- The Reaction Quotient - 1

 Q c is used to determine if any closed system is at equilibrium – and, if not, in which direction the system will shift to reach equilibrium (prediction)  Q c uses measured concentration values of a system in the equilibrium expression 2

 Remember, for the general chemical reaction aA + bB cC + dD 3

Q c is similar to K c, the major difference being that K c is calculated using concentrations at equilibrium, whereas Q c may or not be at equilibrium. Results of calculations MUST be one of three possible situations 4

(1) Q is less than K. The ratio of the concentrations of products to the concentrations of reactants is smaller than when the chemical system is at equilibrium. To move Q toward K, the concentrations of the products must increase while the concentration of the reactants decreases. The chemical system shifts to the right, consuming reactants and forming products until equilibrium is reached and Q equals K. 5

(2) Q is equal to K. The chemical system is at equilibrium, so no shift will occur. 6

(3) Q is greater than K. The ratio of the concentrations of products to the concentrations of reactants is larger than when the chemical system is at equilibrium. To move Q toward K, the concentrations of products must decrease and that of the reactants must increase. The chemical equilibrium system shifts to the LEFT and converts products into reactants until it achieves equilibrium and Q equals K. 7

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Example: Calculating Q c and Comparing it to K c A 50.0-L reaction vessel contains 1.00 mol N 2, 3.00 mol H 2, and mol NH 3. Will more ammonia, NH 3, be formed or will it dissociate when the mixture goes to equilibrium at 400ºC? The equation is N 2(g) + 3H 2(g) 2NH 3(g) K c is at 400 ºC 9

Reaction Quotient Questions p. 465 Practice UC # 1, 2 Solving for Equilibrium Concentrations p. 466 Practice UC # 3, 4 10