Question

± Adding Equilibrium Reactions

Learning Goal:

To understand how to calculate the equilibrium constants for chemical equations that can be produced by the addition of other chemical equations with known equilibrium constants.

For a chemical reaction equation with the general form

aA+bB⇌cC+dD

the equilibrium equation is given by

K1=[C]c[D]d[A]a[B]b

Thus, for a chemical reaction equation with the general form

cC+dD⇌eE+fF

the equilibrium equation is given by

K2=[E]e[F]f[C]c[D]d

If the first two equations are added together such that

aA+bB⇌eE+fF

then the equilibrium equation is given by

K3=[E]e[F]f[A]a[B]b=[C]c[D]d[A]a[B]b⋠[E]e[F]f[C]c[D]d

or

K3=K1⋠K2

Thus, when a chemical equation is the sum of two chemical equations for which equilibrium constants are already known, the equilibrium constant for the reaction is the product of the equilibrium constants for the individual reactions. Keep in mind that equilibrium equations do not include expressions for any pure solids or liquids that may be involved in the reaction.

Part A

Given the two reactions

H2S⇌HS−+H+, K1 = 9.86×10^−8, and

HS−⇌S2−+H+, K2 = 1.25×10^−19,

what is the equilibrium constant Kfinal for the following reaction?

S2−+2H+⇌H2S

Enter your answer numerically.

Kfinal =

7.14•1025

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Part B

Given the two reactions

PbCl2⇌Pb2++2Cl−, K3 = 1.76×10^−10, and

AgCl⇌Ag++Cl−, K4 = 1.13×10^−4,

what is the equilibrium constant Kfinal for the following reaction?

PbCl2+2Ag+⇌2AgCl+Pb2+

Express your answer numerically.

Kfinal =

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