CHEMICAL EQUILIBRIUM Chemical Equilibrium Reversible Reactions A chemical
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 reactants remains unchanged 2 Hg. O(s) 2 Hg(l) + O 2(g) Arrows going both directions ( ) indicates equilibrium in a chemical equation
2 NO 2(g) 2 NO(g) + O 2(g) Remember this from Chapter 12? How do you predict the lines to go from this point forward?
2 NO 2(g) 2 NO(g) + O 2(g)
Law of Mass Action For the reaction: reaction j. A + k. B l. C + m. D Where K is the equilibrium constant, and is unitless
Product Favored Equilibrium Large values for K signify the reaction is “product favored” When equilibrium is achieved, most reactant has been converted to product
Reactant Favored Equilibrium Small values for K signify the reaction is “reactant favored” When equilibrium is achieved, very little reactant has been converted to product
Writing an Equilibrium Expression Write the equilibrium expression for the reaction: 2 NO 2(g) 2 NO(g) + O 2(g) K = ? ? ?
Conclusions about Equilibrium Expressions v The equilibrium expression for a reaction is the reciprocal for a reaction written in reverse 2 NO 2(g) 2 NO(g) + O 2(g) 2 NO 2(g)
Conclusions about Equilibrium Expressions v 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. 2 NO 2(g) 2 NO(g) + O 2(g) NO 2(g) NO(g) + ½O 2(g)
Equilibrium Expressions Involving Pressure For the gas phase reaction: 3 H 2(g) + N 2(g) 2 NH 3(g)
Heterogeneous Equilibria v 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: PCl 5(s) PCl 3(l) + Cl 2(g) Pure solid Pure liquid
The Reaction Quotient For some time, t, when the system is not at equilibrium, the reaction quotient, Q takes the place of K, the equilibrium constant, in the law of mass action. j. A + k. B l. C + m. D
Significance of the Reaction Quotient v If Q = K, the system is at equilibrium v If Q > K, the system shifts to the left, consuming products and forming reactants until equilibrium is achieved v If Q < K, the system shifts to the right, consuming reactants and forming products until equilibrium is achieved
Solving for Equilibrium Concentration Consider this reaction at some temperature: H 2 O(g) + CO(g) H 2(g) + CO 2(g) K = 2. 0 Assume you start with 8 molecules of H 2 O and 6 molecules of CO. How many molecules of H 2 O, CO, H 2, and CO 2 are present at equilibrium? Here, we learn about “ICE” – the most important problem solving technique in the second semester. You will use it for the next 4 chapters!
Solving for Equilibrium Concentration H 2 O(g) + CO(g) H 2(g) + CO 2(g) K = 2. 0 Step #1: We write the law of mass action for the reaction:
Solving for Equilibrium Concentration Step #2: We “ICE” the problem, beginning with the Initial concentrations H 2 O(g) + CO(g) H 2(g) + CO 2(g) Initial: 8 6 0 0 Change: -x -x +x +x Equilibrium: 8 -x 6 -x x x
Solving for Equilibrium Concentration Step #3: We plug equilibrium concentrations into our equilibrium expression, and solve for x H 2 O(g) + CO(g) H 2(g) + CO 2(g) Equilibrium: 8 -x 6 -x x x
Solving for Equilibrium Concentration Step #4: Substitute x into our equilibrium concentrations to find the actual concentrations H 2 O(g) + CO(g) H 2(g) + CO 2(g) Equilibrium: 8 -x Equilibrium: 8 -4=4 6 -x x x 6 -4=2 4 4
Le. Chatelier’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.
Le. Chatelier 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 right to restore equilibrium. _______
Le. Chatelier Example #2 A closed container of N 2 O 4 and NO 2 is at equilibrium. NO 2 is added to the container. N 2 O 4 (g) + Energy 2 NO 2 (g) The system temporarily shifts to the left _______ to restore equilibrium.
Le. Chatelier 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 right to restore equilibrium. _______
Le. Chatelier Example #4 A closed container of N 2 O 4 and NO 2 is at equilibrium. The pressure is increased. N 2 O 4 (g) + Energy 2 NO 2 (g) The system temporarily shifts to the left _______ to restore equilibrium, because there are fewer moles of gas on that side of the equation.
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