1.1 What is Chemistry?

20.3. Free Energy and Equilibrium http://www.ck12.org

20.3 Free Energy and Equilibrium

Lesson Objectives

• Use the reaction quotient to determine which direction a reaction must run in order to reach equilibrium.


• Use the equilibrium constant of a reaction to calculate the change in Gibbs free energy.


• Use changes in Gibbs free energy to calculate equilibrium constants.


• Predict whether the reactants or products of a reaction are favored under a given set of conditions by looking
  at either the equilibrium constant or the change in Gibbs free energy.

Lesson Vocabulary

• reaction quotient (Q): The ratio of the reactant and product concentrations in their non-equilibrium states,
  raised to their respective exponents.

Check Your Understanding

• What data is needed to calculate the change in Gibbs free energy for a reaction?


• What information is needed to calculate an equilibrium constant?

The Reaction Quotient

In the previous chapter, we learned how to set up an equilibrium constant expression. For the generic reaction shown
below, the equilibrium constant expression is constructed as follows:

aA+bBcC+dD

Keq=

[C]c[D]d

[A]a[B]b

It should be emphasized that the equation above is only true when the reaction is at equilibrium. Given enough
time, the indicated ratio of concentrations will eventually converge on the equilibrium constant, but for any reaction
in which net changes are still occurring, this equation is not true. For a reaction that is still in progress, it is often
useful to calculate the same ratio of concentrations, but in a non-equilibrium state. This value is called thereaction
quotient(Q), and comparing the value of Q to the value of Keqtells us which direction the reaction needs to shift in
order to reach equilibrium.

Keq=[C]

c[D]d

[A]a[B]b(Only if the reaction is at equilibrium)