1.7. Reaction Feasibility

Question 1

What does standard mean in standard enthalpy/entropy/free energy?

Answer 1

Standard means the values are derived relatively to a random reference point.

Question 2

What does the standard enthalpy of formation mean?

Answer 2

The enthalpy change involved when one mole of products is formed from its elements in their standard states (an elements most stable for at 1 atm pressure and at 298K.

Question 3

What is the standard enthalpy of formation for an element?

Answer 3

0.

Question 4

How do you calculate the standard enthalpy change of a reaction?

Answer 4

ΔH° = ΣΔH°f (products) - ΣΔH°f (reactants)

You must remember to multiply the standard enthalpies of formation of molecules by their stoichiometric coefficient (mole number).

Question 5

What is entropy?

Answer 5

The measurement of disorder in a system.

Question 6

What does the standard entropy of formation mean?

Answer 6

The entropy of 1 mole of a substance at 1 atm pressure at 298K.

Question 7

What are the units of entropy?

Answer 7

J K^-1 mol^-1

Question 8

What states have the highest entropy?

Answer 8

Gas -> Liquid -> Solid

Question 9

What is the third law of thermodynamics?

Answer 9

At 0K, particles are perfectly ordered as they have no energy to vibrate so entropy value is 0.

Question 10

Just by looking at a reaction we can usually tell if there’s an increase or decrease in entropy. How can we do this?

Answer 10

By looking at the state symbols. If there’s one mole of gas on the reactants side and 2 on the products side there is an increase in entropy.

Question 11

How do we calculate entropy change?

Answer 11

ΔS° = ΣS° (products) - ΣS° (reactants)

Question 12

What is a feasible reaction?

Answer 12

A reaction which always tends towards the products rather than the reactants.

Question 13

What is the 2nd law of thermodynamics?

Answer 13

For a reaction to be feasible, the total entropy change for a reaction system and its surroundings must be positive.

ΔS° (total) = ΔS° (surroundings) + ΔS° (system) = +ve

Question 14

How do we work out the standard entropy change of the surroundings?

Answer 14

ΔS° (surroundings) = -ΔH°/T when T is temperature (298K usually).

Question 15

How do we calculate the standard free energy change?

Answer 15

ΔG° = ΔH° -TΔS°

Question 16

What must ΔG° be if a reaction is to be feasible?

Answer 16

Negative.

Question 17

When do reactions just become feasible?

Answer 17

When ΔG° = 0

Question 18

How do we work out what temperature a reaction becomes feasible at?

Answer 18

With the rearranged equation

T = ΔH°/ΔS°

Question 19

How do we work out the standard free energy change using standard free energy changes of formation?

Answer 19

ΔG° = ΣΔG°f (products) - ΣΔG°f (reactants)

Question 20

What is the standard free energy change for an element in its most stable form?

Answer 20

0

Question 21

Why must we talk about free energy instead of standard free energy once a reaction starts?

Answer 21

Because standard conditions no longer apply.

Question 22

What does ΔG equal for a reaction at equilibrium?

Answer 22

0, be careful as it is ΔG not ΔG°.

Question 23

For feasible reactions, where does the equilibrium lie?

Answer 23

To the right.