1. Entropy, Enthalpy and Spontaneous Reactions

2. Spontaneous Reaction
A reaction that occurs without outside intervention.
Once it occurs it can be fast or slow.
It can go to completion. (Recall the three products produced :
Precipitate (table F), gas, or water.)
Or be in a state of equilibrium.

3. Two factors determine if a reaction is spontaneous
Enthalpy: (heat of reaction )
Entropy : (randomness)

4. Enthalpy: Heat of Reaction
Exothermic Reaction
Endothermic Exothermic
Heat is absorbed
H P > H R
▲H = H P - H R
▲H is positive
Example from table I
Heat is released
H R > H P
▲H = H P - H R
▲H is negative
Example from table I

5. Heat of Formation
Heat absorbed or releases when a mole of a compound is formed from its elements.
Standard heat of formation is at 25 degrees celcius and 1 atm
Stable compounds require a large input of energy to decompose a compound. Therefore more stable the compound the larger the – heat of formation.
Unstable compounds require little or no energy to decompose. They have small –heat of formation or even a + heat of formation.

6. Hess’s Law of Constant Heat Summation

7. Enthalpy and Spontaneous Reactions
At constant temperature and pressure a system tends to undergo a reaction so that in its final state it has a lower energy than its initial state.
Exothermic reactions in nature are favored (-delta H)

8. Entropy S
Measures the disorder, randomness or lack of organization of a system.
Order (-s) Disorder (+S)
Solid liquidaqueous ionsgas
Higher entropy is favored by higher temperatures and more moles of gases.

9. Probability of Disorder
Is there a higher probability your room will be messy or neat as time goes on?

10. Example:
Which has more entropy in its system?
H2O (s) or H2O (g)

11. Entropy and Spontaneous Reactions
At constant temperature a system tends to undergo a reaction so that in its final state it has a higher entropy (greater randomness) than its initial state.

12. Summary Case 1: Exothermic and disorder will always occur
Case 2: Endothermic and order will never occur
Case 3: Exothermic and order will sometimes occur
Case 4: Endothermic and disorder will sometimes occurs.
What determines if it will sometimes occur?

13. Gibbs Free Energy Change
Free energy of a system can be defined as the measure of energy available (free) to do work at a constant pressure and temperature.
In a chemical reaction it measures the extent to which the substances can react.

14. Gibbs Free Energy
G 0 (positive value) reaction will not occur. Favors the reactants.
G  0 (negative values) reaction will occur be spontaneous, Go to completion and favors the products.
G = 0 reaction will be in a state of equilibrium.

15. Summary of Gibbs Free energy

16. Examples:
Physical change: When doe ice melt? Enthalpy_______ Entropy________ Always/never/sometimes Chemical Change: When does ammonia form? Enthalpy________ Entropy_________ Always/never/ sometimes