Entropy

Question 1

Why do enthalpy changes alone not control whether reactions occur?

Answer 1

Some endothermic reactions can occur at room temperature

Question 2

What is entropy?

Answer 2

A measure of the amount of disorder in a system, or the number of ways particles and their energy can be arranged

Question 3

What is the symbol for entropy?

Answer 3

S

Question 4

Which state has the highest entropy?

Answer 4

Gases - the system has more possible arrangements than solids and liquids

Question 5

What are the units of entropy?

Answer 5

J mol^-1K^-1

Question 6

What does the natural direction of change tend towards and what does this mean for spontaneous processes?

Answer 6

Greater disorder, so spontaneous processes involve an increase in total entropy

Question 7

How do changes of state affect entropy?

Answer 7

Going from solid to liquid or gas increases entropy ( particles move more freely so more possible arrangements)

Question 8

How do dissolving ionic solids affect entropy?

Answer 8

Dissolving a crystalline solid into solution increases disorder

Question 9

How does an increase in the formation of gas particles affect entropy?

Answer 9

Reactions where number of gaseous molecules increase show big jump in entropy

Question 10

What is the overall entropy change of a reaction made up of?

Answer 10

• Entropy change from the reaction itself
• Entropy change of the surroundings

Question 11

What is the entropy change of surroundings?

Answer 11

The entropy change due to heat energy exchanged with surroundings

Question 12

How do you calculate the total entropy change of a reaction or process?

Answer 12

Entropy change of system + Entropy change of the surroundings

Question 13

What does it mean if the total entropy change of a reaction or process is positive?

Answer 13

It is thermodynamically feasible (can happen)

Question 14

What does every substance have in a given state?

Answer 14

A standard entropy value

Question 15

What is the equation for the standard entropy change?

Answer 15

(sum of all standard entropies of products) - (sum of all standard entropies of reactants)

Question 16

What happens when a chemical reaction releases or absorbs heat?

Answer 16

It changes the entropy of the surroundings

Question 17

What results in a bigger entropy change in the surroundings?

Answer 17

The greater the heat transfer and the lower the temperature

Question 18

How do we calculate the entropy change of the surroundings?

Answer 18

-(change in energy)/Temperature

change in energy must be J mol^-1

Question 19

What does a large exothermic enthalpy change mean for the entropy change of the surroundings?

Answer 19

Leads to a positive entropy change of surroundings - making the reaction more likely to be feasible

Question 20

What is the equation for Gibbs free energy?

Answer 20

Enthalpy change - (Temperature x entropy change)

Question 21

What does it mean if the Gibbs free energy value is negative?

Answer 21

It is feasible

Question 22

Will a reaction always occur if the Gibbs free energy value is negative?

Answer 22

No as the activation energy may be too high

Question 23

What do we do initially in order to find the minimum temperature when a reaction is feasible?

Answer 23

Set Gibbs free energy = 0

Question 24

What is the equation to find the minimum temperature when a reaction is feasible?

Answer 24

Enthalpy change/Entropy change

Question 25

What does the value of the minimum temperature when a reaction is feasible tell us about the reaction?

Answer 25

- If T is high, reaction only feasible at high temperatures - If T is low, reaction only feasible at lower temperatures

Question 26

What is the direct energy between Gibbs free energy and equilibrium position?

Answer 26

Gibbs free energy = -RTlnK R = 8.31 J/molK K = equilibrium constant

Question 27

What does the direct relationship between gibbs free energy and equilibrium position tell us about the reaction?

Answer 27

- If Gibbs very negative, K is large and reaction lies to right - If Gibbs positive, K is small and reaction lies to left

Question 28

Why may some feasible reactions not occur?

Answer 28

Due to a high activation energy barrier

Question 29

Why may a high activation energy prevent a feasible reaction from occurring?

Answer 29

Rate of reaction may be very slow so the reaction doesn't happen fast enough to observe

Question 30

What is kinetic feasibility?

Answer 30

Whether the reaction rate is fast enough to observe