F325: Module 2: Entropy and Enthalpy

Question 1

C) What is entropy and what makes something have high or low entropy- examples.

Answer 1

• Entropy is a thermodynamic quantity representing the degree of disorder or randomness in a system.
• A highly structured system eg a copper coin will have low entropy as it is very ordered whereas a gas is very disordered so will have a higher entropy

Question 2

C) What is the unit of entropy?

Answer 2

JK^-1mol^-1 (joules per kelvin per mole)

Question 3

C) Why is it more common for the entropy of a system to increase as opposed to decreasing?

Answer 3

An increase in entropy makes the system more energetically stable.
It is more probable that the entropy increases as there are more possible arrangements of the atoms that are more disordered than arrangements that are more ordered

Question 4

C) What is the Gibbs free energy equation?

What does this equation allow us to calculate?

Answer 4

ΔG = ΔH - TΔS

-It allows us to calculate the amount of useful work that can be done by the system

Question 5

C) What is the free energy change of a reaction?

Answer 5

The maximum useful work that can be done by a reaction

Question 6

C) -What must the value of G be in order for the reaction to be spontaneous?

• What happens if the value of G is zero or close to zero (-20kJ to 20kJ)?
• What happens if the value of G is below -20kJ?

Answer 6

• Negative
• The reaction will reach equilibrium with both reactants and products present
• The reaction will be spontaneous and go to completion

Question 7

C) Will a reaction be spontaneous if it is exothermic with a large negative ΔH?

Answer 7

Yes, it will nearly always be spontaneous .

Question 8

C) Will a reaction be spontaneous if it is endothermic with a large positive ΔH?
Why?

Answer 8

If ΔH is a very large positive number is is very unlikely that the reaction will be spontaneous unless there is a very high temperature.

Question 9

C) Will a reaction be spontaneous if ΔS is positive?

Answer 9

If there is an increase in disorder then - TΔS is negative which can contribute to ΔG being negative, meaning the reaction will most likely be spontaneous.

Question 10

C) Will a reaction be spontaneous if ΔS is negative

Answer 10

If there is a decrease in disorder, then- TΔS is positive which can contribute to G being positive which will mean that the reaction will most likely not be spontaneous.

Question 11

C) If ΔS is positive, how may we change T so that the reaction is more likely to be spontaneous and why?

Answer 11

Increasing T will make - TΔS a larger negative number as T increases, making it more likely that G will be negative and the reaction will be spontaneous.

Question 12

C) Why are chemical reactions with a large positive H very rarely spontaneous?
How may a reaction with a small H be spontaneous?

Answer 12

• Because G is almost always positive too

- If S is positive AND T is high enough so that ΔH - TΔS is negative

Question 13

I) How do you calculate the enthalpy of reaction?

Answer 13

The sum of the enthalpies of formation of the Products minus the sum of the enthalpies of formation of the Reactants.

ΔHr = ΔHfp - ΔHfr

Question 14

I) How would you calculate the entropy change in a reaction?

Answer 14

ΔS = ΔSp - ΔSr

Question 15

I) What is the entropy change when:

• The particles in a solution form a gas?
• A solid is disolved?
• The number of gas particles in a closed system decreases?

Answer 15

Positive - Gas molecules have more freedom than those in solution.

Positive - There is more disorder in solution than as a solid.

Negative - There are fewer gas molecules in the product and therefore less disorder (less positions for the particles to be in).

Question 16

C) What two things must you NOT FORGET to do when calculating using ΔG = ΔH - TΔS?

Answer 16

• Convert the value for entropy ΔS into kJK^-1mol^-1 as it is almost always calculated in JK^-1mol^-1
• Substitute the value for temperature in KELVIN not ℃

Question 17

C) Reaction
ΔH=+2879kJmol^-1
ΔS=-256JK^1mol^-1
At 25℃, ΔG is calculated to be +2955kJmol^-1

Why is this reaction not feasible at any temperature?
(1 mark)

Answer 17

-ΔH is positive/ ΔH>0
AND ΔS is negative/ TΔS is negative
AND ΔG will always be positive