Reaching Equilibrium

A reaction at equilibrium has unchanging observable properties. However, a reaction doesn't stop when it reaches equilibrium. It just looks like it has stopped.

Instead, it continues, but it goes just as fast in the direction from reactants to products (the forward direction), as it does from products to reactants (the reverse direction). Understanding equilibrium requires an understanding of reaction rates, which you may wish to review now.

In order for a reaction to reach an equilibrium, the reaction must be reversible. In other words, not only must the reactants be able to turn into products, but the products must be able to turn back into reactants.

The liquid water – water vapor equilibrium described in the previous video is an obvious example of a reversible reaction:

Liquid water can evaporate. The reaction in the left to right direction is called the forward reaction.
H₂O (l)

H₂O (g)

Water vapor can condense. The reaction in the right to left direction is called the reverse reaction.
H₂O (l)

H₂O (g)

If the container is closed, then eventually the water will evaporate and condense at the same speed. When this happens, we'll see no further change to the level of water in the container.

A reaction that has reached equilibrium appears to be doing nothing. But at the molecular level, equilibrium is a dynamic state of equality, where the molecules continue to react at equal but opposite rates.

When we put liquid water into a sealed container, at first it only evaporates. Initially there is no water vapor, so only the forward reaction occurs. The reverse rate of condensation is initially zero. However, as more water vapor accumulates, the reverse rate begins to increase. At some point, equilibrium is reached when the rate in the forward direction = the rate in the reverse direction. At no time will the concentration of the liquid water equal the water vapor.

In this animation the amount of liquid water, and concentration of the water vapor are represented by the color intensity. The rate of the forward and reverse reactions are indicated by the size of the arrows.

Notice how the rate in the reverse direction increases as the concentration of water vapor goes up. The concentration of the liquid and the vapor will not be the same, but the system will still be in equilibrium if the forward and reverse reaction rates are equal.

To show the equality of rate a double arrow symbolizes equilibrium. The water vapor equilibrium would be written as: H₂O (l)

H₂O (g)

Another example of an equilibrium with which everyone is familiar is a saturated solution. If you put enough salt into water, eventually the salt will stop dissolving. No matter how much more salt you add, no more will dissolve. The excess remains undissolved on the bottom of the container. This is an equilibrium, which we could recognize by the constant concentration of the salt solution:

Reactants	NaCl (s)		Na⁺(aq) + Cl^-(aq)	Products
In a saturated solution, no more solid salt appears to dissolve		The rate in the forward and reverse directions is equal	The aqueous ions crystallize, just as fast as they are formed