Topic 13: Energetics II

Question 1

What is the lattice energy?

Answer 1

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

It always has a negative value (it is exothermic)

Question 2

What assumptions do theoretical values for lattice energy?

Answer 2

1. Ions are spherical,
2. Ions are point charges i.e. the charge is evenly distributed,
3. Ions are in contact.
   This would all be the case for pure ionic bonding.

Question 3

Why is there a difference between theoretical and calculated lattice energies?

Answer 3

Covalent character of the ionic bond affects the values. If the positive ion is very small, it has a higher charge density and therefore more polarising ability on the large negative ion.
The larger the negative ion, the more polarisable it is.

This discrepancy causes a difference in values.

Question 4

What is the enthalpy of solution?

Answer 4

enthalpy change when 1
mole of an ionic compound is dissolved in water to
form an infinitely dilute solution

Question 5

What is Enthalpy of hydration?

Answer 5

enthalpy change when 1
mole of gaseous ions are dissolved in sufficient water
to give an infinitely dilute solution. Always negative
values as a result of the formation of strong ion-dipole
attractions.

Question 6

What is meant by lattice enthalpy?

Answer 6

enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions.
Always negative due to the formation of strong ion-ion
attractions.

Question 7

Diagram of an ionic solid dissolving?

Answer 7

Ionic solid —-> Dis. ion in soln
\ /
\ /
Gaseous ions

From gaseous ions -> ionic solid = lattice enthalpy

Gaseous ions –> dissolved in solution = enthalpy of hydration

Ionic solid –> dissolved in solution = Enthalpy of solution

Question 8

What is entropy?

Answer 8

A description of the number of ways that atoms can share quanta of energy.
If the number of ways is high then entropy is high.

Question 9

What is the equation for the total entropy change?

Answer 9

ΔStotal = ΔSsystem + ΔSsurroundings

Question 10

What is the equation for ΔSsystem?

Answer 10

ΔSsystem = ∑S products - ∑S reactants

∑ = sum of

Question 11

What does an increase in entropy mean?

Answer 11

An increase in entropy means that the system becomes energetically more stable
An example of a system that becomes more disordered is when a solid is melted

Question 12

What is more favourable? Higher or lower entropy?

Answer 12

Higher entropy will be energetically favourable (as the energy of the system is more spread out when it is in a disordered state)

Question 13

What is Gibbs free energy?

Answer 13

The feasibility of a reaction is determined by two factors
The enthalpy and entropy change
The two factors come together in a fundamental thermodynamic concept called the Gibbs free energy

Question 14

Gibbs energy equation?

Answer 14

ΔGꝋ = ΔHreaction – TΔSsystem

(under standard conditions)

Question 15

What must the value for Gibbs be in order for the reaction to be feasible?

Answer 15

Less than or equal to 0

Question 16

What are the units for Gibbs?

Answer 16

• The units of ΔGꝋ are in kJ mol–1
• The units of ΔHreactionꝋ are in kJ mol–1
• The units of T are in K
• The units of ΔSsystemꝋ are in J K-1 mol–1(and must therefore be converted to kJ K–1 mol–1by dividing by 1000)

Question 17

What is the equation for ΔSsurroundings?

Answer 17

ΔSsurroundings = -ΔH/T

Question 18

What is the enthalpy change of atomisation?

Answer 18

enthalpy change when one mole of gaseous atoms is produced from its element in its standard state

Question 19

Define electron affinity

Answer 19

The first electron affinity is the energy released when 1 mole of gaseous atoms each acquire an electron to form 1 mole of gaseous 1- ions

always has a negative value

Question 20

What does lattice energy tell us

Answer 20

It provides a measure of ionic bond strength

Question 21

What is the meaning of polarisation when applied to ions?

Question 22

What does the polarising ability of a cation depend on?

Answer 22

Charge - larger charge means more polarising ability
Radius - smaller radius means more polarising ability

Question 23

What does the polarisability of a anion depend on?

Answer 23

Charge
Radius - the larger the radius, the more easily it will be distorted

Question 24

What is the effect of ionic charge and ionic radius on lattice energy?

Question 25

What is the effect of ionic charge and ionic radius on Enthalpy change of Hydration