lattice enthalpy and entropy

Question 1

lattice enthalpy

Answer 1

the enthalpy change when one mole of an ionic compound is formed from its ions in their gaseous state
eg. K+ (g) + Cl- (g) -> KCl (s)

Question 2

significance of the value for lattice enthalpy

Answer 2

the more exothermic the value, the stronger the attraction between the oppositely charged ions (which means a stronger ionic bond)

Question 3

factors affecting lattice enthalpy

Answer 3

ionic radius - smaller ions attract more strongly than larger ions
ionic charge - ions of greater charge attract more strongly than ions of smaller charge

Question 4

enthalpy of atomisation

Answer 4

ΔatH is the enthalpy change when one mole of gaseous atoms is formed from its element

Question 5

first electron affinity

Answer 5

ΔEA1H is the enthalpy change when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions

Question 6

second electron affinity

Answer 6

ΔEA2H is the enthalpy change when one electron is added to each ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions
(endothermic because energy is needed to overcome repulsion between negative electron and negative ion)

Question 7

enthalpy of solution

Answer 7

ΔsolH is the enthalpy change when one mole of compound (solute) is completely dissolved in water under standard conditions

Question 8

enthalpy of hydration

Answer 8

ΔhydH is the enthalpy change when
one mole of isolated gaseous ions is dissolved in water forming one mole of aqueous ions

Question 9

factors affecting enthalpy of hydration

Answer 9

ionic radius - smaller ions attract and bond with H2O molecules more strongly than larger ions
ionic charge - higher charged ions attract and bond with H2O molecules more strongly than lower charged ions

Question 10

entropy, S

Answer 10

a measure of the dispersal of energy in a system
the entropy is greater when the system is more disordered

Question 11

significance of entropy change values

Answer 11

positive values represent a system becoming more disordered, eg liquid to gas
negative values represent a system becoming less disordered, eg liquid to solid

Question 12

entropy change of reaction

Answer 12

ΔSθ is the entropy change that accompanies a reaction in the molar quantities expressed in a chemical equation
ΔSθ = ΔS(products) - ΔS(reactants)

Question 13

when is a reaction feasible?

Answer 13

a process is spontaneous (is feasible) if a chemical system becomes more stable and its overall energy decreases

Question 14

gibb’s free energy equation

Answer 14

ΔG = ΔH - TΔS
ΔH is enthalpy change in kJmol-1
T is temperature in K
ΔS is entropy change in kJK-1mol-1

Question 15

feasibility using ΔG

Answer 15

ΔG < 0 reaction is feasible
ΔG > 0 reaction is not feasible

Question 16

limitations of ΔG

Answer 16

not considered the reaction kinetics (rate of reaction)
so the reaction rate may be so slow that a reaction is not observed