Topic 13: Energetics II

Question 1

What is lattice energy?

Answer 1

• ionic compounds can form regular structures, giant ionic lattices where the positive and negative ions are held together by electrostatic attractions
• when gaseous ions combine to make a solid lattice, energy is given out
• this energy is lattice energy

Question 2

What is the definition of standard lattice energy?

Answer 2

• the standard lattice energy, delta_LEH^ø, is the energy change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions

Question 3

What is standard lattice a measure of?

Answer 3

• it measures ionic bond strength
• the more negative the lattice energy, the stronger the bonding

Question 4

How does ionic charge and size affect lattice energy?

Answer 4

• the higher the charge on the ions, the more energy is released when an ionic lattice forms
• this is due to the stronger electrostatic forces between ions
• more energy released means that lattice energy will be more negative
• compounds with 2+ or 2- ions are more exothermic
• the smaller the ionic radiiof the ions involved, themore exothermic the lattice energy
• smaller ions have a higher charge density and their smaller ionic radii means that the ions can sit closser tofether in the lattice, making attraction between the ions stronger

Question 5

How do you draw a Born-Haber cycle?

Answer 5

• General order:
• Enthalpy of formation
• Atomisation enthalpy
• Ionisation energy
• Electron affinity
• Lattice energy

Question 6

What are the two ways to work out lattice energy?

Answer 6

• experimentally: using experimental enthalpy values in a Born-Haber cycle
• theoretically: doing some calculations based on the purely ionic model of a lattice

Question 7

Why does polarisation lead to covalent character in some ionic lattices?

Answer 7

• e.g. magenisum halides have more covalent character in their ionic bonds than sodium halides
• in a sodium halide, the cation Na⁺, has only a small charge so it can’t really pull electrons from the anion towards itself
• so the charge is distributed evenly around the ions and there’s almost no polarisation
• magnesium halides don’t fit the ionic moodel quite so well because charge isn’t evenly distributed around the ions
• the cation Mg²⁺, has a bigger charge, so it can pull electrons from the anion towards itself, polarising the bond
• in general, the greater the charge density of the cation, the poorer the match will be between experimental and theoretical values for LE

Question 8

What is polarisation?

Answer 8

• when the positive charge on the cation attracts elecrons towards it from the anion

Question 9

Which cations are most polarising and which anions are more polarisable?

Answer 9

• small cations with a high charge are very polarising because they have high charge density
• so the catioin can pull electrons towards itself
• large anions with a high charge are polarised more easily than smaller ones with a lower charge
• this os because their electrons are further away from the nucleus and there is more repulsion between electrons, so electrons can be pulled away more easily
• if a compound contains a cation with a high polarising ability and an anion that is easily polarised, some of the anion’s electron charge cloud will be dragged towards the positive cation
• if the compound is polarised enough, a partially covalent bond is formed

Question 10

What happens when a solid ionic lattice dissolves in water?

Answer 10

• the bonds between the ions break - this is endothermic
• the enthalpy change is the opposite of the lattice enthalpy
• Bonds between the ions and the water are made - this is exothermic
• the enthalpy change here is called the enthalpy change of hydration

Question 11

What is the definition of enthalpy change of hydration

Answer 11

• the enthalpy change of hydration delta_hydH is the enthalpy change when 1 mole of gaseous ions dissolves in water

Question 12

What is the definition of enthalpy change of solution?

Answer 12

• the enthalpy change of solution, delta_solH, is the enthalpy change when 1 mole of solute dissolves in water

Question 13

How to calculate enthalpy change of solution using a cycle?

Answer 13

Question 14

How does ionic charge and ionic radius affect the enthalpy of hydration?

Answer 14

Ions with a greater charge have a greater enthalpy of formation:

• ions with a higher charge are better at attracting water molecules than those with lower charges: the elecstrostatic attraction between the ion and the water molecules is stronger
• this means more energy is released when the bonds are made, giving them a more exothermic enthalpy of hydration

Smaller ions have a greater enthalpy of hydration:

• smaller ions have a higher charge density than bigger ions
• they attract the water molecules better and have more exothermic enthalpy of hydration

Question 15

What is entropy?

Answer 15

• a measure of the disorder of a system
• it tells you the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles
• the more disordered the particles are, the higher the entropy is

Question 16

What affects entropy?

Answer 16

Physical state:

• gas particles have the highest entropy because they have the most random arrangements of particles

Dissolving:

• dissolving a solid also increases iits entropy
• dissolved particles can move freely as theyre no longer held in one place

More particles means more entropy:

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

Question 17

Why do particles try to increase their entropy?

Answer 17

• substances are more energetically stable when there’s more disorder

Question 18

How to calculate the entropy change of a system

Answer 18

• deltaS_system = S_products - S_reactants
• a positive entropy change means reaction is likely to be feasiible

Question 19

What is the total entropy change and how do you calculate it?

Answer 19

• total entropy change is the sum of the entropy of changes of the system and surroundings
• delta S_total = delta S_system + delta Ssurroundings

Question 20

How do you calculate entropy of the surroundings?

Answer 20

• delta S_surroundings = - deltaH (enthalpy change J mol^-1) / T (temperature in K)

Question 21

Calculation example 1

Answer 21

Question 22

Calculation example 2

Answer 22

Question 23

Calculation example 3

Answer 23

Question 24

Definition of standard enthalpy change of formation

Answer 24

• the standard enthalpy change of formation of a compound, delta_fH^Ø is the enthalpy change when one mole of the compoud forms from its elements under standard conditions with the elements and the compound in their standard states

Question 25

Definition of the standard enthalpy change of atomisation

Answer 25

- the standard enthalpy change of atomisation of an element is the energy change needed to produce one mole of gaseous atoms of the elemtent

Question 26

What is the first ionisation energy?

Answer 26

- the first ionisation energy of an element is the energy needed to remove one electron from each atom in one mole of gaseous atoms of the element under standard conditions

Question 27

What is the first electron affinity?

Answer 27

- the first electron affinity of an element is the energy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous ions with a single negative charge