Thermodynamics II

Will the reaction take place? Which way will the reaction go? Thermodynamics II Spontaneity of a Reaction Ch 20 in Silberberg Text

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/Kmol Note: 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 ΔG˚rxn=∑G˚products - ∑G˚reactants 4 KClO3(s) + 3 KClO4(s) + KCl(s) Step 1. Calculate ∆H˚rxn Step 2. Calculate ∆S˚rxn Step 3. Calculate ΔG˚sys

20.4 ΔG, K, and Reaction Direction ΔG˚ = -RT ln K And for conditions other than standard state, ΔG = ΔG˚ + RT ln Q

References • AP College Board Thermodynamics Notes, pp59-75

Thermodynamics II