Lattice enthalpy and free energy

Question 1

What is lattice enthalpy (ΔH_LE)?

Answer 1

The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.

Question 2

What are the key features of lattice enthalpy?

Answer 2

• Lattice enthalpy is always an exothermic change as it is the energy released when ionic bonds are formed between gaseous ions (with no bonds between them).
• Measures the strength of the the ionic lattice so is associated with ionic bond strength. The larger (ore -ve) the lattice enthalpy, the stronger the elctrostatic forces between the ions, the stronger the ionic bonds.
• Cannot be measured directly due to measuring energy released when gaseous ions form lattice being practically impossible.

Question 3

What is the enthalpy change of formation (ΔH_f^ϴ)?

Answer 3

The enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states, under standard conditions (100KPa, 298K).

Question 4

What is the enthalpy change of atomisation (ΔH_at^ϴ)?

Answer 4

The enthalpy change that takes place when one mole of gaseous atoms forms from the element in its standard state.

Question 5

What is first/(n) ionisation energy (ΔH_I1/(n)^ϴ)?

Answer 5

The enthalpy change that accompanies the removal of one electron from each atom in one mole of gaseous (n+) atoms to form one mole of gaseous 1+ [(n+1)+] ions.

Question 6

What is first/(n) electron affinity (ΔH_EA1/(n)^ϴ)?

Answer 6

The enthalpy change that accompanies the addition of one electron to each atom in one mole of gaseous (n-) atoms to form one mole of gaseous 1- [(n+1)-] ions.

Question 7

Why is the first electron affinity exothermic?

Answer 7

The -ve e^- is attracted into the outer shell of the atom by the +ve nucleus, which releases energy.

Question 8

Why is the second electron affinity endothermic?

Answer 8

A -ve e^-particle is being forced into an already negative ion, so energy input is required for the repulsion to be overcome.

Question 9

What is the rule concerning direction of arrows in a Born-Haber cycle?

Answer 9

• All arrows pointing upwards represent endothermic reactions.
• All arrows pointing downwards represent exothermic reactions.

Question 10

What is the enthalpy change of solution (ΔH_S^ϴ)?

Answer 10

The enthalpy change that takes place when one mole of a compound is completely dissolved in water under standard conditions.

Question 11

What are the 2 processes that take place when an ionic solid dissolves?

Answer 11

1. Breaking down the ionic lattice into gaseous ions.
2. Hydration of gaseous ions.

Question 12

What is the enthalpy change of breaking down a lattice?

Answer 12

The opposite of ΔH_LE^ϴ.

Question 13

What is the enthalpy change of hydration (ΔH_hyd^ϴ)?

Answer 13

The enthalpy change that takes place when one mole of isolated gaseous ions are dissolved in water forming one mole of aqueous ions under standard conditions.

Question 14

What causes the enthalpy change of hydration?

Answer 14

Bonds form between +ve ions and the δ- O atoms ion H₂O or between -ve ions and the δ+ H atoms in H₂O. This releases energy, so ΔH_hyd^ϴis always exothermic.

Question 15

What 2 factors affect the value of ΔH_LE^ϴ?

Answer 15

1. Ionic radius.
2. Ionic charge.

Question 16

How does ionic radius affect lattice enthalpy?

Answer 16

The smaller the ionic radius, the smaller the ions and so the closer the ions can be packed in an ionic lattice, the stroger the electrostatic force of attraction between ions (stronger ionic bonds). As ionic radius increases, ionic bond strength decreases so lattice enthalpy becomes less negative.

Question 17

How does ionic chage affect lattice enthalpy?

Answer 17

1. The greater the charge, the greater the electrostatic attraction between the oppositely charge ions.
2. The greater the +ve charge (assuming in the same period), the smaller the ionic radius radius, the closer packed the ions can be in the lattice, the stronger the electrostatic attraction.

As ionic charge increases, lattice enthalpy becomes more negative.

Question 18

What 2 factors affect the value of ΔH_hyd^ϴ?

Answer 18

1. Ionic radius.
2. Ionic charge.

Question 19

How does ionic radius affect ΔH_hyd^ϴ?

