Reversible Reactions and Equilibrium Ch. 18.2 Reversible Reactions and Equilibrium

Reversible Rxn Reversible rxn: in which the conversion of reactants to products and products to reactants occur simultaneously. 2SO2 + O2  2SO3 (Forward rxn) 2SO3  2SO2 + O2 (Reverse rxn) 2SO2 + O2  2SO3 2SO2 + O2 2SO3 (double arrow or 2 half arrows indicates reversible rxn)

Equilibrium Rates of forward and reverse reaction are equal. No net change of concentration occurs in the system. DOES NOT mean equal amounts of products and reactants https://www.youtube.com/watch?v=g5wNg_dKsYY

Middle picture is at equilibrium

Concentration of reactants (green & blue lines) and products(red and aqua lines) with time.

Le Chatelier’s Principle If stress is applied to a system in dynamic equilibrium, the system changes to relieve that stress. Stresses include: Concentration changes Temperature changes Pressure changes (only affects gases)

Guide to Le Chatelier’s principle 2SO2(g) + O2(g) + heat  2SO3(g) Add a substance or heat Equilibrium shifts to opposite side of rxn Remove a substance or heat Equilibrium shifts to same side of rxn Add (increase) pressure (decrease vol) Equil. Shifts to side with fewest gas mc’s remove (decrease) pressure (increase V) Equil shifts to side with most gas mc’s

Effect of LeChatelier’s principle Whichever side the equilibrium shifts to, the concentration increases on that side If equil shifts to reactant side (left) reactant concentration increases The side away from the shift, concentration decreases If equil shifts to reactant side (left) product concentration decreases.

N2(g) + 3H2(g)  2NH3(g) + 22.0 kcal Stress Equil shift [N2] [H2] [NH3] heat 1. Add N2 right  ---------- 2. Add H2 ----------------- 3. Add NH3 ------------- 4. Remove NH3 _______ 5. Remove N2 __________ 6. inc. temp ----------- 7. dec. temp 8. Increase pressure 9. Decrease pressure

18.4 Entropy

Entropy Entropy: Measure of disorder in a system Law of disorder: the natural tendency is for systems to go toward maximum disorder.

What factors promote Entropy? Phase Particle size Number of products Temperature (kinetic energy)

Phase Solid  Liquid  Gas (most entropy)

Particle size More particles (smaller) more entropy

Number of products 2 H2O  2 H2 + O2 Dissolving increases entropy

Temperature Increase of temperature increases kinetic energy which increases entropy

Entropy in Reactions 2 KClO3 (s)  2 KCl (s) + 3 O2 (g) The more entropy the more likely a reaction will occur. S = +# - more entropy 2 KClO3 (s)  2 KCl (s) + 3 O2 (g) H2O (l)  H2O (s) N2 (g) + 3 H2 (g)  2 NH3 (g) HC2H3O2 (l)  H+ (aq) + C2H3O2- (aq) Steam at 105 oC or steam at 115 oC

Free Energy Free Energy: Energy that is available to do work (released by a chemical process/rxn)

Spontaneous Reaction: Occurs naturally Favors the formation of product Releases free energy (combination of enthalpy and entropy) Delta G = + # =nonspontaneous = Endergonic Delta G = - # = spontaneous = Exergonic

Spontaneous Reactions 2 factors determine spontaneous rxns Enthalpy (heat energy of reaction) Exothermic rxns favor spontaneous rxns Entropy (disorder) Increase in entropy (disorder) favor spont. rxns

Is a reaction spontaneous? H S G Spont Dec (-) Inc (+) YES Maybe NO ??