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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
Q

Link difference in actual and theoretical values of lattice enthalpies , covalency and polarisation

A

Larger difference = higher covalency degree = more polarisation

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26
Q

What can the actual and theoretical values of lattice enthalpies be used to find?

A

Degree of polarisation

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27
Q

Why do some ionic compounds not dissolve in water?

A

Strong electrostatic attraction between oppositely charged ions is too large to be overcome by water molecules

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28
Q

Explain how water molecules interact with an ionic compound?

A

Delta positive hydrogen attracted to negative ions

Delta negative oxygen attracted to positive ions

Water molecules vibrate and shake ions free from lattice

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29
Q

What two things occur when ionic compounds dissolve in water?

A

1) Lattice broken apart (endothermic)

2) New bonds form between water molecules and ions (exothermic)

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30
Q

Enthalpy change of solution definition

ionic equation for dissolving ionic solid NaCl

A

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)

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31
Q

Enthalpy of hydration definition

A

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

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32
Q

Enthalpy of lattice dissociation

A

*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

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33
Q

How is lattice enthalpy formation connected to lattice enthalpy dissociation?

A

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

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34
Q

Characteristics of a weak ionic lattice

A

Small cation
Large anion
High charges on ions

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35
Q

What happens to lattice energy as the size of the anion increases?

A

Anion size increase
Charge density decreases
Electrostatic attraction between oppositely charge ions decreases
Value of lattice energy decreases (aka. strength decreases)

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36
Q

What is charge density?

A

Charge per unit volume

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37
Q

What happens to lattice energy as the size of the cation increases?

A

Size of cation increases
Charge density decreases
Less electrostatic attractions
Lattice energy decreases

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38
Q

What happens to lattice energy as the charge on either the cation or anion increases?

A

Charge on ion increases
Charge density increases
Electrostatic attraction increases
Lattice energy increases

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39
Q

Define enthalpy of hydration

A

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)

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40
Q

How does the size of the ion affect enthalpy of hydration?

A 
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

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41
Q

What does a higher charge on ion mean for hydration enthalpy value?

A 
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
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42
Q

Enthalpy of solution equation

A

Enthalpy of solution =

Lattice enthalpy of dissociation + lattice enthalpy of hydration

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43
Q

What is lattice enthalpy of dissociation equivalent to?

A

-ve lattice enthalpy of formation

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44
Q

Picture born-Haber cycle for NaCl

A

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

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45
Q

Enthalpy of solution is turning what to what

A

Solid ionic lattice to aqueous ions

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46
Q

Enthalpy of hydration is turning what to what

A

Gaseous ions to aqueous ions

47
Q

Enthalpy of solution values can be

A

Positive and negative ?

48
Q

A substance is soluble if…

A

Enthalpy of solution is exothermic or slightly endothermic

49
Q

Substance is not soluble if…

A

Enthalpy of solution significantly exothermic (high positive value)

50
Q

Exothermic reactions

A

Exothermic reactions give out heat energy so the products have less energy and are more stable than the reactants

51
Q

Endothermic reactions

A

Energy is taken in an endothermic reactions so the products have more energy and are less stable than the reactants

52
Q

First law of thermodynamics

A

Energy can’t be created or destroyed it is changed from one type to another

53
Q

Hess’ Law

A

Enthalpy change of a chemical reaction is independent of the route taken

(Ie. Whatever steps the enthalpy will always be the same)

54
Q

Entropy is a measure of

A

Disorder

55
Q

Link disorder entropy and freedom of molecules

A

Greater freedom of molecules

Greater disorder

Greater entropy

56
Q

What happens to molecules as entropy increases?

A

They become more randomly distributed

57
Q

Entropy symbol

A

S

58
Q

What happens when molecules gain energy?

A

They gain freedom of movement answer their entropy increases

59
Q

Why do solids have lower entropies than other states?

A

They have more restricted movement of molecules

60
Q

Perfect crystals

A

At 0Kelvin have 0 entropy

61
Q

Why did liquids have higher entropies than solids?

A

Liquid molecules have greater freedom of movement

62
Q

What is the entropy of complex molecules like?

A

Higher entropies than simple molecules because there are more ways their molecules can arrange themselves

63
Q

Values of entropies trend

A

Gas > liquid > solid

64
Q

Trend for orderedness of state

A

Solid > liquid > gas

65
Q

What happens to entropy as gaseous alkane molecules become more complex?

A

Increases

66
Q

Pentane is more complex than butane but it has a lower entropy value, why?

A

Standard state of pentane is liquid so molecules are more ordered so lower entropy value

67
Q

Entropy of a system

A

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
Q

Entropy of a system equation

A

Entropy of system = entropy of products - entropy of reactants

69
Q

For a reaction to be feasible, entropy must…

A

Increase

70
Q

Name 4 changes that cause an increase in entropy

This 4 examples where a reaction is feasible

A

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
Q

Spontaneous change means

A

Feasibility

72
Q

For a spontaneous change to occur, entropy must…

A

Increase!

Entropy must be > 0 AKA a positive value

73
Q

What has happened to entropy if the reactants go from being 5 moles of gas two products of 8 moles of gas?

