/ MYP 10 Chemistry 2012-13
Energetics :
Entropy and Gibbs Free Energy

Name: ______( ) Class: ______Date: ____________

Objectives

1.  Define entropy

2.  List two factors that affect entropy (state of matter and number of particles)

3.  Predict spontaneity based on entropy

4.  Predict spontaneity based on enthalpy and entropy using Gibbs Free Energy

5.  Perform Gibbs Free Energy calculations

Objective 1:

Entropy – amount of disorder, randomness

Symbol: S

Units: kJK–1mol–1

Example:

If you put a cube of sugar in a cup of tea, the sugar will dissolve and never come back to form the cube – the sugar molecules become more disordered

If you spray perfume, the perfume will diffuse throughout the air - the molecules become more disordered.

There is tendency in the universe for greater disorder

How is it measured?

·  It’s a probability calculation

What is it good for?

·  Explaining spontaneous endothermic reactions

Objective 2:

Factors that affect entropy

1.  State of matter

  1. Gas (most) particle motion is more random in a gas
  2. Liquid (middle) particle motion is less random than a gas but more than a solid
  3. Solid (least) particle motion is restricted. Possible positions for molecules are restricted
  4. Examples
  5. (changing state) H2O(l) à H2O(g)
  6. (changing state) H2O(s) à H2O(l)

2.  Temperature

  1. Comparing two gasses, one at 20° C and one at 80° C
  2. Molecules in the 80° C gas have more kinetic energy, they are moving more and colliding more

3.  The number of molecules

  1. More molecules means more possible positions relative to the other molecules
  2. (more moles and change of state) Li2CO3(s) à Li2O(s) + CO2(g)
  3. (more moles) MgSO4·8H2O à Mg2+(aq) + SO42-(aq) + 8H2O(l)

4.  More complex molecules have higher entropy values

Is entropy an extensive or intensive property?

·  Extensive because it depends upon the amount of mass

Objective 3:

Predictions

Which of the following reactions has the largest ΔS value?

1.  CO2(g) + 3H2(g) à CH3OH(g) + H2O(g)

2.  2Al(s) + 3S(s) à Al2S3(s)

3.  CH4(g) + H2O(l) à 3H2(g) + CO(g)

4.  2S(s) + 3O2(g) à 2SO3(g)

Objective 4:

Which is more important enthalpy or entropy?

Gibbs Free Energy

Gibbs Free Energy is the available, useful energy from a process

Spontaneous: a process that is likely to occur without the continuous input of energy

If DG > 0, not spontaneous

If DG < 0, spontaneous

The following equation can be used to predict spontaneity:

DG = DH - TDS

·  Notice that entropy is affected by temperature

DH / DS / DG / Spontaneous / Example
Negative
(exothermic) / Positive
(disordering) / Negative / YES / 2K(s) + 2H2O(l) à 2KOH(aq) + H2(g)
Positive
(endothermic) / Negative
(ordering) / Positive / NO / 16CO2(g) + 18H2O(l0 à 2C8H18 + 25O2(g)
Negative
(exothermic) / Negative
(ordering) / Depends on temp, T / YES at low enough temperature / H2O(g) à H2O(l)
Positive
(endothermic) / Positive
(disordering) / Depends on temp,T / YES at high enough temperature / H2O(s) à H2O(l)

Gibbs Free Energy Calculations

Objective 5:

Example calculation:

Given that the changes in enthalpy and entropy are -139 kJ and 277 J/K respectively for the reaction given below, calculate the change in Gibbs energy. Then state whether the reaction is spontaneous at 25° C

C6H12O6(aq) à 2C2H5OH(aq) + 2CO2(g)

(this reaction is used in baking)

DH = -139 kJ à 139000 J

DS = 277 J/K

T = 25° C à 25 + 273.15 = 298.15 K

DG = ?