound is formed from its elements under standard conditions

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4
Q

Enthalpy of lattice formation (AKA lattice energy) definition

A

Enthalpy change when one mole of a solid ionic lattice is formed from its gaseous ions

Making bonds so exothermic so always NEGATIVE VALUE

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5
Q

Lattice energy is always…

And it is a measure of…

A

Negative

Ionic bond strength

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6
Q

General born-haber cycle to work out lattice energy if have to do calculation?

A

Gaseous ions -lattice enthalpy—> ionic solid (on top)

Elements (on bottom)

Arrow form elements up to gaseous ions - atomisation, ionisation, atomisation, electron affinity etc.

Arrow from elements to ionic solid - enthalpy of formation

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7
Q

Steps to convert elements to gaseous ions

A

1) Change elements in standard form to gaseous state (enthalpy of atomisation)
2) Change gaseous metal atoms to (positive) ions (ionisation energy)
3) Change gaseous non-metal atoms into (negative) ions (atomise first if diatomic then electron affinity)

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8
Q

Standard enthalpy of atomisation definition

A

Enthalpy change when one mole of gaseous atoms are produced from one mole of an element under standard conditions

Eg. 1/2Br2 (l) —-> Br- (g)

Always positive because have to supply energy to atomise

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9
Q

First electron affinity

XXXX

A

Enthalpy change for one mole of gaseous atoms

to gain one mole of electrons

to form one mole of gaseous ions with a 1- charge

Eg. X(g) + e- —-> X-(g)

——————
First electron affinity is exothermic for atoms that normally form negative ions WHY???

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10
Q

Second electron affinity

A

Enthalpy change when one mole of gaseous 1- ions

gains one electron per ion to produce

one mole of gaseous 2- ions

Eg. X-(g) + e- —-> X2- (g)

——————
Second electron affinity is ENDOTHERMIC because energy must be taken in to overcome repulsion between already negative ion and electron you are trying to add

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11
Q

Equation for lattice enthalpy

A

(energy change of elements to ionic solid aka enthalpy of formation) - (energy change the elements to gaseous ions)

Going round the cycle it’s negative gaseous ions to elements + enthalpy of formation of ionic solid from elements

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12
Q

How can we calculate the theoretical lattice enthalpy? And what do we assume?

A

Visualise perfectly spherical ions in a lattice

Calculate all forces of attraction and repulsion in the lattice

Assume perfect ionic model where ions are 100% ionic and spherical and attractions are purely electrostatic.

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13
Q

What kind of values are the values determined by the born-haber cycle?

A

Value is determined by experiment

Reflect the actual value

(We assumed perfect ionic model but if the experimental value is different we know this assumption was wrong and there is covalent character which is causes this difference)

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14
Q

What does a MORE -VE value of lattice enthalpy of FORMATION mean?

And LESS -VE value?

A 
More negative value 
More exothermic
More heat energy released
Stronger bonds made
Stronger lattice 
Less negative value
Less exothermic 
Less heat energy released
Weaker bonds made
Weaker lattice
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15
Q

What does a great a difference between the experimental and theoretical value of the born-haber cycle mean?

A

Greater degree of covalency in the lattice (and thus more polarisation)

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16
Q

Perfect ionic model

A

Ions are 100% ionic and spherical

Attractions are purely electrostatic

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17
Q

Polarised ionic bonding

A

Cation attract electrons

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18
Q

What does the extent of polarisation depend on?

A

How much

the cation pulls the electrons (polarising power)

The anion lets the electrons be pulled away (Polarisability)

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19
Q

Factors which increase polarisation in ionic compounds?

A

1) size of cation (smaller cation attracts e- more so more polarising)
2) size of anion (larger anion more polarisable)
3) increased charge of cation or anion

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20
Q

What type of polarisation would

K+F- have?

A

Low polarisation
K is a large cation
F is a small anion
low charge (both +/-1)

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21
Q

What type of polarisation would

Al3+ (I-)3 have?