A

Entropy has increased

more moles of gas so more ways of arranging the energy of the system over the molecules

74
Q

Reactions involving a decrease in the number of gas molecules

A

Entropy decreases

75
Q

Standard entropy is measured at

A

298K

Pressure of one bar

76
Q

Entropy units

A

J K^-1 mol^-1

77
Q

Special thing to always remember about entropy measurements

A

Measured in JOULES

78
Q

Change in entropy of a reaction is the same as

A

Entropy of a system

79
Q

Change in entropy equation

A

Change in entropy = sum of entropy products - sum of entropy reactants

80
Q

What does the negative entropy value mean?

A

Entropy has decreased

81
Q

Total change in entropy equation

A

Change in entropy of the surroundings + change in entropy of the system

82
Q

Change in entropy of the surroundings equation

A

Change in enthalpy (joules)
- ——————————
Temperature (Kelvin)

83
Q

What to remember when working out the entropy of surroundings?

A

Enthalpy on top of equation is often given in KJ so need to convert to JOULES by x1000

84
Q

Enthalpy change units

VS

entropy change units

A

Enthalpy change
Kj mol^-1

Entropy change
J K^-1 mole^-1

85
Q

Explain why entropy change of the surroundings might be positive even though ENTHALPY change of the reaction is negative.

CHECK XX

A

Reaction is exothermic

Heat given out so entropy of surroundings increases

86
Q

If a reaction ENTHALPY change is negative then entropy of surroundings is always

XXX

A

Positive

Because exothermic reaction releases heat energy so entropy of surroundings increases

87
Q

What do we mean by feasibility

A

A reaction is possible in terms of energy (but this doesn’t necessarily mean it will happen - might be other factors stopping it)

88
Q

For a reaction to be feasible…

A

TOTAL entropy must increase (sign must be +)

89
Q

Dissolving ammonium nitrate crystals in water

A

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
Q

What can entropy of surroundings tell you?

A

If a reaction is exothermic or endothermic

Entropy of surroundings increases, reaction is exothermic
Entropy of surroundings decreases, reaction is endothermic

91
Q

Concentrated ethanolic acid and solid ammonium carbonate equation

A

2CH3COOH + (NH4)CO3 —-> 2CH3COONH4 + H2O + CO2

92
Q

Concentrated ethanolic acid and solid ammonium carbonate

A

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
Q

4 typical reactions from spec where entropy changes occur

A

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
Q

Magnesium ribbon plus oxygen

A

Reaction is feasible
Reaction is not spontaneous (does happen instantly)

So it’s feasible but that doesn’t mean it will necessarily happen

95
Q

Mixing solid barium hydroxide with solid ammonium chloride equation

A

Ba(OH)2.8H2O(s) + 2NH4Cl(s) —> BaCl2(s) + 10H2O(l) + 2NH3(g)

96
Q

Mixing solid barium hydroxide with solid ammonium chloride

A

Entropy of system increases because gas and liquid formed from two solids

97
Q

Using states to predict entropy we are predicting entropy of …

A

The system only

98
Q

Entropy of system if BaCl2(s)

A

+591

99
Q

Entropy of system if BaCl2.H2O(s)

A

+530

100
Q

Name the four main entropy equations

A

🔺Ssystem = 🔺S products -🔺S reactants

🔺Stotal = 🔺Ssurroundings + 🔺Ssystem

                                  🔺H reaction 🔺Ssurroundings  =  - ——————-                                  V.                                     T (in Kelvin)

🔺G = 🔺H - T🔺Ssystem

101
Q

What to remember when working out Gibbs free energy

A

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
Q

Gibbs must be what for reaction to be feasible

Vs what total entropy must be

A

Negative value (🔺G<0)

Total entropy has to be positive value to be feasible

103
Q

Feasibility changes with …

A

Temperature

Eg. Decomposition of CaCO3
Some Kelvin values the 🔺G produced (when you do the calculation) is negative but others it is positive

104
Q

How to calculate a temperature at which 🔺G becomes feasible?

A

🔺G = 🔺H - T🔺S system

Work out enthalpy and entropy of system

Rearrange so = 0

Replace = so finding value < T

105
Q

Equation to find temperature at which reaction involving gases is feasible?

XXX

A

🔺G = -RT

106
Q

What does a large + equilibrium constant mean

A

Reaction highly shifted right because forward reaction happening at faster pace than backwards reaction

107
Q

Why might a thermodynamically feasible reaction not take place?

A

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
Q

Do catalysts affect feasibility?

A

No

109
Q

Do catalysts affect spontaneity (as in instantly)?

A

Yes

110
Q

What is enthalpy

A

Balance of energy left once you have taken bonds broken - bonds made into account

111
Q

Enthalpy changes alone do not…

A

Control whether reactions occur

Entropy is involved as well.

112
Q

When do you increase temperature to achieve feasibility?

A

When entropy change of the system is POSITIVE

113
Q

When do you decrease temperature to achieve feasibility?

A

When entropy change of the system is NEGATIVE

Chemistry (37 decks)

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