Humboldt State University ® Department of Chemistry

Richard A. Paselk
Chem 107

Fundamentals of Chemistry

Fall 2008
Lecture Notes: 4 December

© R. Paselk 2005
  
     
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Equilibria

Chemical equilibria occur when the rates of the forward and the reverse reactions of a chemical system are identical.

Regardless of initial concentrations, systems will approach and reach equilibrium given time [see Figure 15.11, p 450] (overheads, approach to equilibria by carbon monoxide + water vs. carbon dioxide + hydrogen)

Vast numbers of chemical reactions operate at equilibrium in the natural world, and it is frequently essential to be able to understand and to predict their behavior. Qualitatively we can get an idea of how an equilibrium system will behave by using Le Châtelier's Principle.

Le Châtelier's Principle: If stress is applied to a system at equilibrium, the equilibrium will shift in such a way as to relieve the stress. e.g. if the pressure of carbon dioxide is increased over a solution of carbon dioxide in water, more carbon dioxide will dissolve, reducing the pressure increase.

Let's look at an equilibrium system, and try to predict its response using Le Châtelier's Principle and/or the equilibrium expression.

Example: Consider the reaction

CO + NO2 NO + CO2 + heat (226 kJ)

Note that heat appears on the product side - the system is giving up heat, \ DH is negative, DH = - 226 kJ

So, what will happen to [CO2] if:

    • CO is added?
    • NO2 is added?
    • NO is added?
    • T is increased?
    • V is increased?
    • Ar is added?

 

The Equilibrium and Mass Action Expressions

Quantitatively we can look at the relationship in a reaction at equilibrium represented by

A + B C + D

At equilibrium this system consists of two reactions proceeding in opposite directions at the identical rate:

A + B C + D characterized by a constant, k1 to give a rate, r1 = k1[A] [B]

A + B C + D characterized by a constant, k2 to give a rate, r2 = k2[C] [D]

But if r1 = r2, then

k1[A] [B] = k2[C] [D], and gathering constants

k1/k2 = [C] [D]/[A] [B], the ratio of constants is given a new name, the equilibrium constant, Keq.

The Equilibrium Expression is then:

Keq = [C] [D]/[A] [B].

A similar expression is the Mass Action Expression:

Q = [C] [D]/[A] [B].

The mass action expression is algebraically identical to the equilibrium expression, but it applies to a more general case. That is, the equilibrium expression requires that the values in the expression give the equilibrium constant, whereas the mass action expression allows any set of values. Thus the mass action expression is used to describe a system which has not yet reached equilibrium, while the equilibrium expression is a special case of the mass action expression for a system at equilibrium.

 

 

Let's look at a number of examples and see how to use the equilbirium expression to follow the equilibrium process.

Example: Consider the gas phase reaction:

PCl5 PCl3 + Cl2

Keq = 5.0 x 10-2 @ 150 °C

Find [Cl2] if [PCl5] = 0.40 M and [PCl3] = 0.20 M.

 

Example: Consider the reaction of carbon monoxide and water to give carbon dioxide and hydrogen. Calculate the concentrations of all species at equilibrium if we start with 0.341 moles each of carbon monoxide and water in a 2.71 L container @ 600 K. Keq = 302.

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Last modified 4 December 2008