Chemical Equilibrium

Learning Objectives

  1. Define chemical equilibrium.
  2. Recognize chemical equilibrium as a dynamic process.

Consider the following reaction occurring in a closed container (so that no material can go in or out):

H2 + I2 → 2 HI

This is simply the reaction between elemental hydrogen and elemental iodine to make hydrogen iodide. The way the equation is written, we are led to believe that the reaction goes to completion, that all the H2 and the I2 react to make HI.

However, this is not the case. The reverse chemical reaction is also taking place:

2 HI → H2 + I2

It acts to undo what the first reaction does. Eventually, the reverse reaction proceeds so quickly that it matches the speed of the forward reaction. When that happens, any continued overall reaction stops: the reaction has reached chemical equilibrium (sometimes just spoken as equilibrium; plural equilibria), the point at which the forward and reverse processes balance each other’s progress.

Because two opposing processes are occurring at once, it is conventional to represent an equilibrium using a double arrow, like this:

H2 + I2 ⇄ 2 HI 

The double arrow implies that the reaction is going in both directions. Note that the reaction must still be balanced.

Example 1

Write the equilibrium equation that exists between calcium carbonate as a reactant and calcium oxide and carbon dioxide as products.

Solution

As this is an equilibrium situation, a double arrow is used. The equilibrium equation is written as follows:

CaCO3 ⇄ CaO + CO2

Test Yourself

Write the equilibrium equation between elemental hydrogen and elemental oxygen as reactants and water as the product.

Answer

2 H2 + O2 ⇄ 2 H2O 

One thing to note about equilibrium is that the reactions do not stop; both the forward reaction and the reverse reaction continue to occur. They both occur at the same rate, so any overall change by one reaction is cancelled by the reverse reaction. We say that chemical equilibrium is dynamic, rather than static. Also, because both reactions are occurring simultaneously, the equilibrium can be written backward. For example, representing an equilibrium as

H2+ I2 ⇄ 2 HI 

is the same thing as representing the same equilibrium as

2 HI ⇄ H2 + I2

The reaction must be at equilibrium for this to be the case, however.

Key Takeaways

  • Chemical reactions eventually reach equilibrium, a point at which forward and reverse reactions balance each other’s progress.
  • Chemical equilibria are dynamic: the chemical reactions are always occurring; they just cancel each other’s progress.

Exercises

  1. Define chemical equilibrium. Give an example.

  2. Explain what is meant when it is said that chemical equilibrium is dynamic.

  3. Write the equilibrium equation between elemental hydrogen and elemental chlorine as reactants and hydrochloric acid as the product.

  4. Write the equilibrium equation between iron(III) sulfate as the reactant and iron(III) oxide and sulfur trioxide as the products.

  5. Graphite and diamond are two forms of elemental carbon. Write the equilibrium equation between these two forms in two different ways.

  6. At 1,500 K, iodine molecules break apart into iodine atoms. Write the equilibrium equation between these two species in two different ways.

Answers

1.

the situation when the forward and reverse chemical reactions occur, leading to no additional net change in the reaction position; H2+ I2 ⇄ 2 HI (answers will vary)

3.

H2+ Cl2 ⇄ 2 HCl

5.

C (gra) ⇄ C (dia); C (dia) ⇄ C (gra)