13: Lattice energy and Entropy

Question 1

Define lattice energy

Answer 1

The energy change when one mole of an ionic solid is formed from its gaseous ions

Question 2

Define enthalpy of formation

Answer 2

Enthalpy change when one mole of a substance is formed from its constituent elements in their standard states in standard conditions
Exothermic (normally)

Question 3

Define enthalpy of combustion

Answer 3

Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in the standard states in standard conditions
Exothermic

Question 4

Define enthalpy of neutralisation

Answer 4

Enthalpy change when 1 mole of water is formed from in a reaction between an acid and an alkali in standard conditions
Exothermic

Question 5

Define first ionisation enthalpy

Answer 5

Enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ion
Standard conditions
Endothermic

Question 6

Define second ionisation enthalpy

Answer 6

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

Question 7

Define first electron affinity

Answer 7

Enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions
Standard conditions
Exothermic (non-metals)

Question 8

Define second electron affinity

Answer 8

Enthalpy change when each ion in one mole of gaseous 1- ions gain one electron to form one mole of gaseous 2- ions
Standard conditions
Endothermic (adding -ve electron to a -ve ion)

Question 9

Define enthalpy of atomisation

Answer 9

Enthalpy change when one mole of gaseous atoms is produce from an element in its standard state
Standard conditions
Endothermic

Question 10

Define enthalpy of solution

Answer 10

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 don’t interact with one another
Standard conditions
Varies between exo and endo

Question 11

Define hydration enthalpy

Answer 11

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

Question 12

Define bond dissociation enthalpy

Answer 12

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

Question 13

Define lattice enthalpy of formation

Answer 13

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

Question 14

Define lattice enthalpy of dissociation

Answer 14

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

Question 15

What is the relationship between lattice enthalpy of formation and dissociation?

Answer 15

Lattice enthalpy of formation = - (lattice enthalpy of dissociation)

Question 16

Define enthalpy of vaporisation

Answer 16

Enthalpy change when one mole of a liquid is turned into a gas
Standard conditions
Endothermic

Question 17

Define enthalpy of fusion

Answer 17

Enthalpy change when one mole of a solid is turned into a liquid
Standard conditions
Endothermic

Question 18

What is the cycle for enthalpy of solution calculations?

Answer 18

Enthalpy of solution = Ionic solid -> Dissolved ions
Lattice enthalpy of formation = Gas ions -> ionic solid
Hydration enthalpies = Gas ions -> Dissolved ions

Question 19

What is the equation for enthalpy of solution calculations?

Answer 19

Enthalpy of solution = -(lattice enthalpy of formation) + hydration enthalpies (both ions)

Question 20

What does the lattice enthalpy of a compound indicate?

Answer 20

Strength of ionic bonding - larger the magnitude of lattice enthalpy, the stronger the bonding

Question 21

Which ions will have higher lattice enthalpy due to stronger attractions?

Answer 21

Smaller ions, and ions with higher charge

Question 22

How is the experimental lattice enthalpy of formation found?

Answer 22

Born-Haber cycle

Question 23

What are the stages to take elements in normal states to become a ionic compound?

Answer 23

Atomisation of metal and non-metal (endo)
Ionisation of metal atoms (endo)
Electron affinity of non-metal atoms (exo)
Lattice enthalpy of formation (exothermic)

Question 24

What is experimental lattice enthalpy?

Answer 24

Lattice enthalpy calculate using a Born-Haber cycle
All other enthalpy changes found experimentally
This is the real value

Question 25

What is theoretical lattice enthalpy?

Answer 25

Lattice enthalpy calculated using an equation considering: size, charge, etc. Assumes the structure is perfectly ionic Not the real value

Question 26

What is the real lattice enthalpy value?

Answer 26

The experimental lattice enthalpy

Question 27

Why is the theoretical lattice enthalpy not the real value?

Answer 27

Often distortion of the ions (polarisation), meaning the ions are not perfectly spherical Therefore it is not perfectly ionic, said to have covalent character

Question 28

How do cations effect ionic structure?

Answer 28

Positive charge with a high charge density are very good at distorting negative ions

Question 29

How do anions effect ionic structure?

Answer 29

Negative ions that are large or highly charged are easier to distort

Question 30

What is the characteristics of ionic compounds which have covalent character?

