The Foundations of Chemistry

Kc
Kc
 4 .0 10 ^4  2.0 102

You should now work Exercise 30.

THE REACTION QUOTIENT

The reaction quotient, Q,for the general reaction is given as follows.

not necessarily

For aAbB 34 cCdD, Q

[

[

C

A

]

]

c

a

[

[

D

B]

]

b

d

 equilibrium

concentrations

The reaction quotient has the same formas the equilibrium constant, but it involves specific

values that are not necessarilyequilibrium concentrations. If they areequilibrium concen-

trations, then QKc. The concept of the reaction quotient is very useful. We can compare

the magnitude of Qwith that of Kcfor a reaction under given conditions to decide whether

the forward or the reverse reaction must occur to a greater extent to establish equilib-

rium.

We can think of the reaction quotient, Q,as a measure of the progress of the reaction.

When the mixture contains onlyreactants, the concentrations in the numerator are zero,

so Q0. As the reaction proceeds to the right, the product concentrations (numerator)

increase and the reactant concentrations (denominator) decrease, so Qincreases to an in-

finitely large value when all reactants have been consumed and only products remain.

The value of Kcis a particular value of Qthat represents equilibrium mixtures for the

reaction.

If at any time Q Kc, the forward reaction must occur to a greater extent than the

reverse reaction for equilibrium to be established. This is because when Q Kc, the

numerator of Qis too small and the denominator is too large. To increase the numerator

and to reduce the denominator, A and B must react to produce C and D. Conversely, if

Q Kc, the reverse reaction must occur to a greater extent than the forward reaction for

equilibrium to be reached. When the value of Qreaches the value of Kc, the system is at

equilibrium, so no further netreaction occurs.

17-4

[HCl][Br 2 ]1/2



[HBr][Cl 2 ]1/2

The reaction quotient is sometimes
called the mass action expression.

When the forward reaction occurs
to a greater extent than the reverse
reaction, we say that a netforward
reaction has occurred.

716 CHAPTER 17: Chemical Equilibrium

m88 6

(^66)
m88
Reaction proceeds toward right,
forming more products
Reaction proceeds toward left,
forming more reactants
Q = 0
(reactants only)
Increasing Q
Kc
Q = ∞
(products only)
Q Kc Forward reaction predominates until equilibrium is established.
QKc System is at equilibrium.
Q Kc Reverse reaction predominates until equilibrium is established.