1. Chapter 17B Reaction Energy Entropy & Free Energy
West Valley High School
General Chemistry
Mr. Mata

2. Standard 8a
Students will know how enthalpy, entropy, and free energy are related.

3. Essential Questions
What factors influence the enthalpy, entropy, and free energy?

4. Hess’ Law Enthalpy of Reaction
ΔHrxn = Hproducts – Hreactants
- ΔH = exothermic
+ ΔH = endothermic

5. ΔS = Sproducts – Sreactants
Entropy, ΔS
Entropy
measure of relative disorder.
Thermodynamics tells us the universe moves towards disorder (entropy).
ΔS = Sproducts – Sreactants

6. Entropy, ΔS
Entropy helps predict whether a reaction will be spontaneous.
Solids have very low entropy
Gases have very high entropy
Solutions also have high entropy

7. Entropy Values Reaction entropy; 2KClO3(s)  2KCl(s) + 3O2(g)
2 solids  2 solids + 3 gases
Entropy increases in this reaction.

8. Negative Enthalpy (-ΔH) Positive Entropy (+ΔS)
Entropy Values
A positive ΔS = increase in entropy
A negative ΔS = decrease in entropy
Do not confuse entropy and enthalpy!
Moving towards spontaneity:
Negative Enthalpy (-ΔH)
Positive Entropy (+ΔS)

9. Gibb’s Free Energy Measure of system energy change.
Gibbs Free Energy, ΔG
Named after American chemist, J. Willard Gibbs, who developed an equation to relate enthalpy & entropy.

10. ΔG = ΔH -TΔS Free Energy, ΔG
Free energy, ΔG, allows us to assign a value to an entire reaction.
The basic equation is:
ΔG = ΔH -TΔS

11. Gibbs Free Energy, ΔG A negative Gibbs Energy (-ΔG):
Spontaneous, Product favored
A positive Gibbs Energy (+ΔG):
Nonspontaneous, Reactant favored

12. In conclusion… Enthalpy, ΔH Entropy, ΔS Gibbs Free Energy, ΔG
- ΔH = exothermic (heat out)
+ ΔH = endothermic (heat in)
Entropy, ΔS
- ΔS = decrease in entropy (stable)
+ ΔS = increase in entropy (unstable)
Gibbs Free Energy, ΔG
- ΔG = spontaneous
+ ΔG = nonspontaneous

13. Chapter 17 SUTW Prompt
Describe how we calculate enthalpy, entropy, and free energy for chemical reactions.
Complete a sentence paragraph using the SUTW paragraph format. Hilight using green, yellow, and pink.
Due Date: Monday, March 26, 2018 at beginning of your regular class period.