Study Guide

The Equilibrium Constant

The Equilibrium Constant

We're Constantly Amazed By Your Equilibrium

Have you heard about yin and yang? They're total opposites of each other, but they're also connected together in a bunch of deep ways. Not to get all philosophical (too late?), but equilibrium is a little bit like this. A reversible chemical reaction appears to be two polar opposite reactions, but the two opposite reactions are also influenced by the surroundings as well.

We like to think that it was a mighty philosopher-chemist who figured out our next powerful formula. We start with a chemical reaction that has the equation . If it has reached equilibrium at constant temperature and pressure, then the equilibrium constant, Keq, can be described as follows:

What the heck is the equilibrium constant? Well this fancy-shmancy value describes how a reaction will proceed. At equilibrium, this value is calculated as the concentration of the products X and Z, raised to the power of their coefficients, divided by the concentration of the reactants T and V raised to the power of their coefficients. Plug and chug.

For a given reaction under specific conditions, the concentrations of reactants and products will change until the equilibrium constant is reached. In other words, there is a constant value that describes the yin and yang of a chemical reaction. Now let's meditate on that.

Here's a quick tip to keep us on your toes. It's general practice not to include units for the equilibrium constant. Consequently, K has absolutely no units.

How can knowing the equilibrium constant help us in our everyday lives? Oh we're so glad you asked. Think about the following reaction.

Now let's pretend we have Dumbledore-like powers and we can freeze time when the concentrations are exactly [A] = 1M, [B] = 2M and [C] = 1M. Let's say the value of Keq for this particular reaction is 2. Remember K doesn't have units.

Now, let's wave our magic wand and unfreeze time. Will the reaction proceed forward or will the reaction proceed backwards? The answer can be found by peering into our magical equation.

Let's calculate the value of the reaction quotient, Q, at the moment we froze time.

We compare this value to the Keq value, 2, which was so conveniently given to us. The reaction will proceed in the direction that will make Q equal to Keq. In this example, that means the reaction will move in the forward direction.

Nice. One little equation makes us look like the Nostradamus of the chemical reaction world.



Portrait of Nostradamus
I predict that in the year 2029 that the reaction will go from right to left.

(Source)

Here's a quick summary:

Q < Keq means the ratio for the concentrations of products to reactants is too small. To reach equilibrium, reactants must be converted to products. The system proceeds from left to right to reach equilibrium.

Q = Keq means the system is at equilibrium, probably relaxed on the couch with some popcorn, binging on Netflix.

Q > Keq means the ratio for the concentrations of products to reactants is too large. To reach equilibrium, products must be converted to reactants. The system proceeds from right to left to reach equilibrium.

Brain Snack

We don't know about you, but these are pretty similar to the faces we make when we discover something new for the first time.