A

High polarisation
Al3+ is a small cation (more polarising power)
(I-)3 is a large anion (more polarisable)

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22
Q

More +ve value of lattice DISSOCIATION enthalpy means

A 
More +ve value of dissociation enthalpy
More heat energy taken in
More endothermic 
Stronger bonds had to be broken 
Stronger lattice

Less +ve = weaker lattice

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23
Q 
High polarisability (Anion)
High polarisation (cation)
A

LARGE ANION

SMALL CATION

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24
Q

What causes polarisation and what does a greater polarisation mean?

A

Covalency causes polarisation

Greater polarisation means more covalent character

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25
Link difference in actual and theoretical values of lattice enthalpies , covalency and polarisation
Larger difference = higher covalency degree = more polarisation
26
What can the actual and theoretical values of lattice enthalpies be used to find?
Degree of polarisation
27
Why do some ionic compounds not dissolve in water?
Strong electrostatic attraction between oppositely charged ions is too large to be overcome by water molecules
28
Explain how water molecules interact with an ionic compound?
Delta positive hydrogen attracted to negative ions Delta negative oxygen attracted to positive ions Water molecules vibrate and shake ions free from lattice
29
What two things occur when ionic compounds dissolve in water?
1) Lattice broken apart (endothermic) | 2) New bonds form between water molecules and ions (exothermic)
30
Enthalpy change of solution definition ionic equation for dissolving ionic solid NaCl
Exothermic enthalpy change when one mole of ionic compound is dissolved in excess water and ions are well separated & don’t interact with each other Ionic solid —> hydrated ions NaCl(s) + (aq) —> Na+ (aq) + Cl- (aq)
31
Enthalpy of hydration definition
Exothermic enthalpy change when one mole of gaseous ions dissolve in excess water to form an infinitely dilute solution (such that further dilution causes no further heat change) M+(g) + aq —-> M+(aq) X+(g) + aq —-> X+ (aq) M+(g) + X-(g) + aq —-> M+ (aq) + X-(aq) ————— Always exothermic because bonds are MADE between ions and water molecules
32
Enthalpy of lattice dissociation
*opposite of lattice formation enthalpy Enthalpy change to convert 1 mole of solid ionic lattice to 1 mole of its constituent gaseous ions Breaking bonds so always a POSITIVE VALUE
33
How is lattice enthalpy formation connected to lattice enthalpy dissociation?
Lattice enthalpy dissociation is the OPPOSITE of lattice enthalpy formation Lattice enthalpy of formation is exothermic eg. -200 Lattice enthalpy of dissociation is (-ve formation) so -(-200) so +200
34
Characteristics of a weak ionic lattice
Small cation Large anion High charges on ions
35
What happens to lattice energy as the size of the anion increases?
Anion size increase Charge density decreases Electrostatic attraction between oppositely charge ions decreases Value of lattice energy decreases (aka. strength decreases)
36
What is charge density?
Charge per unit volume
37
What happens to lattice energy as the size of the cation increases?
Size of cation increases Charge density decreases Less electrostatic attractions Lattice energy decreases
38
What happens to lattice energy as the charge on either the cation or anion increases?
Charge on ion increases Charge density increases Electrostatic attraction increases Lattice energy increases
39
Define enthalpy of hydration
Enthalpy change when one mole of gaseous ions become hydrated such that further dilution causes no further heat change (Heat energy given out when they dissolve)
40
How does the size of the ion affect enthalpy of hydration?
``` Size of cation/anion increases Charge density of ion decreases Electrostatic attraction between water molecule and ion decreases Weaker bonds made Less energy given out Hydration enthalpy decreases ``` Charge density decreases Hydration enthalpy decreases
41
What does a higher charge on ion mean for hydration enthalpy value?
``` Higher charge Higher charge density Electrostatic attraction between ions and water molecules increases Stronger bonds made More heat energy given out Larger hydration enthalpy value ```
42
Enthalpy of solution equation
Enthalpy of solution = Lattice enthalpy of dissociation + lattice enthalpy of hydration
