3.1.8 Thermodynamics

Question 1

define enthalpy of formation?

Answer 1

this is the enthalpy change when 1 mole of a compound is formed form its constituent elements (EXOTHERMIC)

Question 2

what is enthalpy of atomisation?

Answer 2

the enthalpy change when 1 mole of a gaseous atom is formed from its elements

Question 3

what is meant by ‘1st ionisation energy’?

Answer 3

the energy needed to remove 1 mole of an electron from 1 mole of a gaseous atom to form 1 mole of a gaseous 1+ ion (ENDOTHERMIC)

Question 4

what is meant by ‘1st electron affinity’? (EA1)

Answer 4

this is the energy released when 1 mole of an electron is added to 1 mole of a gaseous atom to form 1 mole of a gaseous 1- ion (EXOTHERMIC)

Question 5

what is meant by ‘lattice enthalpy of formation?

Answer 5

this is the enthalpy change when 1 mole of an ionic compound is formed from its constituent gaseous ions (EXOTHERMIC)

Question 6

what is meant by ‘enthalpy of solution’?

Answer 6

this is the enthalpy change when 1 mole of a compound is dissolved in a large volume of water to form an infinitely dilute solution

Question 7

what is meant by ‘lattice dissociation enthalpy’? (LDE)

Answer 7

energy needed to break 1 mole of an ionic compound to its constituent gaseous ions.

e.g. NaCl(s) -> Na+(g) + Cl-(g)

Question 8

what is meant by ‘enthalpy of hydration’?

Answer 8

enthalpy change when 1 mole of a gaseous ion is attracted to water molecules to form 1 mole of an aqueous ion

Question 9

what is the formula for enthalpy of solution?

Answer 9

ΔHsol = LDE + [(ΔHhyd of cation) + (ΔHof anion)]

Question 10

what is meant by ‘entropy’?

Answer 10

this is the level of disorder

Question 11

what has the lowest entropy and why?

Answer 11

Solids have the lowest entropy because:
- they have the lowest level of disorder
- due to very few ways of rearranging the particles

Question 12

what has the highest entropy and why?

Answer 12

Gases have the highest entropy because:
- they have the highest level of disorder
- as there are many more ways of rearranging the particles

Question 13

CaCO3(s) -> CaO(s) + CO2(g)
has entropy decreased or increased?

Answer 13

entropy has increased because a gas has been formed from a solid

Question 14

N2(g) + 3H2(g) -> 2NH3(g)
has entropy increased or decreased?

Answer 14

entropy has decreased because 2 moles of gas is formed from 4 moles of gas

Question 15

what is the formula for entropy change?

Answer 15

ΔS = ΣS[products]-ΣS[reactants]

Question 16

C3H8(g) -> SO2(g) -> 3CO2(g) + 4H2O(l)
has entropy increased or decreased?

Answer 16

Entropy has decreased because a liquid is formed from a gas

Question 17

how to calculate ΔH using ΔHf?

Answer 17

ΔH = ΣΔHf[products]-ΣΔHf[reactants]

Question 18

how to calculate ΔH using ΔHc?

Answer 18

ΔH=ΣΔHc[reactants]-ΣΔHc[products]

Question 19

what is the formula for Gibb’s free energy?

Answer 19

ΔG=ΔH-TΔS

Question 20

what are the units in the formula for Gibb’s free energy?

Answer 20

• ΔG - KJ/mol
• ΔH - KJ/mol
• T - K
• ΔS - J/K/mol

Question 21

what needs to happen in order for a reaction to be feasible, according to Gibb’s free energy?

Answer 21

ΔG≤0

Question 23

what are the best possible conditions for a reaction to be feasible according to Gibb’s free energy?

Answer 23

ΔH = -ve (EXOTHERMIC)
ΔS = +ve (HIGH ENTROPY)

Question 24

how does the size of an ion affect lattice enthalpy?

Answer 24

• the smaller the size of the ion, the greater the charge density
• stronger attraction to the oppositely charged ion -> LE INCREASES

Question 25

what happens to lattice enthalpy as you go down the group?

Answer 25

• charge of the ion is the same but ion size increases -> charge density FALLS
• so weaker attraction to oppositely charged ions -> LE DECREASES.

Question 26

what happens to lattice enthalpy as you go across the period (groups 1-3)?

Answer 26

• across the period (e.g Na+, Mg2+, Al3+), ion size decreases, charge increases, so charge density is GREATER.
• stronger attraction to oppositely charged ion -> LE INCREASES

Question 27

what happens to lattice enthalpy as you go across the period (groups 5-7)?

Answer 27

• size of the ion decreases, leading to a greater charge density, but charge falls, so charge density is LOWER.

Question 28

how do you compare experimental and theoretical lattice enthalpies?

Answer 28

• theoretical LE = assumes perfect ionic spheres; doesn’t account for covalent character within the ionic compound

Question 29

why do all ionic compounds have covalent character?

Answer 29

• positive ion polarises the negative ion
• this changes/distorts its shape, which induces covalent character