1. Entropy and Gibbs Energy

2. Remember from yesterday:
From Hess’s Law, we got:
H reaction = H products - H reactants
Another Example:
2H2 (g) + 2CO2 (g)  2H2O (g) + 2CO (g)
H reaction = [2(-241.8) + 2(-110.5)]
– [2(0) + 2(-393.5)]
H reaction = 82.4 kJ

3. Now on to Entropy

4. What is Entropy?
Entropy is a measure of randomness or disorder of a system.
It is symbolized by S.
So, if there is a increase in disorder, S is positive
Some Examples of an increase of entropy
Diffusion – the process of dispersion
Reduction of pressure of a gas
Production of more gas in a chemical reaction
Total number of moles of products is greater than the number of moles of reactants [Example:
2NI3 (s)  N2 (g) + 3I2 (g)]

5. Hess’s Law and Entropy
Hess’s law can also be applied to entropy in the same way as it is applied to enthalpy.
Therefore, S reaction = S products - S reactants
Some entropy changes are located on page 359 and 360, but more are located in Appendix A on page 833.
Example:
Calculate the change in entropy for the following reaction using values from page 360 and that CH3OH has an S0 of J/mol.K:
CO (g) + 2H2 (g)  CH3OH (l)

6. How can we tell if a reaction will be spontaneous?
Spontaneous means that a reaction will occur without continuous outside assistance.
Well, if there is a decrease in enthalpy, H is negative. This helps it to be spontaneous.
If there is an increase in entropy, S is positive, that helps too.
A higher temperature also helps.
Josiah Willard Gibbs came up with an answer by putting these facts together.
He came up with what is now known as Gibbs energy, or free energy, or Gibbs free energy

7. Gibbs Energy Equation The equation is G = H - TS,
where G is the change in Gibbs energy, H is the change in enthalpy, T is the temperature, and S is the change in entropy
If G is a negative number, the reaction will be spontaneous.
If G is a positive number, the reaction will be nonspontaneous.
If G is a zero, the reaction is at equilibrium.

8. Relating Enthalpy and Entropy Changes to Spontaneity
Spontaneous?
Negative
Positive
Yes, at all temperatures
Either
Only if T < H/ S
Only if T > H/ S
Never

9. Example
Using the following values, compute the G value for each reaction and predict whether they will occur spontaneously.
Reaction
H (kJ)
Temperature
S (J/K) 1
+95
298 K
+45 2
-96.1
157 K
+119 3
-266
400 C
+54

10. Just Remember H reaction = H products - H reactants
S reaction =  S products -  S reactants
G reaction = G products - G reactants
Just remember the coefficients in front of the compounds in the equation.
Be careful of the units, H and G in the book are in kJ/mol, while S is in J/mol.K

11. Homework
Page 371: 30, 31, 32, 33, 34, 41, 42