Chapter 22: Entropy

Question 1

What is entropy? Include its symbol and units.

Answer 1

• A measure of the dispersal of energy in a system, that is greater the more disordered a system is
• S and J K^-1 mol^-1

Question 2

How does entropy change across the states of matter, and why?

Answer 2

• It increases from solid to liquid to gas
• In solids, particles vibrate around a spot
• In liquids, particles move over each other
• Liquids are therefore more disordered than solids
• In gases, particles move freely and faster, giving them the highest disorder

Question 3

How can you tell if entropy increases or decreases in a reaction? Why?

Answer 3

• If the number of gaseous molecules increase, there is an increase in entropy
• The reverse is also true
• Producing gas increases the disorder of the particles, and energy is spread out more

Question 4

What is used to calculate the entropy change of a reaction? What is its definition?

Answer 4

-S⦵: standard entropy
- The entropy of one mole of a substance under standard conditions

Question 5

What is special about values of standard entropy?

Answer 5

• They’re always positive
• Entropy at 0K is always 0, so entropy increases from 0K to 298K

Question 6

How is the entropy change of a reaction calculated?

Answer 6

• ΔS⦵ = ΣS⦵(products) - ΣS⦵(reactants)

Question 7

What is the term feasibility used to describe?

Answer 7

• Whether a reaction can take place

Question 8

What 2 factors decide the feasibility of a reaction?

Answer 8

• The enthalpy change
• The entropy change at the temperature of the reaction

Question 9

What equation can be used to calculate whether a reaction is feasible? Include units.

Answer 9

• The Gibbs’ equation
• ΔG = ΔH - TΔS
• ΔG in kJ mol^-1
• ΔH in kJ mol^-1
• T in K
• S in kJ K^-1 mol^-1

Question 10

What is ΔG?

Answer 10

• The free energy change
• An energy change that takes into account both the entropy change and the enthalpy change

Question 11

What has to be done before free energy can be calculated?

Answer 11

• The entropy change has to be changed from J K^-1 mol^-1 to kJ K^-1 mol^-1 (by dividing by 1000)

Question 12

How does the value of the free energy change tell you if a reaction is feasible?

Answer 12

• For a reaction to be feasible, ΔG must be negative

Question 13

How can you find the minimum temperature required for a reaction to be feasible?

Answer 13

• The Gibbs’ equation is equated to 0

Question 14

What limitation is there to using values of ΔG to determine whether a reaction takes place?

Answer 14

• Even if ΔG < 0, the reaction may not take place if the activation energy is very high, as this means the reaction will have an extremely slow rate