Answer 19

The smaller the ionic radius, the greater the charge density of the ion, the greater the electrostatic force of attraction between the ion and H₂O. This results in ΔH_hyd^ϴ being more exothermic.

Question 20

How does ionic charge affect ΔH_hyd^ϴ?

Answer 20

The greater the (+ve) ionic charge, the smaller the ionic radius, greater the charge density, the stronger the electrostatic force of attraction between the ions and H₂O, the more exothermic ΔH_hyd^ϴ?

Question 21

What is entropy (S)?

Answer 21

The quantitative measure of the degree of disorder in a system.

Question 22

What are the units for entropy?

Answer 22

JK^-1mol^-1

Question 23

What are the properites of entropy?

Answer 23

• The greater the entropy, the greater the disorder of a system.
• In nature, entropy of a system always increases over time.
• Entropy of a system is always +ve.

Question 24

What are some examples of entropy change in nature?

Answer 24

• Gas spreading throughout room.
• Heat from radiator spreading throughout room.
• Ice melting.
• An ionic salt dissolving in water.

Question 25

What is the effect on entropy when temperature increases?

Answer 25

Increases.

Question 26

What is the effect in entropy when a solid dissolves?

Answer 26

Increases.

Question 27

What is the effect on entropy when there is an increases in the number of moles of gaseous molecules?

Answer 27

Increases.

Question 28

What is the effect on entropy when the number of moles of gaseous molecules decreases?

Answer 28

Decreases.

Question 29

What is the standard entropy change of reaction
(ΔS^ϴ)?

Answer 29

The entropy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants reactants and products being in their standard states.

Question 30

What is the equation for ΔS?

Answer 30

ΔS = ΣS^ϴ_products- ΣS^ϴ_reactants

Question 31

What is the significance of the ΔS value?

Answer 31

• A +ve ΔS indicates an increase in entropy and an increase in disorder of the system.
• A -ve ΔS indicates a decrease in entropy and a decrease in disorder of the system.

Question 32

What is a spontaneous/feasible reaction?

Answer 32

A reaction that will proceed on its own without any external input at a specified temperature.

Question 33

What property do all spontaneous reactions have?

Answer 33

The stability of the chemical species increases as a result of the reaction and overall energy of the system decreases.

Question 34

What 3 factors affect the feasibility of a reaction?

Answer 34

1. Temperature of system (T), in Kelvin.
2. Entropy change of system (ΔS).
3. Enthalpy change (ΔH).

Question 35

What is the name of the variable linking the 3 feasibility factors?

Answer 35

(Gibbs) free energy change (ΔG).

Question 36

What is the equation for ΔG?

Answer 36

ΔG = ΔH - TΔS

Question 37

What are the units of ΔG?

Answer 37

KJmol^-1

Question 38

What condition must ΔG of a reaction satisfy for the reaction to be feasible?

Answer 38

ΔG < 0

Question 39

What is the feasibility of a reaction where ΔH is -ve and ΔS is +ve?

Answer 39

ΔG always -ve so reaction is feasible at any temperature.

Question 40

What is the feasibility of a reaction where ΔH is +ve and ΔS is -ve?

Answer 40

ΔG is never -ve since -TΔS is always positive so reaction is not feasible at any temperature.

Question 41

What is the feasibility of a reaction where ΔH is +ve and ΔS is +ve?

Answer 41

ΔG is -ve only at high temperatures (where TΔS >
ΔH, so reaction is feasible at high temperatures.

Question 42

What is the feasibility of a reaction where ΔH is -ve and ΔS is -ve?

Answer 42

ΔG is -ve only at low temperature (where |TΔS| < |ΔH|), so reaction only feasible at low temperatures.

Question 43

How can an endothermic reaction take place spontaneously?

Answer 43

1. ΔS is positive.
2. High temperatures so that TΔS > ΔH

Question 44

Why do some feasible reactions seem unfeasible in practice?

Answer 44

• Calculated ΔG inaccurate due to reactants and products not being in standard states and/or standard conditions.
• ΔG does not take into account high activation energies. The reaction may be occuring at a very slow rate due to high activation energies so that no visible change in produced.