Chemical systems at equilibrium can be categorized in three major types: phase equilibria, solubility equilibria, and chemical reaction equilibria. Each is a type of dynamic equilibrium, meaning that the reaction rate of the forward reaction is equal to the reaction rate of the backward reaction. Remember, a system in dynamic equilibrium appears to be unchanging. However, microscopic changes are occurring.

1. Phase Equilibrium

   
   A phase equilibrium involves a single chemical substance existing in more than one phase in a closed system. Water placed in a sealed container is a good example of a phase equilibrium. At the macroscopic level, no change is apparent. This is because the rate at which molecules in the liquid phase are evaporating is equal to the rate at which molecules in the gas phase are condensing.

   
   
   

1.  
2. Solubility Equilibrium

   
   A solubility equilibrium is a type of dynamic equilibrium that involves a single chemical solute interacting with a solvent substance. In the saturated solution, the rates of dissolving and the crystallization are equal.

   
   
   

1.  
2.  
3. Chemical Reaction Equilibrium

   
   A chemical reaction equilibrium involves several substances, specifically the reactants and the products of a chemical reaction. At dynamic equilibrium, reactants are being converted into products and products are being converted into reactants. The rate of the forward reaction is equal to the rate of the reverse reaction. This does not mean that the reactants and products end with equal concentrations in the reaction vessel. Remember that the state of dynamic equilibrium refers to equal, opposite rates - and not to equal concentrations.

   
   Consider the chemical reaction shown below. The double arrows communicate that an equilibrium exists between the forward and reverse reactions.

   
   In the chemical reaction below, A and B are reacting to form the products C and D. This is the forward reaction.

   
   As C and D are produced, these two products are reacting to re-form the reactants. This is the reverse reaction.

   
   
   

   
   If the system is closed and if the temperature is held constant, the chemical reaction will achieve dynamic equilibrium. At this point, the reactants are creating the products at the same rate as the products are creating the reactants. In other words, both the forward and the reverse reactions are occurring at the same rate. Remember that at equilibrium the concentrations of every substance in the reaction remain constant and no observable changes occur in the system.

   
   Example

   
   Consider the reversible reaction shown below:

   
   \( \mathrm { H_2(g) + I_2(g) \leftrightharpoons 2HI(g)} \)

   
   Suppose we were to combine the reactants H₂(g) and I₂(g) in a sealed flask and measure the amount of product (HI) formed over time.

   • Initially, hydrogen and iodine react rapidly to form HI(g). The rate of this forward reaction, however, will decrease over time as the concentrations of the reactants decrease.
   • As HI is formed, it will react to re-form H₂(g) and I₂(g). Initially, the rate of this reaction is quite slow because very little HI(g) is in the reaction vessel. However, as the concentration of HI(g) increases (because of the forward reaction), the rate of the reverse reaction increases also.
   • At some point, the rates of the forward and reverse reactions will become equal. At this point, the system is said to be in a state of dynamic equilibrium.

   
   A concentration versus time graph can be used to depict a system approaching equilibrium. For example, the hydrogen-iodine system can be plotted as shown below:

   
   
   

   
   Initially (that is, at time =0), only hydrogen and iodine are present in the reaction vessel. After time 0, however, the reactants begin reacting together to form HI(g). As a result, the concentration of the reactants decreases and the concentration of the HI increases.

   
   Eventually, the curve for each substance flattens, meaning that the concentration of each is no longer changing. At this point the system is in equilibrium.

Check Your Understanding

Complete Question 3 on page 682 of the textbook.