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Equilibrium

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. Definitions.

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Equilibrium

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  1. Equilibrium

  2. 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.

  3. Definitions • Reversible Reaction is one which can take place in both directions, forward and reverse, and does not go to completion. • A Closed System is a system where there is no interchange of particles between the reaction mixture and the outside environment.

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

  5. 3 Kinds of Equilibria • Phase equilibrium – physical • Solution equilibrium – physical • Chemical equilibrium - chemical

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

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

  8. Solution Equilibrium: Gases CO2 in water unopened. CO2(g)  CO2(aq) Favored by high pressure & low temperature.

  9. 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.

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

  11. Reaction Rate • Depends on concentration of reactants. • As concentration of reactants decreases, rate decreases. • As concentration of NH3 increases, rate of reverse rxn increases.

  12. Chemical Equilibrium • State in which forward & reverse rxns balance each other. Rateforwardrxn = Ratereverserxn • At equilibrium, the concentrations of all species are constant. They stop changing. • They are hardly ever equal.

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

  14. 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?

  15. New equilibrium point Lots of product as fast as possible.

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

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

  18. 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.

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

  20. LeChatelier’s Principle “If a system at equilibrium is subjected to a stress, the system will act to reduce the stress.” Stress: Change in… concentration, pressure, or temperature. System tries to UNDO the stress!

  21. Change the System… • Only 2 possible ways: • 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.

  22. 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.

  23. 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.

  24. 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.

  25. 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.

  26. Adding an Inert Gas • No EFFECT! • Does not participate in the reaction so does not affect chemical equilibrium

  27. Application: Haber Process N2(g) + 3H2(g)  NH3(g) Franz Haber The N2 and H2 mixture is brought into a reactor at a high temperature and pressure. The equilibrium mixture is removed from the reactor and cooled in a condenser. Liquid NH3 is removed, and the N2 and H2 mixture is returned to the reactor and mixed with additional reactant gases.

  28. Biological Equilibrium • Ever go high up into the mountains skiing hiking? Then feel tired or lightheaded? Hgb(aq) + O2 (g) Hgb(O2)(aq) • Atmospheric Pressure and [O2] lower. • Equilibrium shifts to produce more oxygen! Equilibrium

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