Topic 13 - ENERGETICS II

Question 1

Definition of lattice energy

Answer 1

The energy released when 1 mole of an ionic compound is formed from gaseous ions

Question 2

Is lattice energy endo/exothermic?

Answer 2

Always EXOthermic (-ve)

Question 3

What is the approximate range for lattice energy and what does it signify?

Answer 3

-500 -> -4000
- (-500) shows weaker ionic bonding
- (-4000) shows stronger ionic bonding

Question 4

Compare the theoretical vs experimental LE for CsF if:
experimental = -739 kJmol-1
theoretical = -737 kJmol-1

Answer 4

• Theoretical and experimental very similar
• Very little covalent character in the bonding
• Because Cs+ polarises F- very little

Question 5

What is the definition of enthalpy of atomisation in formulae and give its sign?

Answer 5

Element -> 1 mol atoms (g)
Endothermic (+ve)

Question 6

What is electron affinity?

Answer 6

1 atom (g) + e- -> ion- (g)

Question 7

What does it mean if the standard enthalpy of formation of a substance is exothermic?

Answer 7

• Lots of energy is released
• Therefore MgCl2 is stable

Question 8

What is the definition of standard enthalpy of hydration?

Answer 8

The enthalpy change when 1 mole of gaseous ions are hydrated by excess water

Question 9

What is the definition of standard enthalpy of solution?

Answer 9

The enthalpy change when 1 mole of ionic solid dissolves in excess water

Question 10

How can we use standard enthalpy of solution to predict solubility?

Answer 10

• If the result is exothermic, the substance is probably soluble
  (Only insoluble if highly endothermic)

Question 11

What is entropy?

Answer 11

Entropy (s) is a measure of the amount of disorder

Question 12

What conditions have an effect on entropy?

Answer 12

• No. of moles
• Temperature increase
• State

Question 13

What is standard entropy change of the system?

Answer 13

Total enthalpy change of products - total enthalpy change of reactants

Question 14

What is standard entropy change of the surroundings?

Answer 14

-△H/T

Question 15

What is △S total?

Answer 15

△S system + △S surroundings

Question 16

Why does △S total matter?

Answer 16

According to the 2nd law of thermodynamics, a reaction is only feasbile if △S total > 0

Question 17

Why might △S total be negative?

Answer 17

• Although △S system is positive, △S surrounding is more negative
• Therefore △system is not positive enough to outweigh the negative △S surrounding

Question 18

What happens when T increases?

Answer 18

• As △H is positive, △S system is positive
• As T increases, △S surroundings becomes less negative
• △S total becomes less negative

Question 19

What is the definition of kinetic stability?

Answer 19

• When a feasible reaction doesn’t happen
• Because the energy (at that temp) is not enough to overcome the activation energy

Question 20

What is thermodynamic stability?

Answer 20

When something just isn’t feasibly because △S total < 0

Question 21

What is △G and what does it show?

Answer 21

△G = △H - T△S system
- Reaction is feasible if △G < 0

Question 22

What is the relation between △G and Kc?

Answer 22

△G = -RT ln(Kc)
or
Kc = e^(-△G/RT)