When you have the balanced equation, and its Kc expression, you can calculate the mathematical value of K if you know the concentration of each substance in the expression at equilibrium.
Note the words in bold, because sometimes in a problem you will know other concentrations instead. These are more complicated to work out.
For now we will stick to the simple problems, when we know all the values at equilibrium.
Example 1: What is the equilibrium constant Kc
for the reaction H2 (g) + I2 (g)  2HI (g) at 450
șC, when at equilibrium, [H2] = 3.4 x 10-2 mol/L, [I2]
= 9.0 x 10-3 mol/L and [HI] = 8.3 x 10-2 mol/L?Step
1: Write out the K expression for the balanced equation
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What about the temperature? Remember that temperature changes the value of Keq, so that is why the temperature is almost always stated in an equilibrium constant problem. However, it is not necessary to know the temperature in order to solve the problem.
| Temperature changes the value of K, but it is not included in the Keq expression. |
What about units? Shouldn't they be included? You'll usually see Kc's expressed without them. For example, in the above calculation, since each concentration was in mol/L units, there actually is no unit.
However, for the equation N2 (g) + 3H2 (g) 2NH3
(g), the units would be:
Thus there really isn't one unit for equilibrium constants, so you will almost never see them written down. In fact, if we were to write our expressions in terms of activities, there would be no unit at all. Equilibrium constants are dimensionless.
| Don't include the units when writing out an equilibrium constant expression. |
| Example 2: What is the equilibrium constant K for the
reaction: 2H2 (g) + O2 (g) 2H2O (g) if at a temperature of 4000
șC, the equilibrium concentrations are: [H2] = 0.0159 mol/L, [O2]
= 0.00794 mol/L, and [H2O] = 0.0281 mol/L? Step 1:
Write out the K expression for the balanced equation
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Try the following problems:
| When answering these problems, use "E" notation to enter numbers in scientific notation on the computer. For example: 2.34 x 105 is 2.34E5, 2.34 x 10-5 is 2.34E-5 |
| At 986 șC, the water gas shift equilibrium can be represented by this equation: H2
(g) + CO2 (g) H2O (g) + CO (g). If the following are the concentrations
(in mol/L) at equilibrium of each substance, then what is the value of Kc? [H2] = 6.92 x 10-4, [CO2] = 4.55 x 10-3, and [H2O] and [CO] are equal and = 2.22 x 10-3 |
| From now on, we won't specifically identify K expressions written in terms of concentration as Kc. When you see the symbol K assume it is a Kc unless told otherwise. |
| At 500 șC, the ammonia synthesis reaction N2 (g) + 3 H2
(g) 2NH3
(g) is in equilibrium with [N2] and [H2] both = 0.120, and [NH3]
= 3.50 x 10-3 mol/L. What is K?
|
| At 500 K, nitrogen dioxide gas will be in equilibrium with nitrogen
monoxide and oxygen gases: 2NO (g) + O2 (g) with [NO] = 1.14 x 10-3, [O2] = 5.70 x 10-4, and [NO2] = 2.42 x 10-2 mol/L. What is K? |
When you know each concentration of every species at equilibrium, simply substitute into the proper K expression, and do the math.
