thermodynamics

Question 1

define lattice enthalpy

Answer 1

the enthalpy change when 1 mole of a salt is formed from/ broken down into its gaseous ions
(basically the strength of the forces between the ions in an ionic lattice)

Question 2

is the lattice enthalpy of dissociation endothermic or exothermic?

Answer 2

endothermic

Question 3

is the lattice enthalpy of association endothermic or exothermic?

Answer 3

exothermic

Question 4

define lattice enthalpy of association

Answer 4

the enthalpy change when one mole of ionic lattice is formed from its gaseous ions under standard conditions

Question 5

define lattice enthalpy of dissociation

Answer 5

the enthalpy change when one mole of ionic lattice dissociates into its constituent gaseous ions under standard conditions

Question 6

2 factors affecting lattice enthalpy

Answer 6

1. charge on the ions- higher the charge, stronger the force
2. atomic radius (shielding): the smaller the radius the stronger the force
   (basically a high charge density causes the lattice enthalpy to be stronger)

Question 7

define enthalpy of formation

Answer 7

the enthalpy change where 1 mole of a compound is formed from its constituent elements in their standard states

Question 8

define ionisation energy

Answer 8

the enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous atoms

Question 9

enthalpy of atomisation

Answer 9

the enthalpy change when 1 mole of an element in its standard state is atomised to produce 1 mole of gaseous atoms

Question 10

mean bond enthalpy

Answer 10

energy required to break 1 mole of a specific covalent bond in a molecule averaged over a range of compounds

Question 11

electron affinity

Answer 11

the enthalpy change when an electron is added to an atom/ion in the gaseous state

Question 12

describe the order of a born-haber cycle

Answer 12

1. atomisation (metal)
2. atomisation (non-metal)
3. ionisation (metal)
4. electron affinity (non-metal)
5. lattice enthalpy
6. enthalpy of formation

Question 13

difference between calculating lattice enthalpy using born-haber cycles and the perfect ionic model?

Answer 13

born-haber cycles uses experimental values for ΔHf, ΔHa, ΔHie etc to calculate lattice enthalpy.
the perfect ionic model assumes:
-perfect ionic bonding between ions (no covalent character, just electrostatic forces)
-perfectly spherical ions
a theoretical value for lattice enthalpy can be calculated based on this.

Question 14

what increases covalent character?

Answer 14

-if a positively charged ion is smaller and more highly charged
- if a negatively charged ion is larger and more highly charged

Question 15

how does covalent character affect lattice enthalpy values

Answer 15

the greater the covalent character, the greater the difference between theoretical and experimental values for lattice enthalpy

Question 16

what is the enthalpy change of solution?

Answer 16

the enthalpy change when 1 mole of an ionic substance dissolves in water to give a solution of infinite dilution in standard conditions

Question 17

what is the equation for enthalpy of solution?

Answer 17

ΔH(sol) = ΔH(lattice dissociation) + Σ(ΔH(hydration))

Question 18

is lattice enthalpy of dissociation endo/exothermic and why?

Answer 18

endothermic (breaking electrostatic attractions required energy)

Question 19

is the enthalpy of hydration endo/exothermic and why?

Answer 19

exothermic- forming electrostatic attractions between the ions and water releases energy

Question 20

what is the enthalpy of hydration?

Answer 20

enthalpy change when 1 mole of gaseous ions forms aqueous ions

Question 21

what does it mean when the enthalpy of solution is positive/negative?

Answer 21

positive value: it’s less likely to be soluble
negative value:it’s more likely to be soluble

Question 22

what is entropy (∆S)

Answer 22

the measure of disorder in a chemical system

Question 23

what is entropy measured in

Answer 23

J.K^-1.mol^-1

Question 24

what 4 things increase the entropy?

Answer 24

solid–>liquid (melting)
liquid–>gas (boiling)
dissolving ((s)–>(aq))
increase in the number of moles present

Question 25

formula for calculating entropy change?

Answer 25

ΔS system = Σ S(products) -Σ S(reactants)

Question 26

what do you have to remember when calculating change in entropy?

Answer 26

to multiply the values for entropy by the number of moles
eg N2 + 3H2 ⇌ 2NH3
ΔS system = 2(193)-[192-3(131)]

Question 27

what does the change in entropy value of a system mean?

Answer 27

the higher/more positive the entropy change, the more likely the reaction is to occur (it’s more feasible)

Question 28

what are the units for gibbs free energy?

Answer 28

ΔG = Jmol^-1

Question 29

what does ΔG indicate?

Answer 29

for a reaction to be feasible, ΔG must be negative/below 0

Question 30

how do you calculate ΔG?

Answer 30

ΔG = ΔH - TΔS
T in K
ΔS in JK^-1mol^-1

Question 31

if ΔH is negative and ΔS is positive, will the reaction be feasible?

Answer 31

yes, always
the ΔG will always be negative

Question 32

if ΔH is positive and ΔS is negative, will the reaction be feasible?

Answer 32

no,never
the ΔG will always be positive

Question 33

if ΔH is negative and ΔS is positive, will the reaction be feasible?

Answer 33

depends on temperature

Question 34

if ΔH is negative and ΔS is positive, will the reaction be feasible?

Answer 34

depends on temperature

Question 35

why may a reaction still not occur even if it is feasible?

Answer 35

it may have a high activation energy which acts as a barrier to the reaction

Question 36

assuming ΔG is 0, how will you calculate the temperature at which a reaction is feasible?

Answer 36

T=ΔH/ΔS

Question 37

how do you calculate the range over which the reaction is feasible

Answer 37

assume ΔG=0
so T= ΔH/ ΔS
range is ≤T