Thermodynamics

Question 1

What does Hess’s 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 elements in their standard states, under standard conditions

Question 3

What is the standard enthalpy of an element?

Answer 3

Zero, 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 elements in their standard state.

Question 6

Define first ionisation energy

Answer 6

Enthalpy change when one mole of gaseous atoms is converted into 1 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 1 mole of gaseous atoms forms 1 mole of gaseous 1- ions

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 (broken into) 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 totally 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

Answer 15

Mg (s) + ½ O2 (g) → MgO (s)

Question 16

Write an example equation for standard enthalpy of combustion

Answer 16

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)

Question 17

Write an example equation for standard enthalpy of atomisation

Answer 17

½ I2 (g) → I (g)

Question 18

Write an example equation for first ionisation energy

Answer 18

Li (g) → Li+ (g) + e-

Question 19

Write an example equation for second ionisation energy

Answer 19

Mg+ (g) → Mg2+ (g) + e-

Question 20

Write an example equation for first electron affinity

Answer 20

Cl (g) + e- → Cl - (g)

Question 21

Write an example equation for second electron affinity

Answer 21

O- (g) + e- → O2- (g)

Question 22

Write an example equation for lattice enthalpy of formation

Answer 22

Na+ (g) + Cl- (g) → NaCl (s)

Question 23

Write an example equation for lattice enthalpy of dissociation

Answer 23

NaCl (s) → Na+ (g) + Cl- (g)

Question 24

Write an example equation for enthalpy of hydration

Answer 24

Na+ (g) → Na+ (aq)

Question 25

Write an example equation for enthalpy of solution

Answer 25

NaCl (s) → Na+ (aq) + Cl- (aq)

Question 26

Write an example equation for mean bond dissociation enthalpy

Answer 26

Br2 (g) → 2Br (g)

Question 27

What is a Born-Haber cycle?

Answer 27

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 28

What factors affect the lattice enthalpy of an ionic | compound?

Answer 28

Size of the ions, charge on the ions

Question 29

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

Answer 29

Smaller ions, since the charge centres will be closer together. Increased charge, since there will be a greater electrostatic 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 30

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

Answer 30

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 31

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

Answer 31

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 32

What is the perfect ionic model?

Answer 32

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

Question 33

Why i the perfect ionic model often not accurate?

Answer 33

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 34

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

Answer 34

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

Question 35

Define the terms spontaneous and feasible

Answer 35

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

Question 36

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

Answer 36

Negative - exothermic

Question 37

Define entropy

Answer 37

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

Question 38

What units is entropy measured in?

Answer 38

JK-1mol-1

Question 39

What is the second law of thermodynamics?

Answer 39

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

Question 40

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

Answer 40

Positive - reactions always try and increase the amount of disorder

Question 41

Compare the general entropy values for solids, | liquids and gases

Answer 41

Solids < liquids < gases

Question 42

How would you calculate the entropy change for a | reaction?

Answer 42

Entropy change = sum of products’ entropy - sum of reactants’ entropy ΔS = ΣS(products) - ΣS(reactants)

Question 43

Define Gibbs free energy using an equation

Answer 43

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

Question 44

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

Answer 44

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

Question 45

What is the significance of the temperature at which | G = 0?

Answer 45

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

Question 46

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

Answer 46

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

Question 47

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

Answer 47

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 48

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

Answer 48

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

Question 49

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

Answer 49

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

Question 50

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

Answer 50

Temperature dependent

Question 51

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

Answer 51

Temperature dependent

Question 52

Why is entropy zero at 0K?

Answer 52

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

Question 53

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

Answer 53

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

Question 54

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

Answer 54

The heat that is given out escapes to the surroundings

Question 55

What is enthalpy?

Answer 55

Heat measured at constant pressure

Question 56

What is mean bond enthalpy?

Answer 56

The enthalpy to break a bond, averaged over different molecules