Thermodynamics

Question 1

What does Hess’ Law state?

Answer 1

The enthalpy change for 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 by definition

Question 4

Define standard enthalpy of combustion

Answer 4

The enthalpy change when one mole of a substance is completely burnt in (excess) oxygen

Question 5

Define standard enthalpy of atomisation

Answer 5

Enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions

Question 6

Define 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

Enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions

Question 8

Define first electron affinity

Answer 8

Enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- moles

Question 9

Define second electron affinity

Answer 9

Enthalpy change when one mole of gaseous 1- ions gains one 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 into its gaseous ions

Question 12

Define enthalpy of hydration

Answer 12

Enthalpy change when one mole of gaseous ions become hydrated / dissolved in water to infinite dilution [water molecules completely surround the ion]

Question 13

Define enthalpy of solution

Answer 13

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

Question 14

Define mean bond dissociation enthalpy

Answer 14

Enthalpy change when one mole of (a certain type of) covalent bonds is broken, with all species in the gaseous state

Question 15

Write an example equation for:
Standard enthalpy of formation
Standard enthalpy of combustion
Standard enthalpy of atomisation

Answer 15

1. Mg (s) + 1/2 O2 (g) —> MgO (s)
2. CH4 (g) + 2O2 (g) —> CO2 (g) + 2H2O (g)
3. 1/2 I2 (g) —> I (g)

Question 16

Write an example equation for:
First ionisation energy
Second ionisation energy
First electron affinity

Answer 16

1. Li (g) —> Li+ (g) + e-
2. Mg+ (g) —> Mg2+ (g) + e-
3. Cl (g) + e- —> Cl- (g)

Question 17

Write an example equation for
Second electron affinity
Lattice enthalpy of formation
Lattice enthalpy of dissociation

Answer 17

1. O- (g) + e- —> O*2- (g)
2. Na+ (g) + Cl- (g) —> NaCl (s)
3. NaCl (s) —> Na+ (g) + Cl- (g)

Question 18

Write an example equation for
Enthalpy of hydration
Enthalpy of solution
Mean bond dissociation enthalpy

Answer 18

1. Na+ (g) —> Na+ (aq)
2. NaCl (s) —> Na+ (aq) + Cl- (aq)
3. Br2 (g) —> 2Br (g)

Question 19

What is a Born-Haber cycle

Answer 19

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

Question 20

What factors affect the lattice enthalpy of an ionic compound

Answer 20

Size of ions
Charge of ions

Question 21

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

Answer 21

Smaller ions since the charge centres will be closer together
Increased charge since there will be greater electrostatic force of attraction between the oppositely charged ions
N.B. Increasing the charge on the Anion has a much smaller effect than increasing the charge on the Cation, since increasing anion size also has the effect of increasing ionic size

Question 22

How can Born-Haber cycles be used to see if compounds could theoretically exist

Answer 22

Use known data to predict certain values of theoretical compounds, and then see if these compounds would be thermodynamically stable
Was used to predict the existence of the first noble gas containing compound

Question 23

What happens when a solid is dissolved in terms of interactions of the ions with water ions

Answer 23

Break lattice - gaseous ions; dissolve each gaseous ion in water. The aqueous ions are surrounded by water molecules (which have a permanent dipole due to polar O-H bond)

Question 24

What is the perfect ionic model

Answer 24

Assumes that ions are perfectly symmetrical and that there is an even charge distribution (100% polar bonds). Act as point charges

Question 25

Why is the perfect ionic model often not accurate

Answer 25

Ions are not perfectly spherical. Polarisation often occurs when small positive ions or large negative ions are involved so the ionic bond gain covalent character. Some lattices are not regular and the crystal structure can differ

Question 26

Which kind of bonds will be the most ionic? Why?

Answer 26

Between large positive ions and small negative ions e.g. CsF

Question 27

Define the terms spontaneous and feasible

Answer 27

If a reaction is spontaneous and feasible it will take place of its own accord; does not take account of rate of reaction

Question 28

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

Answer 28

Negative - exothermic

Question 29

Define entropy

Answer 29

Randomness / disorder of a system Higher value for entropy = more disordered

Question 30

What units is entropy measured in?

Answer 30

J/K/mol or JK^-1mol^-1

Question 31

What is the second law of thermodynamics

Answer 31

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

Question 32

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

Answer 32

Positive - reactions always try and increase the amount of disorder

Question 33

Compare the general entropy values for solids, liquids and gases

Answer 33

Solids < Liquids < Gases

Question 34

How would you calculate the entropy change for a reaction

Answer 34

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

Question 35

Define Gibbs free energy using an equation

Answer 35

ΔG = ΔH - TΔS G = Gibbs free energy H = enthalpy change S = entropy change T = temperature

Question 36

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

Answer 36

If G < 0, reaction is feasible If G = 0, reaction is just feasible If G > 0 reaction is not feasible

Question 37

What is the significance of the temperature at G = 0

Answer 37

This is the temperature in Kelvin at which the reaction becomes feasible

Question 38

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

Answer 38

Rearrange to T = (ΔH)/(ΔS) since G = 0

Question 39

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

Answer 39

Gibbs free energy only indicates if a reaction is feasible. It doesnt take into account the rate of reaction (Kinetics of the reaction) In reality many reactions that are feasible at certain temperature have a rate of reaction that is so slow that effectively no reaction is occuring

Question 40

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

Answer 40

G always negative so reaction is always feasible - product favoured

Question 41

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

Answer 41

G is always positive, so reaction is never feasible - reactant favoured

Question 42

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

Answer 42

Temperature dependent

Question 43

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

Answer 43

Temperature dependent

Question 44

Why is entropy zero at 0K

Answer 44

No disorder - molecules / atoms are not moving or vibrating and cannot be arranged in any other way Maximum possible state of order

Question 45

What are two things to look out for to decide if entropy increase/decreases/stays relatively constant

Answer 45

Number of moles - more moles made ---> increase in entropy Going from to solid ---> liquid/gas or liquid ---> gas

Question 46

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

Answer 46

The heat given out escapes into the surroundings