8. Thermochemistry

Question 1

Enthalpy H

Answer 1

Enthalpy H - property of thermodynamic system. Enthalpy change (ΔH) of any system = sum of change in its internal energy (ΔE) & product of its pressure (P) & volume change (ΔV).

Question 2

Entropy S

Answer 2

• Entropy – measure of system’s thermal energy per unit temperature unavailable for doing useful work. Because work obtained from ordered molecular motion, amount of entropy = measure of molecular disorder (randomness) of system.

All natural processes tend toward increase in energy dispersal = entropy increase within our universe. Thus, chemical system or reaction proceeds in direction of universal entropy increase.

​Disorder is good = positive!

Question 3

Indicators for ↑entropy S = disorder

Answer 3

Indicators for increase in entropy in reaction:

2NH₄NO₃(s) → 2N₂ (g) + O₂ (g) + 4H₂O (g)

↑randomness = ↑disorder = ΔS > 0 = positive entropy

• solid (crystal = structured molecules) turned to gases (unstructured molecules = random motion):
  (s) → (g) + (g) + (g)
• 1 molecule fragmented into 3 molecules:

NH₄NO₃ → N₂ + O₂ + H₂O

• 2 moles of molecule become 7 moles of molecules:

2NH₄NO₃ → 2N₂ + O₂ + 4H₂O

2 last points akin (similar) to having drinking glass, dropping it on floor, and watching it shatter into several pieces = clear ↑randomness = ↑disorder

Question 4

Indicators for ↓entropy S = order

Answer 4

Indicators for decrease in entropy in reaction:

2Cu(s) + O₂(g) → 2CuO(s)

↓randomness = ↑order = ΔS < 0 = negative entropy

• Solid & gas combine to produce solid = ↑order = ↓randomness
  (s) + (g) → (s)
• 2 molecules become 1 molecule = more ordered:

Cu + O₂ → CuO

• 3 moles become 2 = more ordered :

2Cu + O₂ → 2CuO

Question 5

Gibss Free Energy Summary

Answer 5

Gibss Free Energy - energy available to do useful work.

Equation for Gibbs free energy G (Joules) - where H = enthalpy of system in given state (J), T = absolute temperature (in kelvin, K) must always have positive value (CHM 7), S = entropy of system (J/K)

Question 6

Enthalphy Change

Answer 6

Overall enthalpy change = negative (ΔH_net < 0) = exothermic reaction, most likely spontaneous.

Overall enthalpy change = positive (ΔH_net > 0) = endothermic reaction.

Question 7

Gibbs Free Energy Spontaneity

Answer 7

• spontaneous if ΔG < 0 (negative) = exergonic
• not spontaneous if ΔG > 0 (positive) = endergonic
• spontaneous in both directions if ΔG = 0 – state of equilibrium

Question 8

Gibss Free Energy

Answer 8

Equation for Gibbs Free Energy G (Joules):

ΔG = ΔH - T ∙ ΔS

• H = enthalpy of system in given state (J)
• T = absolute temperature (in kelvin, K) must always have positive value
• S = entropy of system (J/K)

Question 9

Gibbs Free Energy

Answer 9

Summary of enthalpy (H) & entropy (S) change with Gibbs free energy (G) & spontaneity.