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Chapter 13 Flashcards

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1
Q

What is lattice energy a measure of?

A

Ionic bond strength

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2
Q

What is the definition of standard lattice energy?

A

Standard lattice energy is the energy change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions

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3
Q

What is the equation for the standard lattice energy of sodium chloride?

A

Na+(g) + Cl-(g) —> NaCl(s)

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4
Q

What are standard conditions?

A

298K and 100kPa

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5
Q

What does the more negative the lattice energy mean?

A

The stronger the bonding

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6
Q

What 2 factors affect lattice energy?

A

Ionic charge and size

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7
Q

What does the higher the charge on the ions mean about the energy releases when an ionic lattice forms?

A

The higher the charge on the ion, the more energy released when an ionic lattice forms, due to the stronger electrostatic forces between the ions

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8
Q

Are lattice energies for compounds with 2+ or 2-ions or compounds with 1+ or 1- ions more exthermic?

A

Compounds with 2+ or 2- are more exothermic

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9
Q

Why does MgCl2 have a higher lattice energy (more negative) than NaCl?

A

Magnesium has a greater charge than sodium

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10
Q

Why does MgS have a higher lattice energy than MgCl2?

A

Because both Mg and S have double charges

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11
Q

How does ionic radius affect the size of the lattice energy?

A

The smaller the ionic radius of the ions involved, the more exothermic (the more negative) the lattice energy is

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12
Q

Why do ions with smaller ionic radius have larger lattice energy values?

A

Smaller ions have higher charge density and their smaller ions can pack more closely together

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13
Q

What does Hess’s law say?

A

The total enthalpy change of a reaction is always the same, no matter the route taken

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14
Q

What is Hess’s law also known as?

A

The conservation of energy

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15
Q

As you can’t calculate lattice energy directly, what do you have to use in order to calculate it?

A

A born-Haber cycle

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16
Q

What can a Born-Haber cycle be used to calculate?

A

Lattice energies

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17
Q

What does a Born-Haber cycle start with on the LHS?

A

The enthalpy of formation

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18
Q

What is the enthalpy of formation?

A

The change of enthalpy during the formation of 1 mole of the substance from its constituent elements, with all substances in their standard states.

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19
Q

What direction does the arrow for the enthalpy of formation face in a Born-Haber cycle?

A

Downward

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20
Q

After the enthalpy of formation, what comes next in a Born-Haber cycle?

A

The enthalpy changes of atomisation and ionisation

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21
Q

In a Born-Haber cycle, what direction do the enthalpy changes of atomisation and ionisation face?

A

Upwards

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22
Q

What is the definition of the enthalpy change of atomisation?

A

The enthalpy change of atomisation is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state

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23
Q

What is the equation for the atomisation of sodium?

A

Na(s) —>Na(g)

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24
Q

What is the equation for the atomisation of chlorine?

A

1/2 Cl2(g) —> Cl(g)

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25
Q

What is the equation for the first ionisation energy of sodium?

A

Na(g) —> Na+(g) + e-

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26
Q

After the enthalpies of atomisation and ionisation, what comes next in a Born-Haber cycle?

A

The electron affinity

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27
Q

What is the equation for the first electron affinity of chlorine?

A

Cl(g)+e- –> Cl-(g)-

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28
Q

What direction does electron affinity face?

A

Downwards

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29
Q

After the electron affinity, what comes next in a Born-Haber cycle?

A

The lattice energy

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30
Q

What is the definition of electron affinity?

A

The amount of energy released when an electron is attached to a neutral atom or molecule in the gaseous state to form a negative ion

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31
Q

When dealing with a Born-Haber cycle containing a G2 element, why do you times the value of the atomisation enthalpy by 2?

A

Because there are 2 moles of the G7 ion in each mole of the compound

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32
Q

When dealing with a Born-Haber cycle containing a G2 element, why do you have to include the second ionisation energy?

A

Because group 2 elements form 2+ ions

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33
Q

When dealing with a Born-Haber cycle containing a G2 element, why do you times the electron affinity by 2?

A

As 2 electrons are being added to the G7 element during the electron affinity

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34
Q

What does a large, negative lattice energy mean in terms of bond strength?

A

The more negative the value, the stronger the bonding

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35
Q

What is the experimental way of calculating Lattice energies?

A

Using experimental enthalpy values in a Born-Haber cycle

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36
Q

What is the theoretical way of calculating Lattice energies?

A

Doing some calculations based on purely the ionic model of a lattice

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37
Q

To work out a theoretical lattice energy, what 2 things do we assume?

A
  • Ions are spherical

- Ions have their charge evenly distributed

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38
Q

What can comparing theoretical and experimental lattice energies tell you?

A

How ionic an ionic lattice is

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39
Q

If the experimental and theoretical values for lattice energies are a pretty close match, what does this tell you, and what does this tell you about the structure?

