Reaction feasibility

Question 1

What is the standard enthalpy of formation?

Answer 1

The enthalpy change when 1 mole of a substance is formed from its elements in their standard states.

Question 2

What is the standard state of a substance?

Answer 2

It’s most stable state at a pressure of 1 atmosphere and at a specified temperature, usually 298K

Question 3

What is the entropy of a system?

Answer 3

A measure of the degree of disorder of the system.

Question 4

The higher the degree of disorder, …

Answer 4

The greater the entropy.

Question 5

Solids have a —— disorder and gases have a —— disorder

Answer 5

low, high

Question 6

How does temperature affect entropy?

Answer 6

Entropy increases as temperature increases.

Question 7

Describe the entropy at the melting point and boiling point of a substance?

Answer 7

Rapid increase in entropy at the melting point of a substance and an even more rapid and large entropy change at boiling point.

Question 8

Entropy (S) units?

Answer 8

J K-1 mol-1

Question 9

What does the 2nd law of thermodynamics state?

Answer 9

The total entropy of a reaction system and its surroundings always increases for a spontaneous process.

Question 10

Heat energy released by the reaction system into the surroundings does what?

Answer 10

Increases the entropy of surroundings.

Question 11

Heat energy absorbed by the reaction system from the surroundings does what?

Answer 11

Decreases the entropy of the surroundings.

Question 12

What does the 3rd law of thermodynamics state?

Answer 12

The entropy of a perfect crystal at 0K is 0.

Question 13

What is the standard entropy of a substance ?

Answer 13

The entropy value for the substance in its standard state.

Question 14

If the change in free energy (ΔG°) between reactants and products is negative…

Answer 14

a reaction may occur and the reaction is said to be feasible

Question 15

What is a feasible reaction?

Answer 15

One that tends towards the products rather than the reactants.

Question 16

Does feasibility indicate the rate of reaction?

Answer 16

No

Question 17

If a reaction is feasible you want?

Answer 17

Reaction to be exothermic (ΔH° negative)
Entropy to increase (ΔS° positive)

Question 18

Under standard conditions any reaction is feasible if ΔG° is ——

Answer 18

Negative.

Question 19

At equillibrium ΔG°= 0 and the reaction is…

Answer 19

Just beginning to become feasible.

Question 20

A reversible reaction will proceed spontaneously until the composition is reached where ΔG° is

Answer 20

= 0

Question 21

The sign of ΔG° depends on?

Answer 21

the positive or negative nature of ΔH° and ΔS°.

Question 22

ΔG° is always negative when?

Answer 22

ΔH° is negative and TΔS° is positive.

Question 23

ΔG° is always positive when?

Answer 23

ΔH° is positive and TΔS° is negative .

Question 24

ΔG° is negative at low temp and positive at high temp when?

Answer 24

ΔH° is negative and TΔS° is negative .

Question 25

ΔG° is positive at low temp and negative at high temp when?

Answer 25

ΔH° is positive and TΔS° is positive .

Question 26

ΔG° for a reaction can give info about?

Answer 26

The equillibrium position in a reversible reaction and the value of K.

Question 27

If ΔG°< 0?

Answer 27

The forward reaction will be feasible and therefore the products will be favoured over the reactants. The equilibrium position will lie to the RHS and K>1.

Question 28

If ΔG° > 0?

Answer 28

The backwards reaction will be feasible and therefore the reactants will be favoured over the products. The equilibrium position will lie to the LHS and K < 1.

Question 29

Suggest why the entropy change for H20 (L) to H20 (g) is greater than the entropy change for CCl4 (L) to CCl4 (g)

Answer 29

H20 has very strong hydrogen bonds between its molecules. CCl4 only has weak LDFs. When H20 evaporates it goes from a highly structured liquid to a much more disordered gas, so the increase in entropy is large. CCl4, already being less structured in the liquid doesn’t have as big a difference in disorder when it becomes a gas.