22 - Enthalpy and entropy

Question 1

What is lattice enthalpy?

Answer 1

enthalpy change when one mole of an ionic compound is formed from its gaseous ions under standard conditions
K+(g) + CL-(g) -> KCl(s)

Question 2

is lattice enthalpy endothermic or exothermic?

Answer 2

it involves bond formation from separate gaseous ions, so it is exothermic, and the value of enthalpy change is always negative

Question 3

What is a Born-Haber cycle?

Answer 3

the indirect determination of lattice enthalpy
-> lattice enthalpy cant be directly measured

Question 4

Routes of a Born-Haber cycle

Answer 4

Route 1
-> formation of gaseous atoms (Endothermic)
-> formation of gaseous ions (Endothermic)
-> lattice formation (Exothermic)

Route 2
-> converts elements in their standard states directly into the ionic lattice, the enthalpy change of formation (Exothermic)

Question 5

standard enthalpy change of formation ΔfH°

Answer 5

enthalpy change when one mole of a compound is formed from its elements under standard conditions

Na(g) + 0.5Cl2(g) -> Nacl(s) =-711klmol-1

Question 6

standard enthalpy change of atomisation ΔatH°

Answer 6

enthalpy change when one mole of gaseous atoms is formed from its elements under standard conditions

Na(s) -> Na (g) = +107 kjmol-1
0.5Cl2 -> Cl(g) = + 121 kjmol-1

Question 7

is ΔatH° endothermic or exothermic?

Answer 7

endothermic as bonds are broken to form gaseuos atoms.

for a gas, ΔatH° refers to the enthalpy of the bond being broken
Cl2(g) -> 2Cl(g)

Question 8

First ionisation energy ΔieH°

Answer 8

enthalpy change required to remove one electron from each atom in one mole of gaseous atom, to form one mole of gaseous 1+ ions, under standard conditions and states

Na(g) -> Na+(g) + e-

Question 9

Are ΔieH° endothermic or exothermic?

Answer 9

Endothermic as energy is required to overcome the attraction between the negative electron and the positive nucleus

Question 10

First electron affinity

Question 11

what does feasibility mean

Answer 11

• describes if a reaction can happen and is energetically feasible

Question 12

what is free energy

Answer 12

the overall change in energy during a chemical reaction
made up of the enthalpy change ΔH (heat transfer between surroundings and chemical surroundings) and the entropy change at the temperature TΔS

Question 13

gibbs equation
- units of each value

Answer 13

ΔG = ΔH - TΔS
T= kelvin (+273)
ΔH= kJmol-1
ΔS= kJK-1mol-1

Question 14

when is a reaction feasible

Answer 14

ΔG < 0

Question 15

limitations of predictions made for feasibility

Answer 15

a very large activation energy results in a slow rate. so it may seem feasible but ΔG doesnt cond=sider the kinetics or rate of reaction