Thermodynamics

Question 1

Lattice Formation Enthalpy

Answer 1

The enthalpy change when one mole of an ionic lattice is formed from its gaseous ions.

Na⁺(g) + Cl⁻(g) → NaCl(s)

Question 2

Enthalpy of Formation

Answer 2

The enthalpy change when one mole of a compound is formed from its constituent elements in their standard states.

Na(s) + 1/2 Cl₂(g) → NaCl(s)

Question 3

Enthalpy of Atomisation

Answer 3

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

Na(s) → Na(g)

Question 4

First ionisation energy

Answer 4

The enthalpy change when each one mole of gaseous atoms loses an electron and forms a 1+ ion.

Mg(g) → Mg⁺(g) + e⁻

Question 5

First electron affinity

Answer 5

The enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions.

O(g) + e⁻ → O⁻(g)

Question 6

Bond dissociation Enthalpy

Answer 6

The bond dissociation Enthalpy is the standard molar Enthalpy change when one mole of a covalent bond is broken into two gaseous atoms.

I₂(g) → 2I(g)

Question 7

Second electron affinity

Answer 7

Second electron affinity is always endothermic as it takes energy to overcome the repulsive force between the negative ion and the electron

Question 8

Enthalpy of hydration

Answer 8

Enthalpy change when one mole of gaseous ions become aqueous ions

X⁺(g)→X⁺(aq)

Always exothermic

Question 9

Enthalpy of solution

Answer 9

The Enthalpy of solution is the standard Enthalpy change when one mole of an ionic solid dissolves in a large enough amount of water to ensure that the dissolved ions are well separated and do not interact with one another

NaCl(s) → Na⁺(aq) + Cl⁻(aq)

Question 10

What factors effect the strength of lattice formation Enthalpy?

Answer 10

The sizes of the ions
The larger the ions the less negative the Enthalpies of lattice formation

The charges of the ions
The larger the charge the greater the attractions between the ions

Question 11

What is the perfect ionic model?

Answer 11

Ions are 100% ionic and spherical and the attractions are purely electrostatic

Question 12

How to determine when a compound shows covalent character?

Answer 12

The difference between the theoretical and Born Haber lattice enthalpies. The larger the difference the more covalent character.

Question 13

What is a spontaneous process?

Answer 13

A process which will proceed on its own without any external influence.

Question 14

What is entropy?

Answer 14

Entropy is a description of the number of ways atoms can share quanta of energy. If number of ways of arranging the energy is high then the system is disordered and the entropy is high.

Question 15

How to calculate ∆S?

Answer 15

∆S = ∑S products -∑S reactants

Question 16

What is Gibbs free energy equation?

Answer 16

∆G = ∆H -T∆S

Question 17

What value for ∆G means the reaction is feasible?

Answer 17

0 or negative

Question 18

What to remember when using Gibbs free energy equation?

Answer 18

Divide ∆S by 1000

Question 19

How to calculate Enthalpy of Solution?

Answer 19

∆H dissociation + ∑∆H hydration

Or

-∆H formation + ∑∆H hydration

Question 20

Are hydration enthalpies Exothermic or Endothermic?

Answer 20

They are exothermic

Question 21

What is interesting about the Enthalpy of solution?

Answer 21

It is usually very close to 0 and is not very exo or endothermic.

Usually if it is exothermic the substance is likely to be soluble and vice versa for endothermic.

Question 22

Explain why there is a difference between the hydration in the hydration enthalpies of the magnesium and sodium ions.

Answer 22

Magnesium ions have a higher charge and a smaller ions and magnesium will attract water more strongly.

Question 23

Why would the gradient the Gibbs Free Energy line change?

Answer 23

The substance could have change the state so the value of entropy changes

Question 24

Suggest why the hydration of the chloride ion is an exothermic process

Answer 24

Water is polar

Chloride ion attracts the water molecules

Question 25

Explain why the theoretical enthalpy of lattice dissociation is different from the experimental value ?

Answer 25

Experimental lattice and the value includes covalent interaction. Theoretical lattice and will be value assumes only ionic interaction.