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Percent Reaction and Q Chem 12 Page 509 & 517-520

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Percent Reaction The yield of product measured at equilibrium compared with the maximum possible yield of products. Example: H 2 + I 2 2HI [HI] at equilibrium =1.56M [HI] quantitative in forward reaction =2.00M %rxn = [HI]eq/ [HI]frxn x 100% = 1.56M/2.00M x 100% = 78%

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Need to Know: < 50% = reactants favored = small K > 50% = products favored = large K > 99% = Quantitative Forward reaction Totally a forward reaction, no equilibrium

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Reaction Quotient, Qc Identical expression to Kc, but its value is calculated using concentrations that aren’t necessarily found at equilibrium. If Qc> Kc = shifts to reactants If Qc< Kc = shifts to products If Qc = Kc = equilibrium

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Practice Problem: Calculate the Q for the following reaction using these equilibrium concentrations: [N 2 ] = 0.10 M; [H 2 ] = 0.30 M; and [NH 3 ] = 0.20 M. Is the system at equilibrium if Kc of the reaction is 0.40? N 2 + 3H 2 2NH 3 Answer: Not at equilbrium, Q>K, so shift to left (reactants)

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Reversible Reactions Reactions are spontaneous if G is negative. If G is positive the reaction happens in the opposite direction. 2H 2 (g) + O 2 (g)

Reversible Reactions Reactions are spontaneous if G is negative. If G is positive the reaction happens in the opposite direction. 2H 2 (g) + O 2 (g)

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