Topic 15.2: Entropy and spontaneity

Question 1

Second law of thermodynamics

Answer 1

a) A reaction is spontaneous if the overall transformation leads to an increase in total entropy (system plus surroundings).
b) This change always increases the total entropy of the universe at the expense of energy available to do useful work.

Question 2

Definition of entropy

Answer 2

Distribution of available energy among the particles in a system
a) The more ways the energy can be distributed, the higher the entropy

Question 3

What happens to the entropy when a system becomes more disordered?

Answer 3

a) The energy in the system can be distributed in more ways among the available particles
b) The entropy of the system is higher

Question 4

Definition of entropy change

Answer 4

Change in the disorder of a system

a) The more disordered a system becomes, the more positive the value of ∆S becomes
b) Systems which become more ordered will have decreasing ∆S values

Question 5

Factors that increase the entropy of a system

Answer 5

a) Changes of state
b) Number of particles
c) Temperature
d) Dissolution

Question 6

Entropy and changes of state

Answer 6

Gas > Liquid > Solid (under same conditions)

Question 7

Entropy in solids

Answer 7

Particles are more constrained, having less ways in which energy is distributed among particles

Question 8

Entropy in gases

Answer 8

Particles are moving at different speeds in different directions, so there is a huge number of ways in which energy is distributed among particles

Question 9

Can absolute values of entropy be measured?

Answer 9

Yes

Question 10

Definition of standard entropy

Answer 10

entropy content of one mole of substance under a standard state (J K-1 mol-1)

Question 11

Formula to calculate entropy change for a reaction

Answer 11

∆S = Sum of S products - Sum of reactants
∆S = q / T

Question 12

Definition of spontaneous reaction

Answer 12

One that occurs without any outside influence

Question 13

Second law of thermodynamics and spontaneity

Answer 13

a) A reaction is spontaneous if the overall transformation leads to an increase in total entropy (system plus surroundings).
b) This change always increases the total entropy of the universe at the expense of energy available to do useful work.

Question 14

∆S Universe

Answer 14

If its value is positive, the entropy of the Universe increases and the reaction occurs spontaneously

Question 15

∆S Universe = ∆S Surroundings + ∆S System

a) ∆S Universe

Answer 15

If its value is positive, the entropy of the Universe increases and the reaction occurs spontaneously

Question 16

∆S Surroundings

a) -∆HSystem
b) 1/T

Answer 16

a) Exothermic reactions result in an increase in the entropy of the surroundings
b) The impact of an addition of heat depends on the present state of disorder, indicated by the absolute temperature

Question 17

Formula to calculate the spontaneity of a reaction using enthalpy change and entropy change of a system

Answer 17

∆G =∆H - T∆S

Question 18

Definition of standard free energy change

Answer 18

Measure of the energy which is free to do useful work rather than just leave a system as heat
a) Spontaneous reactions have negative free energy changes because they can do useful work

Question 19

Formula to calculate standard free energy change at 298K from ∆Gf

Answer 19

a) If ∆G<0 the reaction is spontaneous
b) If ∆G=0 the reaction is at equilibrium
c) If ∆Gθ>0 the reaction is not spontaneous

Question 20

When a reaction will be always spontaneous?

Answer 20

a) - ∆H

b) + ∆S

Question 21

When a reaction will be always non-spontaneous?

Answer 21

a) + ∆H

b) - ∆S

Question 22

When a reaction will be spontaneous at low temperatures and non-spontaneous at high temperatures?

Answer 22

a) - ∆H

b) - ∆S

Question 23

When a reaction will be nonspontaneous at low temperatures and spontaneous at high temperatures?

Answer 23

a) + ∆H

b) + ∆S