Topic 13: Further Energetics

Question 1

Define lattice enthalpy

Answer 1

The enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions.

Question 2

Define the enthalpy change of atomisation

Answer 2

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

Question 3

Define the enthalpy change of 1st and 2nd electron affinity

Answer 3

The enthalpy change when 1 mole of gaseous 1- ions is made from 1 mole of gaseous atoms

The enthalpy change when 1 mole of gaseous 2- ions is made from 1 mole of gaseous 1- ions

Question 4

What is lattice enthalpy? What factors affect it?

Answer 4

Lattice energy is a measure of ionic bond strength. The size of the lattice enthalpy is affected by: Charge density. The number of cation to anion interactions. The sum of their ionic radii. (The type of lattice structure) (The extent of covalent interactions between ions)

Question 5

How can you tell if a compound is stable as an ionic salt?

Answer 5

Construct a born Haber cycle, which is an experimental value. Up arrows are endothermic, down arrows are EXO. If the overall enthalpy change in the cycle is negative, the salt will be stable

Question 6

Why is the lattice energy of MgCl2 much more negative than NaCl?

Answer 6

Mg ion has a 2+ charge whereas Na has a 1+ charge. Hay mas cation to anion interactions in MgCl2 bc hay twice as many chloride ions per cation than in sodium chloride. The Mg2+ ion is smaller than the Na+ ion, reducing the sum of the ionic radii.

Question 7

When calculating theoretical lattice energy, we need to make what assumptions?

Answer 7

When calculating theoretical lattice energy assume that the cmpound has no distortion and is fully ionic.

These assumptions mean theoretical lattice energy and experimental lattice energy may be different. In compounds w a big difference, the bonding in the lattice has considerable covalent character, which makes the experimental value for the lattice energy more negative.

Question 8

Why do some ionic compounds have some covalent character?

Answer 8

Covalency in bonding is caused by polarisation of the anion by the cation.

Polarisation distorts the electron density w in the negative ion, resulting in a higher e density near the cation.

This means hay some electron density between the two ions so sera a degree of covalent bonding.

Question 9

What factors affect polarisation of the anion?

Answer 9

A cation with a higher charge and a small radius (high charge density) has a large polarising power. Anions with a large charge and a large size are polarised most easily. These lead to more covalent character

Question 10

State the definition for enthalpy change of solution

Answer 10

The enthalpy change when one mole of an ionic solid is dissolved in sufficient solvent to form an infinitely dilute solution.

Question 11

What is the point of infinite dilution?

Answer 11

In practice there is a point where further dilution has no measurable effect on enthalpy change of solution. This is called the point of infinite dilution.

Question 12

State the definition for enthalpy change of hydration

Answer 12

Enthalpy change of hydration is the enthalpy change when one mole of an ion in its gaseous state is completely hydrated by water. In practice complete hydration occurs when the solution formed is at infinite dilution.

Question 13

What factors affect size of hydration enthalpy?

Answer 13

Size of the charge: electrostatic attraction between 2+ ions and water molecules will be greater than between singly charged ions and water molecules.

Increasing ionic radius: as ions become larger electrostatic attraction between them and water molecules decreases, so energy released upon hydration decreases.

Charge density: the greater the charge density of the cation the more negative the hydration enthalpy.

Question 14

Why do some reactions occur and some don’t occur at all?

Answer 14

Some reactions do not occur at all, at least not without some help. For some reactions the position of eqm is so far to the right that the reaction has gone to completion.

For some of the reactions significant amounts of both reactants and products are present at eqm.

For other reactions, equm lies so far to the left that they appear not to take place at all. Therefore the position of equilibrium determines whether a reaction occurs.

Question 15

Why do some endo and exo reactions take place spontaneously?

Answer 15

Some endo and exo reactions spontaneously occur.

The driving force for spontaneous ENDO reactions cannot be the formation of more energetically favourable (lower energy) products (bc products of endo reactions are higher energy than reactants) but must involve another factor: entropy.

Entropy is a property of matter that is associated with the degree of disorder or randomness of the particles.

Question 16

What is a spontaneous process?

Answer 16

A spontaneous process is one that takes place without continuous intervention from us.

Question 17

What factors affect entropy?

Answer 17

The greater the degree of disorder, the greater the entropy.

In a spontaneous process, total entropy increases.

Gases have much higher entropies than solids as there are many more ways of arranging the entities, and it is less ordered.

Compounds with more atoms have higher entropy as there are many more ways of arranging the entities, and it is less ordered.

Question 18

What is the equation for total entropy change?

Answer 18

ΔS total = ΔS system + ΔS surroundings

ΔS means entropy change The system is the reactants. The surroundings is everything else.

If a reaction is spontaneous ΔS total must be positive.