A

It tells you that these compounds fit the ‘purely ionic’ model very well. The structure of the lattice is close to being purely ionic

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40
Q

If the experimental and theoretical values for lattice energies are a quite far apart, what does this tell you?

A

The compound isn’t that close to being ‘purely ionic’

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41
Q

What are close to being ‘purely ionic’ out of soidum halides and magnesium halides?

A

Sodium halides

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42
Q

As magnesium halides aren’t as close to being ‘purely ionic’ as sodium halides are, what does this tell you?

A

They have some covalent character, whereas sodium halides have almost no polarisation and little covalent character

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43
Q

What does polarisation of ionic bonds lead to?

A

Covalent character in ionic lattices

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44
Q

What is the equation of charge density?

A

Charge density = Charge / Volume

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45
Q

In a sodium halide, as the cation(Na+) only has a small charge what does this mean?

A

It can’t really pull electrons from the anion towards itself, so the charge is evenly distributed around the ions (almost no polarisation).

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46
Q

What is an anion?

A

A negatively charged ion

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47
Q

What is a cation?

A

A positively charged ion

48
Q

Why do the theoretical calculations for the lattice energies of sodium halides fit so well with the experimental values?

A

Sodium ion can’t really pull electrons from the halide ion (charge of only 1+) so there is almost no polarisation as the charge is evenly distributed across the compound

49
Q

In a magnesium halide, as the cation (Mg2+) has a large charge, what does this mean?

A

Mg2+ has a larger charge than the halide, so it is able to pull electrons from it towards itself a bit, polarising the bond

50
Q

What is the relationship between the charge density of the cation and how well matched the theoretical and experimental values of the lattice energies are?

A

The greater the charge density of the cation, the poorer the values will match

51
Q

What is polarisation?

A

When the positive charge on the cation attracts electrons towards it from the anion

52
Q

Why ae small cations with a very high charge very polarising?

A

They have a high charge density, so they can pull electrons towards itself

53
Q

Why are large anions with a high charge polarised more easily than smaller ones with a lower charge?

A

Because their electrons are further away from the nucleus and there is more repulsion between the electrons, so can be pulled away more easily to the cations

54
Q

If a compound contains a cation with a high polarising ability, and an anion which is easily polarised, what can happen?

A

Some of the anion’s electron charge cloud will be dragged towards the positive cation

55
Q

If a compound is polarised enough, what can be formed?

A

A partially covalent bond

56
Q

What happens if an ionic bond is polarised more and more?

A

The more the bond is polarised, the more covalent character it gains.

57
Q

What happens if an ionic bond starts to gain more covalent characters?

A

It results in compounds with different properties to those with only ionic bonds

58
Q

Is Na+Cl- ionic or covalent?

A

Ionic

59
Q

Is Mg2+Cl- ionic or covalent?

A

It is mostly ionic

60
Q

Is Al3+Cl- ionic or covalent?

A

Mostly covalent

61
Q

Is Si4+Cl- ionic or covalent?

A

Covalent

62
Q

What is the Pauling Scale used to measure?

A

The electronegativity of an atom

63
Q

What is electronegativity?

A

The ability of an atom to attract the bonding electrons in a covalent bond

64
Q

For what value are bonds polar?

A

Bonds are polar when the difference in Pauling electronegativity values are more than about 0.4

65
Q

Predict whether a C-Cl bond will be polar, given that the pauling electronegativity values of carbon and chlorine are 2.5 and 3.0:

A

3-2.5=0.5, so the bond will be slightly polar with the chlorine atom having a slight negative charge and the carbon atom having a slight positive charge

66
Q

When an ionic lattice dissolves in water, what tow things happen?

A
  • The bonds between the ions break

- Bonds between the ions and water are made

67
Q

When an ionic lattice dissolves in water and bonds between the ions break, is this an endothermic or exothermic process?

A

It is exothermic

68
Q

When an ionic lattice dissolves in water and bonds are formed between the ions and water molecules, is this an exothermic or endothermic process?

A

It is an exothermic process

69
Q

When hydrated ions ae formed, why do the water molecules surround the positive charge?

A

The oxygen is slightly negative, so it is attracted to the cation

70
Q

When hydrated ions ae formed, why do the water molecules surround the neagtive charge?

A

The hydrogen atoms are slightly positive, so are attracted to the anion

71
Q

What is the enthalpy change of hydration?

A

The enthalpy change when 1 mole of gaseous ions dissolve in water

72
Q

What is the enthalpy change of solution?

A

The enthalpy change when 1 mole of a solute dissolves in water

73
Q

What is the equation for the enthalpy change of solution for AgCl(s)?

A

AgCl(s) —> Ag+(aq) + Cl-(aq)

74
Q

What is the equation for the enthalpy change of hydration for Ag+(g) + Cl-(g)?