43
What is lattice enthalpy of dissociation equivalent to?
-ve lattice enthalpy of formation
44
Picture born-Haber cycle for NaCl
Top: gaseous ions Na+ (g) + Cl-(g) Midway between: aqueous ions Na+(aq) + Cl-(aq) Very bottom: ionic lattice NaCl(s) 1) gaseous ions to ionic lattice is LATTICE ENTHALPY OF FORMATION 2) gaseous ions to enthalpy of solution is ENTHALPY OF HYDRATION 3) ionic lattice to aqueous ions is ENTHALPY OF SOLUTION
45
Enthalpy of solution is turning what to what
Solid ionic lattice to aqueous ions
46
Enthalpy of hydration is turning what to what
Gaseous ions to aqueous ions
47
Enthalpy of solution values can be
Positive and negative ?
48
A substance is soluble if…
Enthalpy of solution is exothermic or slightly endothermic
49
Substance is not soluble if...
Enthalpy of solution significantly exothermic (high positive value)
50
Exothermic reactions
Exothermic reactions give out heat energy so the products have less energy and are more stable than the reactants
51
Endothermic reactions
Energy is taken in an endothermic reactions so the products have more energy and are less stable than the reactants
52
First law of thermodynamics
Energy can’t be created or destroyed it is changed from one type to another
53
Hess’ Law
Enthalpy change of a chemical reaction is independent of the route taken (Ie. Whatever steps the enthalpy will always be the same)
54
Entropy is a measure of
Disorder
55
Link disorder entropy and freedom of molecules
Greater freedom of molecules Greater disorder Greater entropy
56
What happens to molecules as entropy increases?
They become more randomly distributed
57
Entropy symbol
S
58
What happens when molecules gain energy?
They gain freedom of movement answer their entropy increases
59
Why do solids have lower entropies than other states?
They have more restricted movement of molecules
60
Perfect crystals
At 0Kelvin have 0 entropy
61
Why did liquids have higher entropies than solids?
Liquid molecules have greater freedom of movement
62
What is the entropy of complex molecules like?
Higher entropies than simple molecules because there are more ways their molecules can arrange themselves
63
Values of entropies trend
Gas > liquid > solid
64
Trend for orderedness of state
Solid > liquid > gas
65
What happens to entropy as gaseous alkane molecules become more complex?
Increases
66
Pentane is more complex than butane but it has a lower entropy value, why?
Standard state of pentane is liquid so molecules are more ordered so lower entropy value
67
Entropy of a system
When a chemical reaction occurs the products and reactants will have different entropy values to each other at the end Entropy of system is the same as change in entropy
68
Entropy of a system equation
Entropy of system = entropy of products - entropy of reactants
69
For a reaction to be feasible, entropy must...
Increase
70
Name 4 changes that cause an increase in entropy | This 4 examples where a reaction is feasible
1) formation of gas molecules 2) dissolving solid 3) decomposition of a substance (because resulting components have greater freedom of movement) 4) gases becoming more randomly distributed (diffusion)
71
Spontaneous change means
Feasibility
72
For a spontaneous change to occur, entropy must...
Increase! | Entropy must be > 0 AKA a positive value
73
What has happened to entropy if the reactants go from being 5 moles of gas two products of 8 moles of gas?
Entropy has increased | more moles of gas so more ways of arranging the energy of the system over the molecules
74
Reactions involving a decrease in the number of gas molecules
Entropy decreases
75
Standard entropy is measured at
298K Pressure of one bar
76
Entropy units
J K^-1 mol^-1
77
Special thing to always remember about entropy measurements
Measured in JOULES
78
Change in entropy of a reaction is the same as
Entropy of a system
79
Change in entropy equation
Change in entropy = sum of entropy products - sum of entropy reactants
80
What does the negative entropy value mean?
Entropy has decreased
81
Total change in entropy equation
Change in entropy of the surroundings + change in entropy of the system
82
Change in entropy of the surroundings equation
Change in enthalpy (joules) - —————————— Temperature (Kelvin)
83
What to remember when working out the entropy of surroundings?
Enthalpy on top of equation is often given in KJ so need to convert to JOULES by x1000
84
