1.08 Thermodynamics

Question 1

What does Hess’ law state ?

Answer 1

That the enthalpy change of a reaction is independent of the route taken

Question 2

Define standard enthalpy of formation

Answer 2

The enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions, with all products and reactants in their standard states.

Question 3

What is the standard enthalpy of an element ?

Answer 3

0

Question 4

Define standard enthalpy of combustion

Answer 4

The enthalpy change when one mole of a substance is completely burnt in an excess of O2

Question 5

Define standard enthalpy of atomisation

Answer 5

The enthalpy change when one mole of gaseous atoms is formed from a compound in its standard states and in standard conditions.

Question 6

Define the first ionisation energy

Answer 6

Enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous +1 ions.

Question 7

Define second ionisation energy

Answer 7

The enthalpy change when one mole of electrons is removed from one mole of 1+ ions to form one mole of 2+ ions.

Question 8

Define first electron affinity

Answer 8

The enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- ions.

Question 9

Define second electron affinity

Answer 9

Enthalpy change when one mole of gaseous 1- ions gains another mole of electrons to form one mole of gaseous 2- ions.

Question 10

Define lattice enthalpy of formation

Answer 10

Enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions.

Question 11

Define lattice enthalpy of dissociation

Answer 11

Enthalpy change when one mole of solid ionic lattice is dissociated (broken into) it’s gaseous ions

Question 12

Define enthalpy of hydration

Answer 12

Enthalpy change when one mole of gaseous ions become hydrated (dissolved in water where water molecules completely surround the ion).

Question 13

Define standard enthalpy of solution

Answer 13

The enthalpy change when one mole of solute dissolves completely in a solvent to infinite dilution

Question 14

Define mean bond dissociation enthalpy

Answer 14

The enthalpy change when one mole of a certain type of covalent bond (eg Br-H) is broken, with all species in the gaseous state.

Question 15

What is a Born-Haber cycle ?

Answer 15

•A cycle showing all the enthalpy changes involved in the formation of an ionic compound
•Start with elements in their standard states (enthalpy of 0)

Question 16

What factors affect the lattice enthalpy of an ionic compound ?

Answer 16

Size of the ions and their charge

Question 17

How can you increase the lattice enthalpy of a compound? Why does this increase it ?

Answer 17

•By having smaller ions, as the charge centres will be closer together
•By having an increased charge as there will be a greater electrostatic force

Question 18

How can we use Born-Haber cycles to see if compounds theoretically exist ?

Answer 18

•Use known data to predict certain values of theoretical compounds
•You can then see if these compounds are thermodynamically stable

Question 19

What happens when a solid is dissolved (in terms of interactions of the ions of water molecules)

Answer 19

•The water molecules break the lattice -> the now gaseous ions are then surrounded by the water molecules

Question 20

What is the perfect ionic model ?

Answer 20

•Assumes that ions are perfectly spherical and that there is an equal distribution of charge (100% polar bonds).
•In reality, compounds have covalent character. This increases bond strength.

Question 21

Why is the perfect ionic model often not accurate ?

Answer 21

Ions are not perfectly spherical. Polarisation often occurs when small positive ions or large negative ions are involved, making the ions gain covalent character.

Question 22

Which kinds of bonds are the most ionic? Why?

Answer 22

Between large positive ions and small negative ions eg CsF (atomic radius’ effect on e- loss)

Question 23

Define the terms spontaneous / feasible

Answer 23

If a reaction is spontaneous / feasible, it can take place of its own accord; does not take account of the rate of reaction

Question 24

Is a reaction with a positive or negative enthalpy change more likely to be feasible ?

Answer 24

Negative.

Question 25

Define entropy

Answer 25

Randomness / disorder of a system.
Higher entropy value = more disorder

Question 26

What units is entropy measured in ?

Answer 26

JK-1mol-1

Question 27

What is the second law of thermodynamics ?

Answer 27

Entropy (of an isolated system) always increases, as it is overwhelmingly more likely for molecules to be disordered than ordered.

Question 28

Is a reaction with positive or negative entropy change more likely to be feasible ?

Answer 28

Positive - reactions always favour increasing disorder

Question 29

Compare the general entropy of the different phases

Answer 29

Solid < liquid < gas

Question 30

How would you calculate the entropy change for a reaction

Answer 30

Entropy change = sum of products’ entropy - sum of reactants’ entropy

Question 31

Define Gibbs energy using an equation

Answer 31

•ΔG = ΔH - TΔS
•G = Gibbs free energy
•H = Enthalpy
•S = Entropy
•T = Temperature.

Question 32

What does the value for Gibbs free energy show for a reaction

Answer 32

•If G ≤ 0 then the reaction is feasible
•If G > 0 then the reaction is not feasible.

Question 33

What is the significance of the temperature where G = 0 ?

Answer 33

This is the temperature (in kelvin) at which the reaction becomes feasible.

Question 34

How would you calculate the temperature at which a reaction becomes feasible ?

Answer 34

Rearrange for T = (ΔH)(ΔS) as G= 0

Question 35

What are the limitations of using G as an indicator whether the reaction will occur ?

Answer 35

Gibbs free energy only indicates if a reaction is feasible, it does not account for rate of reaction.
A reaction may theoretically be feasible at a certain temperature but the rate is so slow that it’s basically not happening.

Question 36

If the reaction is exothermic and the entropy increases, what is the value of G and what does it mean ?

Answer 36

G is always negative, so reaction is always feasible - product favoured

Question 37

If a reaction is endothermic and entropy decreases, what is the value of G and what does this mean ?

Answer 37

G is always positive, so reaction is never feasible.

Question 38

If a reaction is exothermic and entropy decreases, what is the value of G and what does this mean ?

Answer 38

The feasibility is temperature dependant

Question 39

If a reaction is endothermic and entropy increases, what is the value of G and what does this mean ?

Answer 39

The feasibility of the reaction is temperature dependant

Question 40

Why is entropy 0 at 0k ?

Answer 40

There is no disorder, molecules are not moving or vibrating and so cannot be arranged in a different way - it is the maximum state of order.

Question 41

What are the two key things to look out for to decide if entropy increases/decreases or stays constant ?

Answer 41

•Increase in number of moles increases entropy
•Going from solid to gas increases entropy

Question 42

How is it possible for the temperature of a substance undergoing an endothermic reaction to stay constant

Answer 42

The heat that is given out escapes to the surroundings.