5ii. Energetics HL

Question 1

What is the first ionisation energy?

Answer 1

The enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous atoms to from 1 mole of gaseous ions with a +1 charge.
X(g)–>X^+(g)+e^- +∆H

Question 2

What is the first electron affinity?

Answer 2

The enthalpy change when 1 mole of electron is gained by 1 mole of gaseous atoms to form 1 mole of gaseous ions with a -1 charge.
X(g)+e^—>X^-(g) -∆H

Question 3

What is a Born-Haber cycle?

Answer 3

Question 4

What do the arrows represent in the Born-Haber cycle?

Answer 4

up arrow is endothermic, down arrow is exothermic

Question 5

Draw a Born-Haber cycle and calculate the lattice enthalpy for potassium iodide, KI using the following values:
∆Hat of K= +90 kJ/mol
∆Hat of I2 = +107 kJ/mol
∆Hf of KI = -328 kJ/mol
1st IE of K =+418 kJ/mol
1st EA of I =-314 kJ/mol

Answer 5

Question 6

What is lattice enthalpy?

Answer 6

Lattice enthalpy represents the enthalpy change when the ions in one mole of a solid ionic compound are
broken apart (lattice enthalpy of dissociation) or brought together (lattice enthalpy of formation).

Question 7

What is lattice enthalpy an indication of?

Answer 7

The lattice enthalpy of a compound is an indication of the strength of the ionic bonding – the greater the
magnitude of the lattice enthalpy, the stronger the bonding. Generally speaking, compounds with smaller ions and/or ions with higher charges have stronger attractions and so greater lattice enthalpy.

Question 8

How to you measure/find lattice enthalpy?

Answer 8

Using a Born-Haber cycle

Question 9

What is Entropy, S?

Answer 9

Entropy, S, is a measure of disorder. More order= solid therefore entropy is lower, gas=less ordered therefore higher entropy

Question 10

What would the entropy be like in an exothermic reaction?

Answer 10

E is transferred to surr, E of surr increases, T of surr increases, Entropy, S, of surr increases

Question 11

What would the entropy be like in an endothermic reaction?

Answer 11

E is absorbed from surr, E of surr decreases, T of surr decreases, Entropy, S, of surr decreases

Question 12

What would this equations entropy be?

Answer 12

Positive entropy is more likely to happen, things want to become disordered.

Question 13

What do the words system and surrounding relate to?

Answer 13

system=reaction
surroundings=everything else, not reaction

Question 14

What would this equations entropy be?

Answer 14

Less likely to happen because the negative change in entropy

Question 15

What would this equations entropy be?

Answer 15

Probably positive change in entropy because solid to no solid. However, this reaction is only thermodynamically(heat) feasible but is kinetically(moving) not feasible(4 carbon bonds so can’t move, requires large amounts of energy to overcome)

Question 16

What are the units for:

Answer 16

Question 17

What is the equation for:

Answer 17

Question 18

When is entropy system feasible?

Answer 18

Question 19

What is the equation for:

Answer 19

Question 20

What is the equation for:

Answer 20

Question 21

How does entropy vary with temperature and state?

Answer 21

• entropy increases with temperature
• there are big increases in entropy on state changes (melting and boiling)
• the entropy increases from liquid to gas is greater than that of solid to liquid due to the large amount of disorder in gases compared to solids to liquids.

Question 22

What happens to entropy change of a system when entropy increases?

Answer 22

Question 23

What happens to entropy change of a system when entropy decreases?

Answer 23

Question 24

What is Gibbs free energy change?

Answer 24

Gibbs free energy change (∆G) combines the two thermodynamic factors of enthalpy change (∆H) and
entropy change (∆S).

Question 25

What is the formula for Gibbs free energy change (∆G) ?

Answer 25

Question 26

What units does each part of Gibbs free energy change (∆G) formula?

Answer 26

∆G=kJ/mol
∆Hr=kJ/mol
T= K
∆Ssys= J/mol/K (need to convert to kJ!)

Question 27

What must ∆G equal to for it to be feasible?

Answer 27

∆G must be negative for a reaction to be feasible, ∆G</=0

Question 28

How does decreasing enthalpy affect ∆G?

Answer 28

A decrease in enthalpy (i.e. ∆H being negative, i.e. an exothermic reaction) is more favourable in a reaction.

Question 29

How does increasing entropy affect ∆G?

Answer 29

An increase in entropy (i.e. ∆S being positive) is more favourable in a reaction.

Question 30

How does temperature affect feasibility of ∆G?

Answer 30

Question 31

How do you calculate temperature from Gibbs free energy?

Answer 31

Question 32

How is changes of state related to Gibbs free energy?

Answer 32

 Changes of state are controlled in the same way.
 Below the melting point of a substance, melting is not feasible as ∆G is positive, but at the melting point it
∆G = 0 and so melting becomes feasible and the substance melts.
 In a similar way, below the boiling point of a substance, boiling is not feasible as ∆G is positive, but at the
boiling point it ∆G = 0 and so boiling becomes feasible and the substance boils.

Question 33

How does equilibrium constants and Gibbs free energy relate?

Answer 33

The position of equilibrium corresponds to the reaction mixture with the maximum value of entropy (of the universe) and the minimum value of Gibbs free energy (of the system).

Question 34

What is the formula with Gibbs free energy and equilibrium constant?

Answer 34

Question 35

What is the graph that relates to Gibbs free energy and equilibrium?

Answer 35

equilibrium position=lowest point/bottom of the curve
∆G for reaction = difference between highest point on both side/axes

Question 36

What units is this formula:

Answer 36

∆G= kJ/mol
R= J/K/mol
T=K
R=8.31 gas constant

Question 37

What is the relationship between Gibbs free energy and equilibrium?

Answer 37

Question 38

Example question Gibbs free energy and equilibrium:

Answer 38