Answer 30

Low solubility in water and lower melting points and electrical conductivity

Question 31

What is the general trend of the difference between experiment and theoretical lattice enthalpy values?

Answer 31

Larger the difference between the values, the greater the covalent character

Question 32

What is polarisation?

Answer 32

The ability of a cation to draw electrons from an anion closer to the cation Anion is no longer spherical

Question 33

What does the enthalpy of a solution have to be for a solid to dissolve?

Answer 33

Exothermic (negative) | However if slightly positive may react due to entropy

Question 34

What is the total hydration enthalpy change when dissolving ionic solids?

Answer 34

Enthalpy of hydration of the cation + anion

Question 35

What are the two steps of dissolving ionic solids?

Answer 35

Ions in solid become separated from each other, and turn into gaseous ions Gaseous ions are hydrated, as ions form bonds with the solvent water molecules

Question 36

What bonding forms between water and ions?

Answer 36

Cations (+) attracted to 𝛿- oxygen atoms | Anions (-) attracted to 𝛿+ hydrogen atoms

Question 37

How does the solubility of group 2 sulphates and carbonates change going down the group and why?

Answer 37

Solubility decreases down the group Radius of cation increases , hydration enthalpy becomes significantly less exothermic Lattice enthalpy decreases, this is NOT significant Overall enthalpy of solution becomes less exothermic

Question 38

How does the solubility of group 2 hydroxides change going down the group and why?

Answer 38

Solubility increases down the group Radius of cation increases, hydration enthalpy NOT significantly less exothermic Lattice enthalpy decreases, significantly Overall the enthalpy of solution becomes more exothermic

Question 39

What means some endothermic reactions occur at room temperatures?

Answer 39

Entropy must be increasing

Question 40

Define entropy

Answer 40

A measure of the disorder of a system

Question 41

What is the natural direction of change of entropy?

Answer 41

Increasing total entropy (+ve entropy change)

Question 42

What entropy will a solid very ordered system have?

Answer 42

Very low entropy

Question 43

What are the entropies of different states?

Answer 43

Solids - low Liquids - higher Gas - much higher

Question 44

What explains the entropies of different states?

Answer 44

Gases have many different combinations which they can be found as they are not tightly packed

Question 45

What happens to entropy as a molecule becomes more complex and why?

Answer 45

Increases in entropy | Due to more combinations which can occur due to more molecules and more electrons so more disordered

Question 46

Why do the size of the atoms affect entropy?

Answer 46

Larger atoms have more entropy due to more different configurations which can occur

Question 47

What is ∆S system?

Answer 47

Entropy change which occurs in the reaction | equation

Question 48

How do you calculate ∆S system?

Answer 48

Entropy of products - reactants

Question 49

What is ∆S of surroundings?

Answer 49

The entropy change of the surroundings when a reaction occurs (doesn't include the changes of the molecules in the reaction)

Question 50

How do you calculate ∆S surroundings?

Answer 50

-∆H / T ∆H - enthalpy change of the reaction T - Temperature in kelvin

Question 51

What is the equation for ∆S total?

Answer 51

∆S system + ∆S surroundings

Question 52

What does ∆S total have to be for a reaction to be feasible?

Answer 52

∆S total must be positive

Question 53

Why do liquids have slightly higher entropy values than solids?

Answer 53

Atoms/molecules are free to move meaning more combionations

Question 54

What are the standard units for ∆S?

Answer 54

J/K/mol

Question 55

What does spontaneous mean in a thermodynamics context?

Answer 55

A reaction in spontaneous if it leads to an overall increase in entropy (which makes it feasible)

Question 56

What is Gibbs free energy equation?

Answer 56

ΔG = ΔH - TΔS system

Question 57

What should Gibbs free energy be for a reaction to be feasible?

Answer 57

The Gibbs free energy value has to be negative for a reaction to be feasible

Question 58

What is the equation for calculating temperature in gibbs?

Answer 58

T = ΔH /ΔS system

Question 59

What is the equation relating ΔS total and the equilibrium constant?

Answer 59

ΔS total = R lnK

Question 60

Predict temp change on equilibrium constant if ΔH = 0, assume ΔS system doesn't change with temp

Answer 60

``` ΔH = 0 Therefore ΔS surrounding = 0 as -ΔH / T ΔS total = ΔS system ΔS total = R lnK ΔS system and total not affected by temp, so K doesn't change ```