1.8 Thermodynamics

Question 1

Define standard enthalpy of atomisation

Answer 1

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

Question 2

Define first ionisation energy

Answer 2

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

Question 3

Define second ionisation energy.

Answer 3

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 4

Define first electron affinity

Answer 4

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

Question 5

Define second electron affinity

Answer 5

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

Question 6

Define lattice enthalpy of formation

Answer 6

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

Question 7

Define lattice enthalpy of dissociation.

Answer 7

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

Question 8

Define enthalpy of hydration.

Answer 8

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

Question 9

Define enthalpy of solution

Answer 9

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

Question 10

Define mean bond dissociation enthalpy

Answer 10

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

Question 11

What is a Born-Haber cycle?

Answer 11

Thermochemical 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 12

What factors affect the lattice enthalpy of an ionic compound?

Answer 12

Size of the ions, charge on the ions

Question 13

How can you increase the lattice enthalpy of a

compound? Why does this increase it?

Answer 13

Smaller ions, since the charge centres will be closer together. Increased charge, since there will be a 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 charge also has the effect of increasing ionic size.

Question 14

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

Answer 14

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 15

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

Answer 15

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 16

What is the perfect ionic model?

Answer 16

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

Question 17

Why is the perfect ionic model often not accurate?

Answer 17

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

Question 18

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

Answer 18

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

Question 19

Define the terms spontaneous and feasible

Answer 19

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

Question 20

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

Answer 20

Negative (exothermic)

Question 21

Define entropy

Answer 21

Randomness/disorder of a system.

Higher value for entropy = more disordered

Question 22

What units is entropy measured in?

Answer 22

JK-1mol-1

Question 23

What is the second law of thermodynamics?

Answer 23

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

Question 24

Is a reaction with positive or negative entropy change

more likely to be spontaneous?

Answer 24

Positive - reactions always try and increase the amount of disorder

Question 25

How would you calculate the entropy change for a

reaction?

Answer 25

ΔS = ΣS(products) - ΣS(reactants)

Question 26

Define Gibbs free energy using an equation

Answer 26

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

Question 27

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

Answer 27

If G < 0, reaction is feasible. If G = 0, reaction is JUST feasible. If G > 0, reaction is not feasible.

Question 28

What is the significance of the temperature at which

G = 0?

Answer 28

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

Question 29

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

Answer 29

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

Question 30

What are the limitations of using G as an indicator of

whether a reaction will occur?

Answer 30

Gibbs free energy only indicates if a reaction is feasible. It does not take into account the rate of reaction (the kinetics of the reaction). In reality, many reactions that are feasible at a certain temperature have a rate of reaction that is so slow that effectively no reaction is occurring.

Question 31

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

Answer 31

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

Question 32

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

Answer 32

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

Question 33

If the reaction is exothermic and entropy decreases,

what is the value of G and what does this mean?

Answer 33

Temperature dependent

Question 34

If the reaction is endothermic and entropy increases,

what is the value of G and what does this mean?

Answer 34

Temperature dependent

Question 35

Why is entropy zero at 0K?

Answer 35

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

Question 36

What are the two key things to look out for to decide

if entropy increases/decreases/stays relatively constant?

Answer 36

Number of moles - more moles made → increase in entropy

Going from solid → liquid/gas or liquid → gas

Question 37

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

Answer 37

The heat that is given out escapes to the surroundings