Equilibrium State of balance Condition in which opposing forces exactly balance/equal each other Need 2-way or reversible situation Need a closed system

Dynamic Equilibrium Macroscopic level Microscopic level looks like nothing is happening Microscopic level lots going on

3 Kinds of Equilibria Phase equilibrium – physical Solution equilibrium – physical Chemical equilibrium - chemical

Phase Equilibrium Phase changes are reversible processes H2O(l)  H2O(g) H2O(l)  H2O(s) Same substance on both sides but phase is different

Examples - Phase Equilibrium Water & water vapor in sealed water bottle Perfume in partially full, sealed flask Ice cubes & water in insulated container Dry ice & CO2(g) in a closed aquarium

Solution Equilibrium: Solids Saturated solution = dynamic equilibrium Dissolving & Solidification occur at equal rates

Solid in Liquid NaCl(s)  NaCl(aq) Favored a little bit by higher temperature

Solution Equilibrium: Gases CO2 in water CO2(g)  CO2(aq) favored by high pressure & low temperature

Reversible Reactions N2(g) + 3H2(g)  2NH3(g) Forward: N2 & H2 consumed; NH3 produced 2NH3(g)  N2(g) + 3H2(g) Reverse: NH3 consumed; N2 & H2 produced

Reversible Reactions: 1 Equation N2(g) + 3H2(g)  2NH3(g) Forward reaction: reactants on L Read left to right Reverse reaction: reactants on R Read in reverse: right to left Reaction runs in both directions all the time

N2(g) + 3H2(g)  2NH3(g) Concentration H2 NH3 N2 Time Why is this point significant? Concentration H2 NH3 N2 Time

Reaction Rate Depends on concentration of reactants As concentration reactants ↓, rate forward reaction ↓ As concentration product ↑, rate reverse reaction ↑

NO! Chemical Equilibrium State in which forward & reverse rxns balance each other Rateforward rxn = Ratereverse rxn Does this mean concentrations reactants/products are equal? NO!

Chemical Equilibrium Rateforward rxn = Ratereverse rxn At equilibrium: concentrations all species are constant stop changing rarely ever equal

Reversible Reactions vs. Reactions that “Go to Completion” If goal is to maximize product yield: Easier in reaction that goes to completion Use up all reactants Left with only product Reversible reactions are different Look at Conc/time picture again

N2(g) + 3H2(g)  2NH3(g) Concentration H2 NH3 N2 Time Original Equilibrium Point Concentration H2 NH3 N2 Time

Reversible Reactions Once reach equilibrium, don’t produce any more product bad news if product is what you’re selling Can you change the equilibrium concentrations? If so how can it be done? For example, how can you maximize product?

What you would really like to see…

New equilibrium point Lots of product as fast as possible

Affecting Equilibrium Equilibrium can be changed or affected by: any factor that affects forward and reverse reactions differently

What factors affect rate of reaction? Concentration/Pressure Temperature Presence of catalyst

Catalyst same effect on both forward & reverse reactions Equilibrium reached more quickly, but “equilibrium point” not shifted equilibrium concentrations are same with or without catalyst

Concentration, Pressure, Temperature Changes in concentration, pressure, temperature affect forward & reverse reactions differently Composition of equilibrium mixture will shift to accommodate these changes

LeChatelier’s Principle “If system at equilibrium is subjected to stress, the system will act to reduce stress” stress = change in concentration, pressure, or temperature System tries to undo stress

System Only 2 possible actions Shift to the right & form more product forward reaction speeds up more than the reverse reaction Shift to the left & form more reactant reverse reaction speeds up more than the forward reaction

A + B  C + D (at equilibrium) If ↑ concentration A, how will system react? How does new equilibrium mixture compare to original equilibrium mixture? Use logic: If you ↑ [A]: the system wants to ↓ [A] It has to use A up, so it speeds up the forward reaction

A + B  C + D DEC  ______ INC  Left [B] Right [C] [D] [A] [D] Equil. Shift Stress

Changes in Temp Exothermic reaction: A + B  C + D + heat If ↑ temperature, system shifts to consume heat so shifts to left Endothermic reaction: A + B + heat  C + D If ↑ temperature, system shifts to consume heat so shifts to right

Changes in Pressure N2(g) + 3H2(g)  2NH3(g) If ↑ pressure, system shifts to side with fewer moles of gas left side: 4 moles of gas; right side: 2 moles ↑ pressure cause shift to the right If ↓ pressure, system shifts to side with more moles of gas ↓ pressure cause shift to the left

H2(g) + I2(g)  2HI(g) This system has 2 moles gas on left & 2 moles gas on right Systems with equal moles gas on each side cannot respond to pressure changes so no shift occurs