Enthalpy change units VS entropy change units
Enthalpy change Kj mol^-1 Entropy change J K^-1 mole^-1
85
Explain why entropy change of the surroundings might be positive even though ENTHALPY change of the reaction is negative. CHECK XX
Reaction is exothermic | Heat given out so entropy of surroundings increases
86
If a reaction ENTHALPY change is negative then entropy of surroundings is always XXX
Positive Because exothermic reaction releases heat energy so entropy of surroundings increases
87
What do we mean by feasibility
A reaction is possible in terms of energy (but this doesn’t necessarily mean it will happen - might be other factors stopping it)
88
For a reaction to be feasible...
TOTAL entropy must increase (sign must be +)
89
Dissolving ammonium nitrate crystals in water
Crystal breaks up into ions and mixes with water Endothermic (takes in energy, making bonds) Entropy change of surroundings decrease But entropy of system increases more than this so total entropy is positive and reaction is feasible.
90
What can entropy of surroundings tell you?
If a reaction is exothermic or endothermic Entropy of surroundings increases, reaction is exothermic Entropy of surroundings decreases, reaction is endothermic
91
Concentrated ethanolic acid and solid ammonium carbonate equation
2CH3COOH + (NH4)CO3 —-> 2CH3COONH4 + H2O + CO2
92
Concentrated ethanolic acid and solid ammonium carbonate
Entropy of surroundings decreases Entropy of system increases ‘Increase in entropy due to formation of gaseous carbon dioxide outweighs fall in entropy of surroundings’ Total entropy increases, reaction is feasible
93
4 typical reactions from spec where entropy changes occur
Dissolving ammonia nitrate in water Concentrated ethanolic acid and solid ammonium carbonate Magnesium ribbon and oxygen Mixing solid barium hydroxide with solid ammonium chloride
94
Magnesium ribbon plus oxygen
Reaction is feasible Reaction is not spontaneous (does happen instantly) So it’s feasible but that doesn’t mean it will necessarily happen
95
Mixing solid barium hydroxide with solid ammonium chloride equation
Ba(OH)2.8H2O(s) + 2NH4Cl(s) —> BaCl2(s) + 10H2O(l) + 2NH3(g)
96
Mixing solid barium hydroxide with solid ammonium chloride
Entropy of system increases because gas and liquid formed from two solids
97
Using states to predict entropy we are predicting entropy of ...
The system only
98
Entropy of system if BaCl2(s)
+591
99
Entropy of system if BaCl2.H2O(s)
+530
100
Name the four main entropy equations
🔺Ssystem = 🔺S products -🔺S reactants 🔺Stotal = 🔺Ssurroundings + 🔺Ssystem 🔺H reaction 🔺Ssurroundings = - ——————- V. T (in Kelvin) 🔺G = 🔺H - T🔺Ssystem
101
What to remember when working out Gibbs free energy
Gibbs answer must be in Kj mol ^-1 🔺H is in Kj mol^-1 But 🔺S system is in J K^-1 mol^-1 so you have to DIVIDE ENTROPY OF SYSTEM BY 1000 TO CONVERT FROM JOULES TO KJ
102
Gibbs must be what for reaction to be feasible Vs what total entropy must be
Negative value (🔺G<0) Total entropy has to be positive value to be feasible
103
Feasibility changes with ...
Temperature Eg. Decomposition of CaCO3 Some Kelvin values the 🔺G produced (when you do the calculation) is negative but others it is positive
104
How to calculate a temperature at which 🔺G becomes feasible?
🔺G = 🔺H - T🔺S system Work out enthalpy and entropy of system Rearrange so = 0 Replace = so finding value < T
105
Equation to find temperature at which reaction involving gases is feasible? XXX
🔺G = -RT
106
What does a large + equilibrium constant mean
Reaction highly shifted right because forward reaction happening at faster pace than backwards reaction
107
Why might a thermodynamically feasible reaction not take place?
It is inhibited by kinetic factors such as If activation energy is significantly high If collisions don’t produce enough energy to meet activation energy Strength of bonds
108
Do catalysts affect feasibility?
No
109
Do catalysts affect spontaneity (as in instantly)?
Yes
110
What is enthalpy
Balance of energy left once you have taken bonds broken - bonds made into account
111
Enthalpy changes alone do not...
Control whether reactions occur Entropy is involved as well.
112
When do you increase temperature to achieve feasibility?
When entropy change of the system is POSITIVE
113
When do you decrease temperature to achieve feasibility?
When entropy change of the system is NEGATIVE

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