Spontaneity & Entropy 19.1-19.5

Spontaneous Changes H2O(s) ---> H2O(l) Occurs with no outside intervention; the rate of change may be fast or slow H2O(s) ---> H2O(l) Reverse rxn is NOT spontaneous And equilibrium

NonSpontaneous Changes Occur with outside intervention H2O --> H2 + O2 Reverse rxn IS spontaneous Reactions are spontaneous in the rv H2O --> H2 + O2 Demo decomposition of H2O

Equilibrium “Product Favored” - more products around at equilibrium “Reactant Favored” more reactants around at equilibrium May be spontaneous or not

Heat and Spontaneity Exothermic rxn are _____________ Endothermic rxn are ____________

Entropy (S) measures the disorder or randomness in a system. Nature favors disorder 2nd Law of Thermodynamics - in any process, the entropy of the universe increases

Entropy Happens

Entropy (S) Universe includes the system & surroundings ∆Suniv = ∆ Ssys + ∆ Ssurr The entropy of a system may decrease, as long as the surroundings increase

Entropy (S) and Spontaneity For a spontaneous process: ∆ Suniv > 0 For a non-spontaneous process: ∆ Suniv < 0 At equilibrium: ∆ Suniv = 0 Entropy is not conserved, it is continually increasing.

Entropy Changes of Surroundings (∆Ssurr°) ∆ Ssurr = - ∆Hrxn T ∆Hsys° = ∆H°f(Products) −  ∆ H°f (Reactants) Kelvin T of surroundings

Entropy Changes of Surroundings (∆Ssurr°) Calculate the ∆Ssurr° using Appendix L in this reaction: N2(g) + 3H2(g) --> 2NH3(g)

Entropy Changes of System (∆Ssys°) ∆Ssys° = S°products − S°reactants Example: N2(g) + 3H2(g) --> 2NH3(g) Calculate the ∆Ssys° using Appendix L S = -198.1 J H= + 91.80 kJ

More About Entropy Larger S, more entropy (disorder) State function 3rd Law of Thermodynamic is a reference point A crystal at 0 Kelvin has no entropy (S=0), its perfectly ordered Nature tends to be disorganized or randomness

The entropy of any substance can be obtained by measuring the heat added to a substance to raise its temp. from 0 K

Standard Entropy , S° Entropy gained by converting a crystal at 0 K to standard conditions Units: J/K mol Generalizations: For similar substances: Increasing S Solutions are comparable to liquids S l g

Generalizations Boiling has much greater change in entropy than melting

Generalizations Larger molecules have more entropy than smaller molecules Entropy increases as temperature increases

Generalizations Any rxn that increases the # gas molecules has higher entropy at the end 2 H2O (l) --> 2H2(g) + O2(g)