The Foundations of Chemistry

When we add more A to an equilibrium mixture, this rate increases so that it no longer

matches the rate of the reverse reaction. As the reaction proceeds to the right, consuming

some A and B and forming more C and D, the forward rate diminishes and the reverse

rate increases until they are again equal. At that point, a new equilibrium condition has

been reached, with more C and D than were present in the originalequilibrium mixture.

Not all of the added A has been consumed when the new equilibrium is reached, however.

If a reactant or product is removedfrom a system at equilibrium, the reaction that

produces thatsubstance occurs more rapidly and to a greater extent than its reverse. If

some C or D is removed, then Q Kc, and the forward reaction is favored until equilib-

rium is reestablished. If some A or B is removed, the reverse reaction is favored.

Direction of Shift of

Stress Q AB 34 CD

Increase concentration of A or B Q K left88nright

Increase concentration of C or D Q K left right

Decrease concentration of A or B Q K left right

Decrease concentration of C or D Q K left88nright

88n

88n

The terminology used here is not
as precise as we might like, but it is
widely used. When we say that the
equilibrium is “shifted to the left,”
we mean that the reaction to the left
occurs to a greater extent than the
reaction to the right.

This tabulation summarizes a lot of
useful information. Study it carefully.

722 CHAPTER 17: Chemical Equilibrium

Effects of changes in concentration on the equilibrium

[Co(OH 2 ) 6 ]^2 4Cl 34 [CoCl 4 ]^2 6H 2 O

A solution of CoCl 2 T6H 2 O in isopropyl alcohol and water is purple (not shown) due to

the mixture of [Co(OH 2 ) 6 ]^2 (pink) and [CoCl 4 ]^2 (blue). When we add concentrated HCl,

the excess Clshifts the reaction to the right (blue, right). Adding AgNO 3 (aq) removes

some Clby precipitation of AgCl(s) and favors the reaction to the left (produces more

[Co(OH 2 ) 6 ]^2 ; the resulting solution is pink (left). Each insert shows the structure of the

cobalt complex species present in higher concentration; other ions and solvent molecules

are not shown.