CH17 Thermodynamics

Question 1

What is Hess’s law

Answer 1

The enthalpy change for a reaction is independant of the route taken

Question 2

Standard enthalpy of formation

Answer 2

Enthalpy change when 1 mole of a compound is formed from its constituent elements in standard conditions

Question 3

What is standard enthalpy of an element

Answer 3

0 by definition

Question 4

Define standard enthalpy of combustion

Answer 4

The enthalpy change when 1 mole of a substance is completely burnt in excess oxygen

Question 5

Define standard enthalpy of atomisation

Answer 5

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

Question 7

Define second ionisation energy

Answer 7

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

Question 8

Define first electron affinity

Answer 8

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

Question 9

Define second electron affinity

Answer 9

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

Question 10

Define lattice enthalpy of formation

Answer 10

Enthalpy change when 1 mole of solid ionic lattice is formed from its constituent gaseous ions

Question 11

Define lattice enthalpy of dissociation

Answer 11

Enthalpy change when 1 mole of solid ionic lattice is dissolved into its gaseous ions

Question 12

Define enthalpy of hydration

Answer 12

Enthalpy change when 1 mole of gaseous ions become hydrated to infinite dilution

Question 13

Define enthalpy of solution

Answer 13

Enthalpy change when 1 mole of solute dissolves completely in a solvent to infinite dilution

Question 14

Define mean bond enthalpy

Answer 14

Enthalpy change when 1 mole of covalent bonds is broken, with all species in the gaseous state

Question 15

Example equations for:
Standard enthalpy of formation
Standard enthalpy of combustion

Answer 15

Standard enthalpy of formation - Mg (s) + 1/2 O2 (g) –> MgO (s)
Standard enthalpy of combustion - CH4 (g) + 2O2 (g) –> CO2 (g) + 2H2O (l)

Question 16

Example equation for:
Standard enthalpy of atomisation
First ionisation energy

Answer 16

Standard enthalpy of atomisation - 1/2 I2 (g) –> I(g)

First ionisation energy - Li (g) –> Li+ (g) + e-

Question 17

Example equations for:
Second ionisation energy
First electron affinity

Answer 17

Second ionisation - Mg+ (g) –> Mg2+ (g) + e-

First electron affinity - Cl (g) + e- –> Cl- (g)

Question 18

Example equations for:
Second electron affinity
Lattice enthalpy of formation

Answer 18

Second electron affinity - O- (g) + e- –> O2- (g)

Lattice enthalpy of formation - Na+ (g) + Cl- (g) –> NaCl (s)

Question 19

Example equations for:
Lattice enthalpy of dissociation
Enthalpy of hydration

Answer 19

Lattice enthalpy of dissociation - NaCl (s) –> Na+ (g) + Cl- (g)
Enthalpy of hydration - Na+ (g) –> Na+ (aq)

Question 20

Example equations for:
Enthalpy of solution
Mean bond dissociation enthalpy

Answer 20

Enthalpy of solution - NaCl (s) –> Na+ (aq) + Cl- (aq)

Mean bond dissociation enthalpy - Br2 (g) –> 2Br (g)

Question 21

What is a Born-Haber cycle

Answer 21

Thermochemical cycle showing all the enthalpy changes involved in the formation of an ionic compound. State with elements in their standard states

Question 22

What factors affect the lattice enthalpy of an ionic compound

Answer 22

Size of the ions

Charge of the ions

Question 23

How can you increase lattice enthalpy of a compound - why does this increase it

Answer 23

Smaller ions - charge centres will be closer together so greater electrostatic forces of attraction

Question 24

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

Answer 24

Use known data to predict certain values of theoretical compounds, and then see if these compounds will be thermodynamically stable

Question 25

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

Answer 25

The lattice is broken into gaseous ions - dissolve each gaseous ion in the water. The aqueous ions are surrounded by water molecules

Question 26

What is the perfect ionic model

Answer 26

Assumes that ions are perfectly spherical and that there is an even charge distribution. Act as point charges

Question 27

Why is this often not accurate

Answer 27

Ions are not perfectly spherical. Polarisation occurs

Question 28

Which kind of bonds will be mostly ionic

Answer 28

Between large positive ions and small negative ions

Question 29

Define terms spontaneous and feasible

Answer 29

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

Question 30

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

Answer 30

Negative - exothermic

Question 31

Define entropy

Answer 31

Disorder/randomness of a system

Question 32

Units of entropy

Answer 32

JK-1 mol-1

Question 33

What is the second law of thermodynamics

Answer 33

Entropy always increases, s it is overwhelmingly more likely for molecules to be disordered than ordered

Question 34

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

Answer 34

Positive - Reactions always try and increase the amount of disorder

Question 35

Compare the general entropy values for solids, liquids and gases

Answer 35

Solids < liquids < gases

Question 36

How would you calculate the entropy change for a reaction

Answer 36

Entropy change = sum of product's entropy - sum of reactant's entropy ΔS = ΣS (products) - ΣS (reactants)

Question 37

Define Gibbs free energy using an equation

Answer 37

``` ΔG = ΔH - TΔS G = Gibbs free energy H = Enthalpy change S = Entropy change T = Temperature ```

Question 38

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

Answer 38

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

Question 39

Significance of temperature at which G = 0

Answer 39

Temperature at which reaction becomes feasible

Question 40

Limitations of using G as an indicator of whether a reaction will occur

Answer 40

Only indicated if reaction is feasible. Doesnt take into account rate of reaction

Question 41

If reaction is exothermic what is value of G

Answer 41

G is always negative, so reaction is feasible

Question 42

If reaction is endothermic and entropy decreases, what is value of G

Answer 42

G always positive, so reaction is never feasible

Question 43

If reaction is exothermic and entropy decreases, what is value of G

Answer 43

Temperature dependant

Question 44

If the reaction is endothermic and entropy increases what is value of G

Answer 44

Temperature dependant

Question 45

Why is entropy 0 at 0K

Answer 45

No disorder - molecules/atoms not moving

Question 46

2 key things to look out for to decide if entropy decreases, increases or stays the same

Answer 46

number of moles - more moles made --> increase in entropy | Going from solid - liquid or liquid - gas