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Equilibrium and Reversible Reactions

Equilibrium and Reversible Reactions. Many reactions are reversible. Once product is formed, it can go back and reform reactants. http://www.youtube.com/watch?v=br8lKynV1Hc&safe=active (2:45 min). What is Equilibrium?. A state of balance exists between the forward and reverse reactions.

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Equilibrium and Reversible Reactions

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  1. Equilibrium andReversible Reactions • Many reactions are reversible. • Once product is formed, it can go back and reform reactants. http://www.youtube.com/watch?v=br8lKynV1Hc&safe=active (2:45 min)

  2. What is Equilibrium? • A state of balance exists between the forward and reverse reactions. Rate of forward rxn = Rate of reverse rxn https://www.youtube.com/watch?v=dUMmoPdwBy4 What is Chemical equilibruim? (3:24)

  3. What Happens Once Equilibrium is Reached? • Concentration of reactants and products become constant at equilibrium. • This does not mean the amount or quantities are equal to each other. • Concentrations constant, not equal • Con Con, Requal!!!

  4. To ReachEquilbrium • Must have a closed system. • This is so no reactants or products (especially gases) can escape. • If they did you could not reach equilibrium • The reaction will instead “go to completion” in one direction http://www.youtube.com/watch?v=LMIbJ-B92Ho&safe=active (1:25 min)

  5. Dynamic Equilibrium • Dynamic Process: the reactions (forward/reverse) have not stopped. • You can no longer see changes in materials because the forward/reverse rxns happening at same rate. Dynamic Equilibrium https://www.youtube.com/watch?v=wlD_ImYQAgQ (4 min) http://www.youtube.com/watch?v=JsoawKguU6A&safe=active (1:47 min)

  6. Types of Equilibrium Phase Equilibrium: Gas/liquid: Rate of vaporization = rate of condensation Ex: H2O (l) ↔ H2O (g) Results in constant amount of vapor at a specific temperature we measure as “equilibrium vapor pressure”

  7. Phase Equilibrium: Solid/liquid: Rate of melting (fusion) = rate of solidification Ex: H2O (s) ↔ H2O (l)

  8. Types of Equilibrium Solution Equilibrium: Rate of dissolving = rate of crystallization • Ex: NaCl (s) ↔ NaCl (aq) • Occurs in saturated solutions • Solution holds maximum solute it can at that temp. • Even if you add more solute the conc. will not change

  9. Types of Equilibrium Chemical Equilibrium: Happens during chemical reactions • Forward/reverse reactions occur at same rate. • Concentrations become constant • No longer see observable changes Ex: N2(g) + 3H2(g) ↔ 2NH3(g) • http://www.kentchemistry.com/links/Kinetics/Equilibrium.htm

  10. Going to Completion • Reactions will not reach equilbrium if: • You remove product preventing reverse reaction • If system left open and gas leaves • If an insoluble product forms (ppt) • If water is formed

  11. Practice Questions

  12. Which balanced equation represents a phase equilibrium? (1) H2(g) + I2(g) ↔ 2HI(g) (2) 2NO2(g)  ↔ N2O4(g) (3) Cl2(g)  ↔ Cl2(l) (4) 3O2(g) ↔   2O3(g)

  13. Which statement about a system at equilibrium is true? (1) The forward reaction rate is less than the reverse reaction rate. (2) The forward reaction rate is greater than the reverse reaction rate. (3) The forward reaction rate is equal to the reverse reaction rate. (4) The forward reaction rate stops and the reverse reaction rate continues.

  14. Which quantities must be equal for a chemical reaction at equilibrium? (1) the activation energies of the forward and reverse reactions (2) the rates of the forward and reverse reactions (3) the concentrations of the reactants and products (4) the potential energies of the reactants and products

  15. Given the equation representing a phase change at equilibrium: H2O(s)  ↔ H2O(l) Which statement describes this equilibrium? (1) The H2O (s) melts faster than the H2O(l)  freezes. (2) The H2O (l) freezes faster than the H2O (s) melts. (3) The mass of H2O(s) must equal the mass of H2O(l). (4) The mass of H2O (l) and the mass of H2O (s) each remain constant.

  16. Le Chatelier’s Principle • When a system at equilibrium is subjected to a stress, the system “shifts” to relieve effects of the stress and restore equilibrium • “Shift” = forward or reverse reaction speeds up • “Stress” = something that causes a reaction (forward or reverse) to speed up or slow down. • Possible Stresses: • Change in Conc. • Change in Pressure • Change in Temperature https://www.youtube.com/watch?v=dIDgPFEucFM (2 min) https://www.youtube.com/watch?v=XmgRRmxS3is 4 min

  17. Effects of Stresses on Equilibrium Ex: A + B ↔ C + D • If forward rate increases • C + D produced at greater rate • amounts increase • A + B used up faster • amounts decrease The substances at the “head of the arrow” increase!

