Chapter 15 – Chemical Equilibrium

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enough background energy to allow both reactions proceed at an appreciable rate, so both materials are present in the reaction vessel at all times. E n e r g y E a (forward) reactants Reaction progress Figure 1 E a (reverse) products If we start with pure A in a container, initially the forward rate will be large and the reverse rate zero. The forward rate will then slow (A is used up) and the reverse rate will accelerate (B is formed) until they become equal. If pure B is used, then the “reverse” rate will be large and the “forward” rate will be zero. Likewise, the large rate will slow down and the small rate will increase until they equalize. At equilibrium k f [A] = k r [B] The following graphs show representative plots for the change in concentration for two species with time (Figure 2) and the change in the rates of A disappearance and B appearance as the system approaches equilibrium (Figure 3). concentration 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 [A] [B] 0 5 10 15 20 25 30 time rate 1 0.8 0.6 0.4 0.2 0 k f [A] k r [B] 0 5 10 15 20 25 30 time Figure 2 Figure 3
3 The previous equality can be rearranged to produce a useful equation. k k f r [ B] = = K [ A ] eq where K eq is the equilibrium constant. Always remember that equilibria are dynamic processes, both the forward and reverse reactions are always occurring. 15.2 The Equilibrium Constant The previous equation can be generalized for any combination of materials in solution at equilibrium: aA + bB K eq = c [C] [D] a [A] [B] cC + dD d b If the same reactants were in the gas phase, the partial pressures of the gases are used in place of concentrations and the equilibrium constant becomes: K eq = P P c d C PD a b A PB It is important to use the “eq” as a subscript to help differentiate between the equilibrium and rate constants (handwriting sometimes makes distinguishing a capital K from a little k difficult). Example: N 2 (g) + 3 H 2 (g) 2 NH 3 (g) PNH3 K eq = 3 P P N2 2 H2 Thus, the equilibrium constant expression depends only on the reaction stoichiometry. For this reason, it is convention to write out the stoichiometry using only whole numbers. Using fractions can change the equilibrium constant. For example: [C] A + 2 B C K eq = 2 [A][B]
Page 1: Chapter 15 - Chemical Equilibrium C
Page 5 and 6: 5 As you can see, the pressures of
Page 7 and 8: 7 Units of Equilibrium Constants As
Page 9 and 10: Example: A mixture of 5.000 x 10 -
Page 11 and 12: 11 The other extreme occurs when th
Page 13 and 14: 13 x 1 = 1.47 x 10 - 3 atm and x 2
Page 15 and 16: 15 If some A is added, once again t
Page 17: 17 The Effect of Catalysts First re

Chapter 15 – Chemical Equilibrium

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