1.2 Enthalpy, Entropy and Free Energy

Question 1

Define feasible/spontaneous changes, with an example

Answer 1

changes that have a natural tendency to occur without any external help, eg. Aqueous silver ions and aqueous iodide ions spontaneously form a yellow precipitate of silver iodide.

Question 2

how do feasible changes relate to stability of a system?

Answer 2

A feasible change occurs due to stability - feasible changes move a system from a less stable state to a more stable state.

Question 3

What is the temperature change in most feasible reactions, and when is this not the case?

Answer 3

• Most feasible reactions are exothermic
• Some can be endothermic, but there must be an additional factor that determines whether or not the reaction will take place.

Question 4

Give an example of a feasible endothermic reaction

Answer 4

Ammonium carbonate + ethanoic acid
This reaction shows an increase in disorder as a solid and solution produces a gas and solution - hence there is an increase in entropy. The reaction is therefore feasible despite being endothermic as it results in a change in entropy.

Question 5

How does feasibility relate to entropy?

Answer 5

feasible reactions increase the disorder/entropy of a system (eg. changes of state, increased number of gas molecules etc)

Question 6

How do the states of matter relate to entropy?

Answer 6

In terms of randomness/disorder:

solid < liquid < gas

Question 7

define entropy (S)

Answer 7

a measure of disorder or randomness of a system

Question 8

Why is entropy always a positive value?

Answer 8

all substances possess some degree of disorder, as particles are always in constant motion

Question 9

state the second law of thermodynamics

Answer 9

The sum of the entropy changes for a chemical system and its surroundings must be positive for a process to be feasible.

Question 10

define standard entropy (S^θ)

Answer 10

The absolute value of a substance’s entropy under standard conditions

Question 11

Describe the relative standard entropy values of different states of matter

Answer 11

Gases generally have larger standard entropies than liquids, which have larger values than solids

Question 12

Define standard entropy change (∆S^θ)

Answer 12

the entropy change per mole for conversion of reactants in their standard states into products in their standard states, at a stated temperature

Question 13

state the equation to calculate the standard entropy change

Answer 13

∆S^θ = S^θproducts - S^θreactants

Question 14

state the equation used to calculate free energy change

Answer 14

∆G= ∆H – T∆S

Question 15

define Gibbs’ Free Energy

Answer 15

The energy associated with a chemical reaction that can be used to do work

Question 16

What is the criterion for a feasible reaction?

Answer 16

∆G^θ < 0

Question 17

what can be assumed if ∆G is positive?

Answer 17

the reaction is not feasible at that temperature, although it may be feasible at another temperature

Question 18

what can be assumed if ∆G=0?

Answer 18

the system is in a state of equilibrium, ie. there is no thermodynamic tendency for the reaction to proceed in either direction.

Question 19

What units should ∆S be if used in the free energy equation?

Answer 19

kJ/K/mol

Question 20

What can be assumed about the feasibility of a system if ∆H is negative and ∆S is positive?

Answer 20

∆G is always negative

therefore feasible at any temperature

Question 21

What can be assumed about the feasibility of a system if ∆H is positive and ∆S is negative?

Answer 21

∆G is always positive

therefore not feasible at any temperature

Question 22

What can be assumed about the feasibility of a system if ∆H is negative and ∆S is negative?

Answer 22

∆G can be positive or negative

feasible below certain temperatures

Question 23

What can be assumed about the feasibility of a system if ∆H is positive and ∆S is positive?

Answer 23

∆G can be positive or negative

feasible above certain temperatures

Question 24

What features must a reaction system have to proceed satisfactorily?

Answer 24

• Thermodynamically stable (ie. negative ∆G value)

- Kinetically stable (ie the reaction must have a low activation energy)

Question 25

why might a system with a negative ∆G value still react very slowly, and how can this be amended?

Answer 25

Although it is thermally feasible, it might not be kinetically stable. A catalyst can be used to increase the rate of reaction by lowering the activation energy.