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Chp7_ExtensiveApx

# Chp7_ExtensiveApx - When Might a Reaction be Safely...

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When Might a Reaction be Safely Approximated as “Extensive”? Let us consider the hypothetical reaction: D C B A + + for which the equilibrium constant is K . Let us further imagine initial concentrations (designated by ‘naught’) given by: [A] 0 = 7.00 mM[B] 0 = 9.00 mM [C] 0 = [D] 0 = 0.00 Case I. If the equilibrium constant is “large”, we assume the reaction is extensive and construct the following reaction table: A + B = C + D i 7.00 9.00 0.00 0.00 –7.00 –7.00 +7.00 +7.00 f ~0.00 2.00 7.00 7.00 The final concentrations must satisfy the constraint of the equilibrium constant. A literal zero for [A] = is unacceptable. To compute this, we assume that the reaction “back reacts” a SMALL amount: small here meaning that the amount that “back reacts” to form A is small compared to the amounts of B, C, and D. Hence those values remain effectively unchanged and thus we use them in the equilibrium constant as follows (all concentrations are equilibrium values, I only use the subscript ‘=’ for emphasis): [ ] [ ] [ ] [ ] B A D C K × × = or [ ] [ ] [ ] [ ] B K D C A × × = =

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