Barrons SAT Subject Test Chemistry, 13th Edition

TIP

Notice that as the forward reaction rate decreases, the reverse reaction rate increases until

they are at “equilibrium.”

We see that, for the given reaction and the given conditions, Keq is a

constant, called the equilibrium constant. If Keq is large, this means that

equilibrium does not occur until the concentrations of the original reactants are
small and those of the products large. A small value of Keq means that equilibrium

occurs almost at once and relatively little product is produced.
The equilibrium constant, Keq, has been determined experimentally for many

reactions, and the values are given in chemical handbooks.
Suppose we find Keq for reacting H 2 and I 2 at 490°C to be equal to 45.9.

Then the equilibrium constant for this reaction

H 2 (g) + I 2 (g) 2HI(g) at 490°C

is

Sample Problem

Three moles of H 2 (g) and 3.00 moles of I 2 (g) are introduced into a 1-liter box at a

temperature of 490°C. Find the concentration of each substance in the box when
equilibrium is established.
Initial conditions:

The reaction proceeds to equilibrium and

At equilibrium, then,

[H 2 ] = (3.00 – x)

mol/L

(where x is the number of moles of H 2 that are in the form of HI

at equilibrium)