The most obvious reaction is a reaction where it seems to proceed as a forward reaction, where the products are different than the reactants, such as fire burning wood (combustion reaction) or rusting of a nail (oxidation reaction). This is a one way reaction in which the product is irreversible. An irreversible reaction is one where the reaction will continue until all the reactant is used up to make the product.
4 Fe(s) + 6 H2O(l) + 3 O2(g) -> 4 Fe(OH)3(s) (chemical equation of rust)
As you can see, the arrow points to the right, hence a one-way (irreversible) reaction.
In dynamic equilibrium, the reactions are known to be reversible. A reversible reaction is one where the products break apart to reform the reactants. Therefore, the reaction proceeds to the right (products) and the reaction proceeds to the left (reactants) until equilibrium is reached. Equilibrium exists when the reaction moves to the right (products) at the same rate it moves to the left (reactants), thus there is no net change. It is important to realize that this is an active process – although it seems stagnant due to the zero net change.
Note the arrow points in both directions, signifying a reaction in dynamic equilibrium.
Consider the dynamic equilibrium between nitrogen dioxide gas and dinitrogen tetroxide gas:
The equation above is a reversible reaction in dynamic equilibrium. Nitrogen dioxide gas (NO2) is produced in the exhaust of a car. Nitrogen dioxide gas in the atmosphere then converts to dinitrogen tetroxide gas, N2O4. The reaction in which NO2 (a brownish component of smog) forms N2O4 (a colorless component of smog) is the forward reaction. The reaction in which N2O4 forms NO2 is the reverse reaction. When this reaction is at equilibrium, it will have reached the active process of the reactants forming the products and the products reforming the reactants at a constant rate.
This constant rate means that the concentration of NO2 and N2O4 is also constant. Thus, dynamic equilibrium is the state in which the concentration of the products and reactants are constant because the rate at which the products are formed and the rate at which the reactants are reformed is constant. Consider the picture below:
It is important to realize that dynamic equilibrium does not mean that the number of moles or the concentration of the products and reactants are the same. It is the rate at which the products are formed and the reactants are reformed that is the same.
Dynamic Equilibrium Review:
1. What characterizes a reversible reaction?
2. Describe what it means when a reaction is in a state of dynamic equilibrium.
3. How do the rates of a forward and reverse reaction change throughout a reaction?
4. How do the concentrations of the forward and reverse reaction relate?