physical chem; thermodynamics

Question 1

define the term enthalpy change of formation

Answer 1

the enthalpy change when 1 mol of a compound is formed from it’s elements in their standard states under standard conditions

Question 2

define the term lattice enthalpy of formation

Answer 2

the enthalpy change when 1 mole of a solid ionic compound is formed from it’s gaseous ions under standard conditions

Question 3

define the term lattice enthalpy of dissociation

Answer 3

the enthalpy change when 1 mole of a solid ionic compound is dissociated into it’s gaseous ions under standard conditions

Question 4

define the term enthalpy change of dissociation

Answer 4

the enthalpy change when 1 mole of bonds of the same type of molecule in the gaseous state is broken

Question 5

define the term enthalpy change of 1st ionisation

Answer 5

the enthalpy change when 1 mole of gaseous 1+ ions are made from 1 mole of gaseous atoms

Question 6

define the term enthalpy change of 2nd ionisation

Answer 6

the enthalpy change when 1 mole of +2 ions are made from 1 mole of gaseous +1 ions

Question 7

define the term enthalpy change of atomisation

Answer 7

the enthalpy change of when 1 mole of gaseous atoms is made from an element in it’s standard state

Question 8

define the term enthalpy change of 1st electron affinity

Answer 8

the enthalpy change when 1 mole of gaseous -1 ions are made from 1 mole of gaseous atoms

Question 9

define the term enthalpy change of 2nd electron affinity

Answer 9

the enthalpy change when 1 mole of gaseous -2 ions are made from 1 mole of gaseous -1 ions

Question 10

what are born-haber cycles used for?

Answer 10

to calculate lattice enthalpies

Question 11

what do theoretical & experimental lattice enthalpy values depend on?

Answer 11

how pure an ionic compound is as some ionic compounds have covalent characteristics

Question 12

what is a perfect ionic model?

Answer 12

1. perfectly spherical ions
2. the charge is evenly distributed in the sphere (point charges)

Question 13

why do some ionic compounds have covalent character?

Answer 13

the +ve ion distorts the charge distribution in the -ve ion (+ve ion polarises -ve ion). the more polarisation there is, the more covalent character there will be

Question 14

what do lattice enthalpy values tell us about ionic substances?

Answer 14

how purely ionic a substance is

Question 15

how can we know that an ionic substance contains some covalent character from looking at lattice enthalpy values?

Answer 15

if the experimental value shows a higher lattice enthalpy than the purely ionic theoretical value

Question 16

define the term enthalpy change of solution

Answer 16

the enthalpy change when 1 mole of an ionic substance is dissolved in the minimum mount of solvent to ensure no further enthalpy change is observed upon further dilution

Question 17

name the requirements needed for a substance to dissolve

Answer 17

1. substance bonds must break (endothermic)
2. new bonds formed between the solvent & substance (exothermic)

Question 18

explain what happens when ionic substances dissolve in a polar solvent (e.g. water)?

Answer 18

the δ+ charge is attracted to the -ve ions & the δ- oxygen is attracted to +ve ions - structure starts to break down.
the water molecules surround the ions in a process called hydration.

Question 19

define the term enthalpy change of hydration

Answer 19

the enthalpy change when 1 mole of aqueous ions is made from 1 mole of gaseous ions

Question 20

what is entropy?

Answer 20

the measure of disorder in a system - more disorder = higher level of entropy

Question 21

state the disorder of the 3 states in order from lowest to highest

Answer 21

solids - lowest disorder as particles are tightly packed to one another
liquids
gases - highest as particles are free

Question 22

what affects entropy change?

Answer 22

the no.of particles
if a reaction is in the same state but more moles are produced then entropy increases

Question 23

explain how enthalpically unfavourable reactions can react spontaneously

Answer 23

increasing entropy is energetically favourable for endo reactions (enthalpically unfavourable) & can still spontaneously react if changes in entropy overcome changes in enthalpy

Question 24

how do we know if a reaction is entropically favourable?

Answer 24

more moles with the products
increased disorder (e.g. change in state from solid to liquid/gas)

Question 25

how do we know if a reaction is enthalpically favourable

Answer 25

if its exothermic

Question 26

how is entropy change calculated?

Answer 26

∆S = S (products) - S (reactants)

Question 27

how do we know if a reaction is entropically feasible?

Answer 27

if the entropy value is +ve

Question 28

what is gibbs free energy?

Answer 28

tells us if a reaction is feasible or not

Question 29

how is gibbs free energy calculated?

Answer 29

∆G = ∆H - T∆S

∆H - enthalpy change
T - temp
∆S - entropy change

Question 30

how do we know if a reaction is feasible by using ∆G value?

Answer 30

a reaction is feasible in theory if ∆G is -ve or 0

Question 31

state whether the reaction is feasible if ∆H is -ve & ∆S is +ve & state whether the ∆G value is +ve or -ve

Answer 31

yes, its feasible at any temp
∆G always -ve

Question 32

state whether the reaction is feasible if ∆H is +ve & ∆S is -ve & state whether the ∆G value is +ve or -ve

Answer 32

never feasible at any temp
∆G always +ve

Question 33

state whether the reaction is feasible if ∆H is -ve & ∆S is -ve & state whether the ∆G value is +ve or -ve

Answer 33

yes, feasible at lower temps
∆G -ve at lower temps

Question 34

state whether the reaction is feasible if ∆H is +ve & ∆S is +ve & state whether the ∆G value is +ve or -ve

Answer 34

yes, feasible at higher temps
∆G is -ve at higher temps

Question 35

how is the temp of a reaction that becomes just feasible calculated?

Answer 35

a reaction is just feasible when ∆G = 0 so the equation can be rearranged to T = ∆H / ∆S