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

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

Equilibrium Rate of forward process = rate of reverse process. Hallmark: Looks like nothing is happening. Variables describing system are constant.

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

Phase Equilibrium Phase changes are reversible processesPhase changes are reversible processes. H 2 O(l)  H 2 O(g) H 2 O(l)  H 2 O(s) Same substance on both sides. Phase is different.

Examples - Phase Equilibrium Water & water vapor in a sealed water bottle. Perfume in a partially full, sealed flask. Ice cubes & water in an insulated container. Dry ice & CO 2 (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 CO 2 in water unopened. CO 2 (g)  CO 2 (aq) Favored by high pressure & low temperature.

Reversible Reactions N 2 (g) + 3H 2 (g)  2NH 3 (g) ForwardForward: N 2 & H 2 consumed. NH 3 produced. 2NH 3 (g)  N 2 (g) + 3H 2 (g) ReverseReverse: NH 3 consumed. N 2 & H 2 produced.

Reversible Reactions, 1 Equation N 2 (g) + 3H 2 (g)  2NH 3 (g) Forward rxn, reactants are on left. Read left to right. Reverse rxn, reactants are on right. Read in reverse – right to left. Rxns run in both directions all the time.

Time Concentration NH 3 H2H2 N2N2 N 2 (g) + 3H 2 (g)  2NH 3 (g) Why is this point significant?

Reaction Rate Depends on concentration of reactants. As concentration of reactants decreases, rate decreases. As concentration of NH 3 increases, rate of reverse rxn increases.

Chemical Equilibrium State in which forward & reverse rxns balance each other. Rate forward rxn = Rate reverse rxnRate forward rxn = Rate reverse rxn Does it say anything about the concentrations of reactants & products being equal? NO!

Chemical Equilibrium Rate forward rxn = Rate reverse rxn constantAt equilibrium, the concentrations of all species are constant. They stop changing. They are hardly ever equal.

Reversible Reactions vs. Reactions that “Go to Completion” If your goal is to maximize product yield: Easier in a reaction that goes to completion. Use up all the reactants. Left with nothing but product. Reversible reactions are different. Look at  Conc/  time picture again.

Time Concentration NH 3 H2H2 N2N2 N 2 (g) + 3H 2 (g)  2NH 3 (g) Original Equilibrium Point

Reversible Reactions Once you reach equilibrium, you don’t produce any more product. This is bad news if the product is what you’re selling. How can you change the equilibrium concentrations? For example, how can you maximize product?

How can you get from here

Lots of product as fast as possible. New equilibrium point To here?

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

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

Catalyst Has the same effect on the forward & reverse reactions. Equilibrium is reached more quickly, but the “equilibrium point” is not shifted. The equilibrium concentrations are the same with or without a catalyst.

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

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

N 2 (g) + 3H 2 (g)  2NH 3 (g) N2N2 H2H2 NH 3 Stress: Increased [N 2 ] Original Equilibrium New Equilibrium

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

A + B  C + D, at equil. If I increase the concentration of A, how will the system react? How does the new equilibrium mixture compare to the original equilibrium mixture? Use logic. If you increase [A], the system wants to decrease [A]. It has to use A up, so it speeds up the forward reaction.

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

Changes in Temp Exothermic rxn: A + B  C + D + heat consumeIf you increase the temperature, the system shifts to consume heat. So here, it shifts to the left. Endothermic rxn: A + B + heat  C + D If you increase the temperature, the system shifts to consume heat. So here, it shifts to the right.

Changes in Pressure N 2 (g) + 3H 2 (g)  2NH 3 (g) If you increase pressure, the system shifts to the side with fewer moles of gas. Here, the right hand side has only 2 moles of gas while the LHS has 4. Increasing pressure will cause a shift to the right. If you decrease pressure, the system shifts to the side with more moles of gas.

H 2 (g) + I 2 (g)  2HI(g) This system has 2 moles of gas on the LHS & 2 moles of gas on the RHS. cannotSystems with equal moles of gas on each side cannot respond to pressure changes.