PHYSICAL - thermodynamics

Question 1

what does hess’s law state?

Answer 1

the enthalpy change for a reaction is independant 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

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

the enthalpy change when one mole of gaseous atoms is formed from its elements in its standard state and 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 moles 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 moles of gaseous 2+ ions.

Question 8

define first electron affinity?

Answer 8

enthalpy change where one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous 1- ions

Question 9

define second electron affinity?

Answer 9

enthalpy change where one mole of gaseous 1- ions gain 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

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 covalent bonds is broken, with all species in the gaseous state.

Question 15

what is born haber cycle?

Answer 15

thermochemical cycle showing all the enthalpy changes, involved in the formation of an ionic compound. start with elements in their standard states.

Question 16

what factors affect the lattice enthalpy of an ionic compound?

Answer 16

size of the ions
charge on the ions

Question 17

how can you increase the lattice enthalpy of a compound? why does this increase?

Answer 17

• smaller ions = charge centres will be closer together so stronger electrostatic forces of attraction between the oppositely charged ions

Question 18

what is the perfect ionic model?

Answer 18

assumes that ions are perfectly spherical and that there is an even charge distribution

Question 19

why is the perfect ionic model innacurate?

Answer 19

polarisation only occurs when small +ive ions and large -ive ions are involved

Question 20

define the terms spontaneous and feasible?

Answer 20

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

Question 21

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

Answer 21

negative - exothermic

Question 22

define entropy?

Answer 22

measure of disorder in a system

higher value for entropy = more disorder

Question 23

what units is entropy measured in?

Answer 23

JK-1mol-1

Question 24

what is the second law of thermodynamics?

Answer 24

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

Question 25

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

Answer 25

Positive - reactions always try and increases the amount of disorder

Question 26

compare the general entropy values for solid liquids and gases?

Answer 26

solid< liquid < gas

Question 27

how will you calculate entropy change for a reaction?

Answer 27

sum of products entropy - sum of reactants entropy

Question 28

define Gibbs free energy using an equation?

Answer 28

ΔG = ΔH - T ΔS

Question 29

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

Answer 29

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

Question 30

what is the significance of Temperature at which G=0 ?

Answer 30

this is temperature at which the reaction becomes feasible.

Question 31

how will you calculate temperature?

Answer 31

T = ΔH/ ΔS

Question 32

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

Answer 32

• does not take into account of the rate of reaction
• in reality many reactions which are feasible has a slow rate of reaction = so no reaction is actually occurring

Question 33

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

Answer 33

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

Question 34

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

Answer 34

G is always possible, reaction is never feasible - reactant favoured

Question 35

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

Answer 35

It is temperature dependent

Question 36

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

Answer 36

It is temperature dependent

Question 37

why is entropy zero at 0K?

Answer 37

No disorder - molecules aren’t vibrating