Thermodynamics

Question 1

define enthalpy change of formation

Answer 1

enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

Question 2

define lattice enthalpy of dissociation

Answer 2

enthalpy change when 1 mole of a solid ionic compound is dissociated into its gaseous ions under standard conditions

Question 3

define lattice enthalpy of association

Answer 3

enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions

Question 4

define bond dissosciation enthalpy

Answer 4

the enthalpy change when 1 mole of covalent bonds is broken into 2 gaseous atoms (or free radicals)

Question 5

define enthalpy change of atomisation

Answer 5

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

Question 6

define the enthalpy change of 1st ionisation

Answer 6

enthalpy change when 1 mole of electrons are removed from 1 mole of atoms in their gaseous states to produce 1 mole of 1+ ions also in their gaseous states

Question 7

define the enthalpy change of 2nd ionisation

Answer 7

enthalpy change when 1 mole of electrons are removed from 1 mole of 1+ ions in their gaseous states to produce 1 mole of 2+ ions also in their gaseous states

Question 8

define 1st electron affinity

Answer 8

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

Question 9

define 2nd electron affinity

Answer 9

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? why?

Answer 10

• used to calculate lattice enthalpies
• can’t calculate from experiments

Question 11

what is the formula to calculate lattice enthalpy of dissociation?

Answer 11

-ΔH°f + ΔH°at + In + Ea

Question 12

describe the perfectly ionic model

Answer 12

• ions that are perfectly spherical
• charge is evenly distributed in this sphere (point charges)

Question 13

what is the perfectly ionic model used for?

Answer 13

calculating theoretical lattice enthalpies

Question 14

what does it mean if the experimental lattice enthalpy of a compound is different to the theoretical lattice enthalpy?

Answer 14

• compound doesn’t follow the perfectly ionic model so has some covalent characteristics

Question 15

why might a compound not follow the perfectly ionic model?

Answer 15

• positive ion polarises the negative ion (distorts the charge distribution in the negative ion)
• more polarisation = more covalent character

Question 16

what is entropy a measure of?

Answer 16

disorder in a system

Question 17

what symbol represents entropy?

Answer 17

S

Question 18

what is entropy?

Answer 18

number of ways energy can be shared between particles

Question 19

state the link between disorder and entropy

Answer 19

more disorder = higher level of entropy

Question 20

list 4 factors that increase entropy

Answer 20

• inc number of molecules
• inc temperature
• inc randomness of molecule (s -> l -> g)
• inc complexity of molecule

Question 21

what does enthalpically unfavourable mean?

Answer 21

endothermic

Question 22

will a reaction tend to more or less disorder? what effect does this have on entropy?

Answer 22

more disorder + therefore inc entropy

Question 23

give an equation for measuring entropy change

Answer 23

entropy change = entropy of products - entropy of reactants
ΔS = S_products - S_reactants

Question 24

what is entropy change measured in?

Answer 24

J K^-1mol^-1

Question 25

what does it mean if the entropy of a reaction is positive?

Answer 25

the reaction is entropically feasible

Question 26

what is the symbol for Gibbs Free Energy?

Answer 26

ΔG

Question 27

what does ΔG tell us?

Answer 27

whether or not a reaction is feasible

Question 28

give an equation to calculate Gibbs Free Energy

Answer 28

ΔG = ΔH - TΔS gibbs free energy (J mol^-1) = enthalpy change (J mol^-1) - temp (K) x entropy change (J K^-1 mol^-1)

Question 29

what equation is used to calculate the temperature at which a reaction becomes feasible?

Answer 29

T = ΔH / ΔS - make sure ΔH is in J mol^-1

Question 30

when is a reaction considered feasible in terms of ΔG?

Answer 30

reaction feasible if ΔG is negative or zero

Question 31

why may a reaction not be observed to be occuring, despite it being calculated to be feasible?

Answer 31

- activation energy may be too high - rate of reaction may be very slow

Question 32

if ΔH value is negative and ΔS value is positive, what is the ΔG value? at what temp is the reaction considered feasible?

Answer 32

- ΔG always negative - feasible at any temp

Question 33

if ΔH value is positive and ΔS value is negative, what is the ΔG value? at what temp is the reaction considered feasible?

Answer 33

- ΔG always positive - never feasible at any temp

Question 34

if ΔH value is negative and ΔS value is negative, what is the ΔG value? at what temp is the reaction considered feasible?

Answer 34

- ΔG negative at lower temps - feasible at lower temps

Question 35

if ΔH value is positive and ΔS value is positive, what is the ΔG value? at what temp is the reaction considered feasible?

Answer 35

- ΔG negative at higher temps - feasible at higher temps

Question 36

define enthalpy change of solution

Answer 36

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

Question 37

what conditions need to be met for a substance to dissolve?

Answer 37

- substance bonds must break (endothermic) - new bonds form between solvent and substance (exothermic) - new bonds formed must have same strength or greater than those broken

Question 38

describe what happens when an ionic lattice in solid form is dissolved in water

Answer 38

- substance bonds broken to create free moving ions - partially pos H is attracted to negative ions and partially neg oxygen is attracted to positive ions - structure starts to break down - water molecules surround the ions (hydration) - bonds formed between ions and water. the ions are hydrated

Question 39

are soluble substances more likely to have exothermic or endothermic enthalpies of solution? why?

Answer 39

- exothermic - new bonds formed must have same strength or greater than those broken

Question 40

give a formula to calculate enthalpy change of solution

Answer 40

LE_diss + enthalpy change of hydration

Question 41

define enthalpy change of hydration

Answer 41

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