Question 19

How do you calculate entropy change of the system and the surroundings?

Answer 19

ΔS° = ΣS(products) - ΣS(reactants)

ΔS surroundings = -ΔH / T

ΔH is the enthalpy change of reaction, T=temp in kelvin.

For an exo reaction where ΔH is negative, entropy change of the surroundings will be positive, so the entropy of the surroundings increases.

For an endo reaction entropy change of the surroundings will always be negative so S of the surroundings decreases.

Question 20

How does temperature affect entropy?

Answer 20

Supplying heat energy to a v cold object will make molecules move around more, increasing entropy. If we supply the same amount of heat energy to a much hotter object, entropy still increases but not by as much as w the cold object. This is bc in the hot object the molecules already move around vigourously and hay LESS increased degree of movement.

Question 21

Solid ammonium carbonate is added to ethanoic acid. The temperature falls considerably and gas bubbles are produced. Is this reaction thermodynamically spontaneous?

Answer 21

Since the reaction is endothermic, entropy change of the surroundings is negative.

However hay large increase in entropy of the system bc a gas is produced from a liquid and solid.

The size of the entropy change of the system is greater than the entropy change of the surroundings which makes total entropy positive. Therefore the reaction is thermodynamically spontaneous.

Question 22

How does entropy change when the number of moles in a reaction changes?

Answer 22

If the number of moles are increased, the number of particles are automatically increased. This results in an increase in the number of ways that particles can be arranged which increases the entropy of the system, making ΔS system positive.

Question 23

2CH3C00H + (NH4)2C03 –> 2CH3C00NH4 + C02 +H20

Mg + 1/2 O2 –> MgO

Ba(OH)2.8H20 + 2NH4Cl –> BaCl2 + 10H20 + 2NH3

For the reactions, state if ΔS system is positive or negative

Answer 23

1) moles of reactants: 3 Moles of products: 4. Therefore ΔS system is positive
2) moles of reactants: 1.5 Moles of products: 1. Therefore ΔS system is negative
3) moles of reactants: 3 Moles of products: 13. Therefore ΔS system is positive

Question 24

How does entropy change when an ionic solid dissolves in water?

Answer 24

When an ionic solid dissolves in water, the lattice structure is broken down and the ions become hydrated. Number of moles also increase, increasing entropy.

The hydration of the ions is exothermic but breaking down of the lattice structure is endothermic, and is the reverse of the lattice energy.

Water molecules arrange themselves in an orderly manner around + and - ions. The increase in order of the water molecules decreases entropy of the water.

Question 25

How is the solubility of an ionic solid determined?

Answer 25

The solubility of an ionic solid is determined by total entropy change for the solid: ΔS total = ΔS system + ΔS surroundings

OR ΔS total = ΔS system-ΔH sol / T

OR If Gibbs energy change of solution is negative then the products are favoured at eqm and the salt is soluble.

If Gibbs energy change of solution is + then the solid salt is favoured at eqm and the salt is insoluble/sparingly soluble.

Question 26

Describe and explain Standard Gibbs free energy change.

Answer 26

ΔG = ΔH - TΔ System. Entropy change is calculated in joules, but ΔG and ΔH are both measured in kJ. You must change the entropy change value into kJ before you start!!

If ΔG is negative, the reaction IS thermodynamically feasible/spontaneous. The reactant is NOT stable.

If ΔG is 0, the reaction is in eqm. This calculation is identical to finding ΔS total.

Question 27

What is the equation relating Gibbs free energy and eqm?

Answer 27

ΔG = -RT ln K

R = 8.314 J K-1mol-1. T is the temp of the reaction in Kelvin. You must convert your standard free energy value into joules by x the kJ value by 1000.

Rearranged to find K: K = e^(-ΔG / RT). e is a button on the calculator.

If ΔG is negative, the expression in brackets is positive. The eqm constant will be>1 so the products are favoured (vice versa if ΔG is +)

If K is very large the eqm position will almost totally favour the products.

Question 28

Sometimes, even thermodynamically feasible reactions don’t occur. Why is this?

Answer 28

A reaction may have a negative ∆G value but require a large AE that’s not provided by the reaction conditions. Thus the reaction won’t occur spontaneously.

Alternatively, a reaction may be thermodynamically feasible but kinetic factors may lead it to occur at ​such a slow rate​ that, in practice, it doesn’t appear to occur.

∆G is calculated for standard conditions. Sometimes reactions become thermodynamically feasible because ∆G is negative under non-standard conditions.

Question 29

Answer 29

Question 30

Answer 30

Question 31

Using the data below, contruct a born haber cycle for LiCl and find the lattice enthalpy

Answer 31

Question 32

Answer 32

Two moles going to two moles but one of them is a solid.

Hence RHS more ordered than the left side, so entropy change of the system must be negative