A

Ag+(g) + Cl-(g) –> Ag+(aq+ + Cl-(aq)

75
Q

When do substances generally dissolve?

A

When the energy released is roughly the same, or greater than the energy taken in

76
Q

Do soluble substance tend to have endo or exo enthalpies of solution?

A

Exothermic

77
Q

Describe how the enthalpy change of solution can be calculated:

A
  • Put ionic lattice and dissolved ions at the top, connected by enthalpy change of solution
  • Connect ionic lattice to the gaseous ions by lattice energy
  • Connect gaseous ions to dissolved ions by hydration enthalpies
78
Q

What 2 factors affect the enthalpy of hydration?

A

Ionic charge and ionic radius

79
Q

Why do ions with a higher charge have a greater enthalpy of hydration?

A

Ions with a higher charge are better at atttacting water molecules than those with lower charges , so the electrostatic attraction is greater, so more energy is required to break the bonds, giving them a more exothermic enthalpy of hydration

80
Q

Why do smaller ions have a greater enthalpy of hydration?

A

They have higher charge densities so they attract water molecules more easily and have a more exothermic enthalpy of hydration

81
Q

How many more water molecules surround a cation with a 2+ charge compared to a cation with a 1+ charge?

A

Double the amount

82
Q

What will have a more exothermic enthalpy of hydration, magnesium or sodium?

A

Magnesium as it is smaller and has a higher charge

83
Q

What is entropy a measure of?

A

The disorder of a system- it tells you the number of ways particle can be arranged and the number of ways that energy can be shared between the particles

84
Q

What happens as the entropy increases?

A

The more disordered the particles are

85
Q

What does a large, positive value of entropy show?

A

High levels of disorder

86
Q

What are the three main things that affect entropy?

A
  • Physical state
  • Dissolving
  • Number of particles
87
Q

Why do solids have the lowest entropy?

A

Solid particles wobble about a fixed point, so there is hardly any randomness, so they have the lowest entropy

88
Q

Why do gas particles have the highest entropy?

A

They move around wherever they like, so they have the most random arrangements of particles, so they have the most entropy

89
Q

If magnesium (s) reacts with Oxygen(g) to form solid magnesium oxide, what happens to the levels of entropy?

A

The levels of entropy lower

90
Q

Why does dissolving a solid increase its entropy?

A

Dissolved particles can move more freely as they are no longer in place

91
Q

Fit solids, liquids and gases into random, some disorder and ordered:

A

Solids are ordered
Liquid have some disorder
Gases are random

92
Q

Why do more particles mean more entropy?

A

The more particles you have, the more ways they and their energy can be arranged

93
Q

In the reaction N2O4(g) –> 2NO2(g), why does the entropy increase?

A

Because the number of moles has increased

94
Q

When are substances more energetically stable?

A

When there’s more disorder

95
Q

What will particles move to try and increase?

A

Their entropy, so that they are more energetically stable

96
Q

Why are some reaction feasible when their energy change is exothermic?

A

An increase in entropy overcomes the change in enthalpy

97
Q

What are the units of entropy?

A

JK-1mol-1

98
Q

What does delta S system represent?

A

Entropy change

99
Q

What does delta S sytstem equal?

A

S products - S reactants

100
Q

What is S products?

A

The entropy of the products

101
Q

What is S reactants?

A

The entropy of reactants

102
Q

Why does the entropy of the surroundings change aswell?

A

Because energy is transferred to or out from the system

103
Q

What is the total entropy change a sum of?

A

The entropy changes of the system and surroundings

104
Q

What does delta S total equal?

A

Delta S system + Delta S surrounding

105
Q

What does delta S system equal?

A
  • deltaH / T
106
Q

What is delta H?

A

Enthalpy change

107
Q

What is delta G?

A

The free energy change

108
Q

For feasible reactions, what must delta G be?

A

Negative or zero

109
Q

What does delta G equal?

A

delta H -T(delta S system)

110
Q

As when delta G is zero or negative the reaction is feasible, what equations allows us to calculate at which temperature the reaction is feasible?

A

T = delta H / delta S system

111
Q

What is the equilibrium constant a measure of?

A

The ratio of the concentration of products to reactants at equilibria for a reversible reaction

112
Q

What must the value for the equilibrium constant be for reaction with delta G <0?

A

Greater than 1

113
Q

What must the value for the equilibrium constant be for reaction with delta G > 0?

A

Smaller than 1

114
Q

What equation links free energy change and the equilibrium constant?

A

delta G = -RT lnk

115
Q

Why doesn’t negative delta G always guarantee a reaction?

A

The value of the free energy change doesn’t tell you anything about the rate, and the activation energy may be extremely high

Chemistry (25 decks)

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