Ch 22 Enthalpy And Entropy

Question 1

Why are solid ionic compounds very stable

Answer 1

Strength of ionic bonds, electrostatic attractions creates a substantial energy barrier that must be overcome

Question 2

The Born-Haber cycle

Answer 2

Lattice enthalpy (ionic bond formation from separate gaseous ions) can’t be measured directly so this cycle used 
See diagram 1 
Route 2 converts elements directly to ionic lattice, this is enthalpy change of formation and is exothermic

Question 3

In its solid lattice a compound will always be…

Answer 3

Ionic

Question 4

Standard enthalpy change of atomisation

Answer 4

See diagram 2

It’s always endothermic because bonds broken to form gaseous atoms

Question 5

Born Haber cycle examples

Answer 5

See diagram 3 and 4

Question 6

Successive electron affinities

Answer 6

See diagram 5

Question 7

Enthalpy of solution is exo or endo

Answer 7

Can be either

Question 8

In q=mcT mass is

Answer 8

Mass of solution not mass of water

Question 9

Determination of enthalpy change of solution

Answer 9

See diagram 6

Question 10

Enthalpy change of solution diagram

Answer 10

See diagram 7

Question 11

Enthalpy change of solution examples

Answer 11

See diagram 8. And 9

Question 12

Properties of ionic compounds

Answer 12

High melting and boiling points
Soluble in polar solvents
Conduct electricity when molten or in aqueous solution

Question 13

Indication of m.p. Of ionic compound

Answer 13

Magnitude of lattice energy

Question 14

Effect of ionic charge and size across period 3

Answer 14

See diagram 10

Question 15

To predict solubility

Answer 15

To dissolve ionic compound in water quantity of energy must equal lattice enthalpy
H2O molecules attracted to ions surrounding them and releasing energy equal to hydration enthalpy
If sum of hydration enthalpies is larger then magnitude of lattice enthalpy overall enthalpy change will be exothermic and compound should dissolve but solubility also depends on temp and entropy

Question 16

Effect of entropy on dispersal of energy

Answer 16

Greater entropy greater dispersal of energy and greater disorder

Question 17

At 0k

Answer 17

No energy and all substances would have an entropy val of zero
Above this temp energy becomes dispersed amongst particles and substances have positive entropy

Question 18

If a System changes to become more random

Answer 18

Energy spreads out more and entropy change will be more positive

Question 19

Production of a gas

Answer 19

Increases disorder of particles
Energy spread out more
Entropy change positive

Question 20

Decrease in number of gas molecules

Answer 20

Decrease in randomness of particles
Energy spread out less
Entropy change is negative

Question 21

Standard entropies

Answer 21

Units JK^-1 mol^-1

Always positive

Question 22

Free energy change

Answer 22

Change in energy during reaction

Question 23

In Gibbs equation all units must be changed to

Answer 23

kJK^-1mol^-1

Question 24

At room temp delta H

Answer 24

Much larger than TDeltaS