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CHEMICAL EQUILIBRIUM

CHEMICAL EQUILIBRIUM. Chemical Equilibrium. Reversible Reactions :. A chemical reaction in which the products can react to re-form the reactants. Chemical Equilibrium :. When the rate of the forward reaction equals the rate of the reverse reaction and the concentration of products and

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CHEMICAL EQUILIBRIUM

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  1. CHEMICAL EQUILIBRIUM

  2. Chemical Equilibrium Reversible Reactions: A chemical reaction in which the products can react to re-form the reactants Chemical Equilibrium: When the rate of the forward reaction equals the rate of the reverse reaction and the concentration of products and reactants remains unchanged 2HgO(s)  2Hg(l) + O2(g) Arrows going both directions (  ) indicates equilibrium in a chemical equation

  3. 2NO2(g)  2NO(g) + O2(g) Remember this? Why was it so important to measure reaction rate at the start of the reaction (method of initial rates?)

  4. Law of Mass Action For the reaction: jA + kB  lC + mD Where K is the equilibrium constant, and is unitless

  5. Product Favored Equilibrium Large values for K signify the reaction is “product favored” k > 1 When equilibrium is achieved, most reactant has been converted to product

  6. Reactant Favored Equilibrium Small values for K signify the reaction is “reactant favored” k < 1 When equilibrium is achieved, very little reactant has been converted to product

  7. Writing an Equilibrium Expression Write the equilibrium expression for the reaction: 2NO2(g)  2NO(g) + O2(g) K =???

  8. Conclusions about Equilibrium Expressions • The equilibrium expression for a reaction is the reciprocal for a reaction written in reverse 2NO2(g)  2NO(g) + O2(g) 2NO(g) + O2(g)  2NO2(g)

  9. Conclusions about Equilibrium Expressions • When the balanced equation for a reaction is multiplied by a factor n, the equilibrium expression for the new reaction is the original expression, raised to the nth power. 2NO2(g)  2NO(g) + O2(g) NO2(g)  NO(g) + ½O2(g)

  10. Equilibrium Expressions Involving Pressure For the gas phase reaction: 3H2(g) + N2(g)  2NH3(g) K and Kp are NOT interchangeable!

  11. Heterogeneous Equilibria • The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present Write the equilibrium expression for the reaction: PCl5(s)  PCl3(l) + Cl2(g) Pure solid Pure liquid

  12. CAN you….. • Write an Keq expression? • Tell how Keq changes as the stoichiometry changes? • Convert Keq in terms of KcvsKp? • Explain what K means for the rxn?

  13. The Reaction Quotient When the reaction is NOT at equilibrium for a given time, the reaction quotient, Q takes the place of K, the equilibrium constant, in the law of mass action. jA + kB  lC + mD Useful in determining what will happen under special conditions.

  14. Significance of the Reaction Quotient • If Q=K, the system is at equilibrium • If Q > K, the system shifts to the left, consuming products and forming reactants until equilibrium is achieved • If Q < K, the system shifts to the right, consuming reactants and forming products until equilibrium is achieved

  15. Solving for Equilibrium Concentration Consider this reaction at some temperature: H2O(g) + CO(g)  H2(g) + CO2(g) K = 2.0 Assume you start with 8 molecules of H2O and 6 molecules of CO. How many molecules of H2O, CO, H2, and CO2 are present at equilibrium? Here, we learn about “RICE” – the most important problem solving technique in the second semester. You will use it for the next 4 chapters!

  16. Solving for Equilibrium Concentration H2O(g) + CO(g)  H2(g) + CO2(g) K = 2.0 Step #1: We write the law of mass action for the reaction:

  17. Solving for Equilibrium Concentration Step #2: We “RICE” the problem, beginning with the R for balanced reaction Reaction H2O(g) + CO(g)  H2(g) + CO2(g) 8 6 0 0 -x -x +x +x 8-x 6-x x x

  18. Solving for Equilibrium Concentration Step #3: We plug equilibrium concentrations into our equilibrium expression, and solve for x H2O(g) + CO(g)  H2(g) + CO2(g)

  19. Solving for Equilibrium Concentration Step #4: Substitute x into our equilibrium concentrations to find the actual concentrations H2O(g) + CO(g)  H2(g) + CO2(g)

  20. LeChatelier’s Principle When a system at equilibrium is placed under stress, the system will undergo a change in such a way as to relieve that stress. Translated: The system undergoes a temporary shift in order to restore equilibrium.

  21. LeChatelier Example #1 A closed container of ice and water is at equilibrium. Then, the temperature is raised. Ice + Energy  Water The system temporarily shifts to the _______ to restore equilibrium. right

  22. LeChatelier Example #2 A closed container of N2O4 and NO2 is at equilibrium. NO2 is added to the container. N2O4 (g) + Energy  2 NO2(g) The system temporarily shifts to the _______ to restore equilibrium. left

  23. LeChatelier Example #3 A closed container of water and its vapor is at equilibrium. Vapor is removed from the system. water + Energy  vapor The system temporarily shifts to the _______ to restore equilibrium. right

  24. LeChatelier Example #4 A closed container of N2O4 and NO2 is at equilibrium. The pressure is increased. N2O4 (g) + Energy  2 NO2(g) The system temporarily shifts to the _______ to restore equilibrium, because there are fewer moles of gas on that side of the equation. left

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