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Equilibrium. 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.
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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 processes. • H2O(l) H2O(g) • H2O(l) H2O(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 & 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 unopened. 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 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.
N2(g) + 3H2(g) 2NH3(g) Why is this point significant? Concentration H2 NH3 N2 Time
Reaction Rate • Depends on concentration of reactants. • As concentration of reactants decreases, rate decreases. • As concentration of NH3 increases, rate of reverse rxn increases.
Chemical Equilibrium • State in which forward & reverse rxns balance each other. • Rateforwardrxn = Ratereverserxn • Does it say anything about the concentrations of reactants & products being equal? NO!
Chemical Equilibrium • Rateforwardrxn = Ratereverserxn • At equilibrium, the concentrations of all species are constant. They stop changing. • They are hardly ever equal.
Reversible Reactions vs. Reactions that “Go to Completion” • Ifyour 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.
N2(g) + 3H2(g) 2NH3(g) Original Equilibrium Point Concentration H2 NH3 N2 Time
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?
New equilibrium point Lots of product as fast as possible. 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.
N2(g) + 3H2(g) 2NH3(g) Original Equilibrium New Equilibrium H2 NH3 N2 Stress: Increased [N2]
System • Only 2 possible actions • Shift to the right & form more product. The forward rxn speeds up more than the reverse rxn. • Shift 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.
Changes in Temp • Exothermic rxn: • A + B C + D + heat • If 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 • N2(g) + 3H2(g) 2NH3(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.
H2(g) + I2(g) 2HI(g) • This system has 2 moles of gas on the LHS & 2 moles of gas on the RHS. • Systems with equal moles of gas on each side cannot respond to pressure changes.