  18. Change in Concentration Will affect only gas and aqueous • Changing amounts of a pure liquid or solid will not cause a shift. • It will not change their concentrations, just their amounts. INCREASE Concentration • Reaction shifts in direction that consumes extra amount A + B ↔ C + D (Increase [A], shifts ) DECREASE Concentration • Reaction shifts in direction that produces more of what was taken away A + B ↔ C + D (Decrease [B], shifts )

  19. Change in Temperature Increase Temp. Favors ENDOTHERMIC • Direction that consumes excess heat • A + B ↔ C + D + energy Decrease Temp. Favors EXOTHERMIC • Direction that produces heat • A + B ↔ C + D + energy

  20. Pressure Changes • Affect gases only • Count total MOLES of gas on each side of equation • Increase pressure = Decrease volume • Shifts in direction that forms fewer moles of gas • Decrease pressure = Increase volume • Shifts in direction that forms greater moles of gas NOTE: If moles of gas equal on both sides NO SHIFT

  21. http://www.kentchemistry.com/links/Kinetics/LeChatelier.htm • Ex: N2(g) + 2O2(g) ↔ N2O4(g) 3 moles gas1 mole gas • Increase Pressure: Shifts • Decrease Pressure: Shifts

  22. How might you optimize the production of ammonia in the Haber reaction? N2(g) + 3H2(g) ↔ 2NH3(g) + 91.8kJ What could we do to: concentration, temp. and pressure? To shift right Increase [N2], [H2], Decrease [NH3] Decrease Temp Increase Pressure https://www.youtube.com/watch?v=Tt6diRJli6Q Haber reaction http://www.youtube.com/watch?v=NWhZ77Qm5y4&safe=active

  23. Common Ion Effect • Substance is added to an equilibrium system that has a “common ion” with the reaction. • This will increase the concentration of that aqueous ion and will cause a shift. • Ex: Saturated solution of AgCl AgCl(s) ↔ Ag+1(aq) + Cl-1(aq) • Stress: Add NaCl (s). • It breaks apart in water forming Na+1 and Cl-1 • Common Ion = Cl-1 • Shift • This causes a decrease in the solubility of AgCl

  24. Effect of Catalyst Increases rate of forward and reverse reactions equally. • There is no overall “shift” • Lowers activation energy the same for forward and reverse reaction

  25. Practice Regents Questions Given the a system at equilibrium: N2(g) + 3H2(g) ↔ 2NH3(g) + energy Which changes occur when the temperature of this system is decreased? (1) The conc. of H2(g) increases and the conc. of N2(g) increases. (2) The conc. of H2(g) decreases and the conc. of N2(g) increases. (3) The conc. of H2(g) decreases and the conc. of NH3(g) decreases. (4) The conc. of H2(g) decreases and the conc. of NH3(g) increases.

  26. Given the equation representing a reaction at equilibrium: N2(g) + 3H2(g)↔2NH3(g) + energy Which change causes the equilibrium to shift to the right? (1) decreasing the concentration of H2(g) (2) decreasing the pressure (3) increasing the concentration of N2(g) (4) increasing the temperature

  27. Given the system at equilibrium: 2POCl3(g) + energy ↔ 2PCl3(g) + O2(g) Which changes occur when O2(g) is added? (1) The equilibrium shifts to the right and the concentration of PCl3(g)increases. (2) The equilibrium shifts to the right and the concentration of PCl3(g)decreases. (3) The equilibrium shifts to the left and the concentration of PCl3(g) increases. (4) The equilibrium shifts to the left and the concentration of PCl3(g) decreases.

  28. Given the reaction at equilibrium: N2(g) + 3H2(g) -->2NH3(g) + 91.8 kJ What occurs when the conc. of H2(g) is increased? (1) The rate of the forward reaction increases and the concentration of N2(g) decreases. (2) The rate of the forward reaction decreases and the concentration of N2(g) increases. (3) The rate of the forward reaction and the concentration of N2(g) both increase. (4) The rate of the forward reaction and the concentration of N2(g) both decrease.

  29. Given the system at equilibrium: N2(g) + O2(g) + energy ↔ 2 NO(g) Which changes will result in a decrease in the amount of NO(g) formed? (1) decreasing the pressure. (2) decreasing the concentration of N2(g). (3) increasing the concentration of O2(g) . (4) increasing the temperature

  30. Given the equilibrium reaction in a closed system: H2(g) + I2(g) + heat ↔ 2 HI(g) What will be the result of an increase in temperature? (1) The equilibrium will shift to the left and [H2] will increase. (2) The equilibrium will shift to the left and [H2] will decrease. (3) The equilibrium will shift to the right and [HI] will increase. (4) The equilibrium will shift to the right and [HI] will decrease.

  31. Given the reaction at equilibrium: the concentration of A(g) can be increased by A.  lowering the temperature B.  adding a catalyst C. increasing the concentration of AB(g) D.  increasing the concentration of B(g)

  32. Equilibrium Overview (Good Review) https://www.youtube.com/watch?v=yFqYrBxbURY Le Chats: (Honors) http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/lechv17.swf Le Chat’s Overview Part 1 http://www.youtube.com/watch?v=7zuUV455zFs&safe=active Le Chat’s Overview Part 2 http://www.youtube.com/watch?v=XhQ02egUs5Y&safe=active Crash Course: Equilibrium http://www.youtube.com/watch?v=g5wNg_dKsYY&safe=active Le Chat’s (University of Surrey) http://www.youtube.com/watch?v=dIDgPFEucFM&safe=active

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