Thermodynamics II

Spontaneity of a ReactionCh 20 in Silberberg Text

Which way will

the reactio

n go?

Will the reaction take place?

What is a spontaneous reaction?

• Happens without any outside help.

• Have a natural direction of reaction.

What is Entropy?

• “A measure of the dispersal of energy over the states available to a system”

• Or, the tendency of a system to undergo disorder

• ∆S˚: change in entropy as a system undergoes a change in the number of possible particle arrangements.

• Units of entropy: Joule/KmolNote: the ˚ represents

standard conditions of 25C and 1 atmosphere

Factors which affect the entropy of a substance

• Temperature• Physical state• Lattice energy of a crystal• Atomic and molecular size• Freedom of rotation of a molecule

Identify whether entropy is increasing or decreasing in each of these reactions

• Br2(l) Br2(g)

• H2 O(l) H2O(s)

• Precipitation of a solid after mixing two liquids• CuCO3(s) CuO(s) + CO2(g)

• 2SO2(g) + O2(g) 2SO3(g)

• Ba(OH)2(s) + 2NH4Cl(s) 2NH3(g) + 2H2 O(l) +

Ba2+(aq) + 2Cl-

(aq)

∆S ˚ Calculations

• 20.2 ∆S˚rxn = ∑S˚products - ∑S˚reactants

Example: N2(g) + 3H2(g) 2 NH3(g)

Prediction: Solution: ∆S˚rxn = [2 molx 193J/Kmol ] – [(1molx191.5J/Kmol) +

(3 mol x 130.6J/Kmol)] = -197J/K

Second Law of Thermodynamics

• In all irreversible processes, the total entropy of a system must increase.

Gibbs Free Energy

• ΔG is the measure of spontaneity of a reaction; it is the measure of the energy used to do work.<0 spontaneous>0 nonspontaneous=0 at equilibrium

Will it react? Calculations of ∆G˚

∆H˚rxn = ∑H˚products - ∑H˚reactants

∆S˚rxn = ∑S˚products - ∑S˚reactants

ΔG˚rxn = ΔH˚rxn - T S˚rxn

4 KClO3(s) + 3 KClO4(s) + KCl(s)

Step 1. Calculate ∆H˚rxn

Step 2. Calculate ∆S˚rxn

Step 3. Calculate ΔG˚sys

ΔG ˚rxn = ∑G˚products - ∑G˚reactants

20.4 ΔG, K, and Reaction DirectionComparison of Q and K

Q/K ratio Reaction direction ΔG Value

Q<K Q/K <1 Forward <0

Q>K Q/K >1 Reverse >0

Q=K Q/K = 1 No reaction = 0

ΔG˚ = -RT ln K

And for conditions other than standard state,

ΔG = ΔG˚ + RT ln Q

References

• AP College Board Thermodynamics Notes, pp59-75