Chem – Equilibrium Definition

What is an Equilibrium?

This is one of the most confusing concepts and chapters in chemistry. The reason is not many people tend to think of an equilibrium in their lives. An equilibria or an equilibrium is creating a balance. This does not have to be an equal balance. Here is example of a non-chemical equilibrium. You strike a balance in your every day life. The balance is between work, sleep, and school. Not everyone creates a perfectly equal balance. That is, not everyone works 8 hours, sleeps 8 hours, and goes to school 8 hours a day. Some work 4 hours, sleep 7 hours, and go to school 11 hours a day. Yet that is still creating an equilibrium. It is a day to day or week to week routine that they keep relatively constant. People create a balance in their life that works to maintain whatever goals they have.


Lets try another word based example of an equilibrium. You and your employer maintain an equilibrium with your hourly wages. You make 10 dollars an hour and your employer makes 20 dollars an hour. As long as the same hourly wages continue you are striking an equilibrium. You are maintaining a constant ratio between your payment and the employer’s payment.


An equilibrium is simply a balance to maintain some kind of condition or achieve some kind of goal.


In chemistry the definition of an equilibrium is when you achieve a relatively stable concentration of products and reactants. Another way to say it is when the rate of the forward reaction equals the rate of the reverse reaction. How we measure or describe an equilibrium in chemistry is we say it maintains a balance between the ratio of the concentration of the products to the concentration of the reactants. Whenever anything is in brackets ( [ ] ) in chemistry it means concentration in molarity. In a math formula the equilibrium looks like what is in the box below:


Equilibrium = [ products ]
[ reactants ]


Books and tests tend to refer to equilibrium by a number and they use a constant letter to reference that number. The letter is K (the equilibrium constant). Whether they call it Keq, or Kc, or Kp, or Ka, or Kb, or Ksp it is really all the same thing. Do not let that fool you. So if we put K into the equation it tends to look like this:


K = [ products ]
[ reactants ]


Examples: Give the equilibrium constant for the following examples.


Apples <—> Oranges

If the concentration of apples is 2 and the concentration of oranges is 10

[ 10 ] / [ 2 ] K = 5
Boys <—> Girls

If the concentration of girls is 3 and the concentration of boys is 9

[ 3 ] / [ 9 ] K = 1/3


No matter how fancy the problem sounds always remember that K is nothing more than a ratio.


PRACTICE PROBLEMS: Give the equilibrium constant for the problems below.


If the concentration of cars in the city is 5 and the concentration of people is 20. What is the ratio or equilibrium constant for the equation below?

Cars <—> People

Answer: 20 / 5 = K = 4


If the concentration of birds in a forest is 500 and the concentration of trees is 400. What is the ratio or equilibrium constant for the equation below?

Birds <—> Trees

Answer: 400 / 500 = K = 0.8


If the concentration of teacher at a school is 25 and the concentration of students is 125. What is the ratio or equilibrium constant for the equation below?

Students <—> Teachers

Answer: 25 / 125 = K = 0.2


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