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
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