The
reaction quotient Q is an expression that is sometimes referred to as the
equilibrium constant (K) expression. This expression is different for different chemical reactions.
How do I write Q for a particular chemical reaction? Q is shown for the general chemical reaction: aA + bB → cC + dD
where substances A, B, C and D are all solutes or gases.
AND a, b, c, d are the numbers required to balance the equation
How do I recognize which substances are solutes and gases? Solutes in aqueous systems have (
aq) after their formula in the equation for the reaction
Gases have (
g) after their formula in the equation for the reaction
What about substances in the equation with (s) or (l) after their formula?Solids and pure liquids do not appear in
Q expressions because their standard states are pure solid and pure liquid. Therefore when these are divided by their standard state, the result is 1.
What does [ ] mean? [A] means concentration of A in
divided by the standard state concentration (1
). Thus
Q is a number. The
c in
Qc indicates that concentrations have been used.
What now?Practice constructing
Q expressions on the next few pages.
Later in the module you will learn
how Q relates to K and
how comparing Q and K can allow you to make some useful predictions.