Chemistry - A Molecular Science

Chapter 9 Reaction Energetics

directions. Writing the reaction as CH

I + OH 3

1-^

U

CH

OH + I 3

1- shows that it is an

equilibrium process that consists of a forward reaction (CH

I + OH 3

1-^

→

) and a reverse

reaction (

←

CH

OH + I 3

1-) occurring simultaneously.

As a result of the back reaction, there will

always be at least some of the reactants

(substances on the left side of the chemical equation) present at equilibrium;

i.e.

, the

limiting reactant does not disappear complete

ly. How far the reaction proceeds before

reaching equilibrium is established by the

equilibrium constant, K

, for the reaction. The

equilibrium constant expression can be de

rived from the thermodynamic definition of

equilibrium, but we use the kinetic definition in the following.

At equilibrium, the rate of the forward reacti

on equals the rate of the reverse reaction,

so R

= Rf

. Substitution of the rate laws for the r

forward and reverse reactions, produces the

following for an equilibrium system:

k[CHf

I][OH 3

1-] = k

[CHr

OH][I 3

1-]

where the concentrations are the equilibrium c

oncentrations. Next, we divide both sides of

the equation by k

and then by [CHr

I][OH 3

1-] to get the constants (k

and kf

) on one side and r

the equilibrium concentrations on the other. The ratio of two constants

is a constant, so the

ratio

kf/

is set equal to K, the equilibrium constant. kr

1-

3

f

1-

r

3

[CH OH][I ]

k

= K, the equilibrium constant

k

=[CH I][OH ]

Eq. 9.7

The equilibrium mixture must satisfy the e

quilibrium constant regardless of how it is

formed. Thus, the same concentrations are pr

oduced by the reaction of one mole each of

CH

OH and I 3

1- or one mole each of CH

I + OH 3

1-.

The equilibrium constant expression for a reaction in which all of the

reactants and

products are in aqueous solution

is the product of the concentrations of the substances on

the right side of the reaction (products) divide

d by the product of the concentrations of the

substances on the left side of the reaction (reactants). The concentration of each substance is raised to an exponent equal to its

coefficient in the balanced equation.

In the reaction

between CH

I and OH 3

1-, all of the coefficients are one

, so the exponents do not appear

explicitly. Most of the reactions studied in th

e remainder of the text take place in aqueous

solutions, so consider the following generic equilibrium in water* and its equilibrium constant expression.

* Recall that the abbreviation ‘aq’ is used to show that a substance

is in an aqueous solution.

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