Solve homogeneous equilibrium problems using the expression for Kc
A homogenous equilibrium system is when all the substances involved in the reaction are in the same state. For the reaction:
The equilibrium constant expression is:
This is valid, provided the temperature and concentrations are constant. The larger the equilibrium constant is, the more of the products there will be. Ensure that you quote the temperature with this value, otherwise it cannot be reused for other calculations!
If Kc >> 1, the reaction goes almost to completion If Kc << 1, the reaction hardly proceeds
Kc can never equal 0.
The equilibrium constant always has different units, so these must be calculated each time. They are found by cancelling the units in the equation.
We can use data from experiments to work out the equilibrium constant, telling us the concentrations of the substances when they reach equilibrium. If we obtain data from multiple experiments, then we can find the average equilibrium constant for all of them.
Using the equation, we can find the value of Kc. DON’T PANIC: you will never be asked to perform a calculation of Kc that involves quadratic formulas.
Other things to consider:
- Since you are not always given the information in this format, you may have to find the concentrations of the reactants and products yourself, using other data. When the questions become longer like this, then make sure you set out your work properly!
- If the equilibrium is found while the reaction is still progressing, then:
- If it is increasing, it is continuing towards the right
- If it is decreasing, then it is continuing towards the left