Thermodynamics

Question 1

Standard enthalpy of solution

Answer 1

Enthalpy change when 1 mol of an ionic compound is dissolved in solvent/ H20 to make aq ions.

Question 2

Enthalpy of hydration

Answer 2

Enthalpy change when 1 mol of gaseousions is converted into aqueous ions

Question 3

What are the units for entropy?

Answer 3

JK-1mol-1

Question 4

Definition for enthalpy change

Answer 4

Heat change at a constant pressure
Gases have high entropy due to random arrangement of particles

Question 5

How is disorder measured?

Answer 5

JK-1mol-1
Symbol (S)
From a solid to aqueous ions= entropy is positive as there is more disorder
From gas to liquid= entropy is negative as less disorder

Question 6

What does it mean when a reaction is feasible?

Answer 6

Reaction can take place. Overall if Gibbs free is less than or equal to to 0 then reaction is feasible.

Question 7

Why might a reaction that is feasible not take place?

Answer 7

May have a high Ea that’s too high to overcome.

Question 8

Favourable conditions for Gibbs free energy reaction

Answer 8

1) Enthalpy change= Negative
2) Entropy value= Positive

Question 9

What has zero entropy?

Answer 9

Only perfect crystals at absolute zero (T = 0 K) will have zero ∆S entropy.

Normally substances don’t have zero entropy.

Question 10

How does the size and charge of the ion affect the size of the lattice enthalpy of an ionic solid?

Answer 10

The smaller the ion and the higher its charge, the stronger the lattice

Question 11

How will a significant entropy change occur?

Answer 11

1) If there is a change of state from solid or liquid to gas
2) There is a significant increase in number of molecules between products and reactants.

Question 12

What happens to the feasiblity of a reaction when increasing entropy?

Answer 12

If the reaction involves an increase in entropy (∆S is +ve) then increasing temperature will make it more likely that ∆G is negative and more likely that the reaction occurs e.g. NaCl + aq Na+ (aq) + Cl - (aq)

Question 13

Example Q

State why there is a difference between the theoretical and experimental values.

Answer 13

because the compound may have some covalent character

Question 14

Example Q

Explain why the enthalpy of hydration becomes less exothermic from Li+ to K+

Answer 14

because the size of the ionic radius increases so the attraction between the outer electron and the nucleus decreases
so less energy is required

Question 15

Define enthalpy of formation

Answer 15

Define enthalpy of formation
The enthalpy change when 1 mole of a substance is formed from its constituent elements when all products and reactants are in their standard states under standard conditions

Question 16

Define standard enthalpy of combustion

Answer 16

The enthalpy change when 1 mole of a substance is burned completely in excess oxygen with all products and reactants in their standard states under standard conditions

Question 17

Define enthalpy of atomisation

Answer 17

The enthalpy change when one mole of gaseous atoms are formed from the element in its standard state

Question 18

Define enthalpy of lattice formation

Answer 18

The enthalpy change when 1 mole of an ionic compound is formed from its component gaseous ions

Question 19

Define enthalpy of lattice dissociation

Answer 19

The enthalpy change when 1 mole of ionic compound is (infinitely) separeted to form its constituent gaseous ions

Question 20

Why is the phrase ‘infinitely separated’ used of enthalpy of lattice dissociation ?

Answer 20

All bonding in the ions must be broken

Question 21

Which enthalpy changes are endothermic ?

5

Answer 21

• Enthalpy of lattice dissociation
• Enthalpy of atomisation
• 1st/2nd Ionisation enthalpy
• 2nd electron affinity
• Bond dissociation enthalpy

Question 22

Which enthalpy changes are exothermic ?

2

Answer 22

• 2nd electron affinity
• Enthalpy of formation (mostly)

Question 23

What is the relationship between enthalpy of atomisation and bond dissociation enthalpy?

Answer 23

Enthalpy of atomisation produces 1 mole of gaseous atoms
Bond dissociation enthalpy produces 2 moles of gaseous atoms

BDE = EA X 2

Question 24

Define first electron affinity

Answer 24

The enthalpy change when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of 1- ions

Question 25

Why is the lattice enthalpy greater for smaller ions ?

Answer 25

Smaller ions are more closely packed in the lattice
There is a stronger attraction

Question 26

Why is the lattice enthalpy greater for ions with a high charge?

Answer 26

There is a stronger attraction

Question 27

What does the perfect ionic model assume?

Answer 27

The bonding in the compound if 100% ionic
The ions are perfect spheres/point charges and not distorted

Question 28

Why do some compounds have covalent character?

3

Answer 28

One of the ions is small and has a large charge
This distorts the other ion
The ions are not spherical

Question 29

What happens when an ionic compound dissolves in water?

Answer 29

The ionic lattice breaks up (enthalpy of lattice dissociation)
The polar water molecules form bonds with the ions (enthalpy of hydration)

Question 30

Define enthalpy of hydration

Answer 30

The enthalpy change when 1 mole of gaseous ions is converted to 1 mole of aqueous ions

Question 31

Why is the enthalpy of hydration exothermic ?

Answer 31

Energy is released when ions are attracted to the polar water molecules