CH 117 Spring 2015Worksheet 17
Chapter 16
1. What do Gibbs free energy and entropy indicate about spontaneity of a chemical reaction?
If G is negative ® reaction is spontaneous
If G is positive ® reaction is nonspontaneous
If S (entropy) is negative ® reaction is nonspontaneous
If S (entropy) is positive ® reaction is spontaneous
2. What is the definition of a state function? What are some examples of state functions?
A state function is a property that depends on the present state of the system and not its previous history. The path that the function took doesn’t matter, only the two points it travels in between. Some state functions include: Gibbs free energy (G), enthalpy (H), entropy (S), pressure, volume, and temperature.
3. What is the 3rd law of thermodynamics?
The entropy of a perfectly pure, crystalline solid substance (one that is perfectly ordered) is 0 at absolute zero (0 K).
4. Consider the following reaction at 25oC for which the standard entropy is +186.0 J/K. Calculate the standard entropy of formation for C (s), given the entropy values: CO (g) = 197.7 J/molK and O2 (g) = 205.0 J/molK.
2 C (s) + O2 (g) ® 2 CO (g)
Remember calculating the standard entropy change of a reaction is the same process as calculating enthalpy from last semester: sum of all the products – sum of all the reactants. (Same formula can be used for Gibbs free energy as well!)
186.0 J/K = products – reactants
186.0 J/K = 2(197.7 J/molK) – [205.0 J/molK + 2(x)]
-209.4 J = -205.0 J – 2x
-4.4 J = -2x ® x = 2.2 J/molK
5. Label each of the following reactions as having a positive or negative entropy change.
Trick here is to remember that gases have highest entropy values, then liquids, then solids!
a). H2O (g) ® H2O (l) negative
b). BF3 (g) + NH3 (g) ® F3BNH3 (s) negative
c). NH4Br (s) ® NH3 (g) + Br2 (l) positive
d). 2 NH4NO3 (s) ® 2 N2 (g) + 4 H2O (g) + O2 (g) positive
e). 2 H2O2 (l) ® 2 H2O (l) + O2 (g) positive
f). N2 (g) + 3H2 (g) ® 2 NH3 (g) negative