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Chapter 13 | Fundamental Equilibrium Concepts 685
together) over the reactants (also multiplied together), with each concentration raised to the power of the coefficient of that substance in the balanced chemical equation. For example, the reaction quotient for the reversible reaction
is given by this expression:
Example 13.1
Writing Reaction Quotient Expressions
Write the expression for the reaction quotient for each of the following reactions: (a)
(b)
(c)
Solution
(a)
(b)
(c)
Check Your Learning
Write the expression for the reaction quotient for each of the following reactions: (a)
(b)
(c)
Answer: (a) (b) (c)
The numeric value of Qc for a given reaction varies; it depends on the concentrations of products and reactants present at the time when Qc is determined. When pure reactants are mixed, Qc is initially zero because there are no products present at that point. As the reaction proceeds, the value of Qc increases as the concentrations of the products increase and the concentrations of the reactants simultaneously decrease (Figure 13.6). When the reaction reaches equilibrium, the value of the reaction quotient no longer changes because the concentrations no longer change.
