Reversible Reactions and Chemical Equilibrium
 

Objectives:

  • explain how reversible and irreversible reactions differ
  • describe how the rate for the forward reaction changes during a reversible reaction
  • describe how the rate for the reverse reaction changes during a reversible reaction
  • sketch and interpret a plot of reaction rate for forward and reverse reaction versus time
  • give an example of an irreversible reaction
  • give an example of a reversible reaction
 

Reversible Reactions and Chemical Equilibrium

When reactions are analyzed in kinetics, there are two types of reactions. There are nonreversible reactions and reversible reactions.

The nonreversible reactions occur so that the reactants are completely consumed in the reaction, the products are formed and the process comes to an end. These kinds of reactions are said to go to "completion". Burning a piece of paper or metabolizing glucose, C6H12O6(s), is a nonreversible reaction. Once the oxygen and paper form CO2 and H2O the process is over.

C6H12O6(s) + 6 O2 (g) ------> 6 H2O(g) + 6 CO2

Some ambitious plant or bacterium somewhere may take in the water and carbon dioxide to make cellulose again, but that doesn't fit the idea of a reversible reaction.

A reversible reaction is an interesting process. The reaction really involves two reactions. There is a "forward" reaction and a mirror image "reverse" reaction.

 

H2(g) + F2(g) ------> 2 HF(g) forward
H2(g) + F2(g) <------ 2 HF(g) reverse

Reversible chemical reactions follow a similar pattern. The reactants are initially the only molecules around. They react to form products. The amount of reactants dwindles and the forward reaction slows down. The product amounts increase at the same time the reactants are disappearing. They products "decompose" to form reactants. The rate for this reverse reaction increases as the amount of product grows. Ultimately there comes a time when the forward reaction rate and the reverse reaction rates are equal. The mixture is at equilibrium.

At equilibrium the numbers of reactant and product molecules stays constant. The identity of individual molecules keeps changing.

 

Online Introductory Chemistry

Dr. Walt Volland, Bellevue Community College, All rights reserved, 1998
last modified March 18, 1999