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Nonreversible Reactions

Explores the factors that cause some reactions to be driven to completion instead of reaching equilibrium.

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Nonreversible Reactions

A fire is an irreversible reaction

Credit: User:Awesomoman/Wikimedia Commons
Source: http://commons.wikimedia.org/wiki/File:Fire.JPG
License: CC BY-NC 3.0

Where’s the fire?

Fires are a part of life. Some fires clear the land and allow new growth. Other fires provide warmth on a cold night. Unfortunately, many fires are destructive, leaving damage in their wake. All fires leave the environment changed, never to revert back to the original situation. The carbon dioxide and water generated by a fire go off into the atmosphere and do not return. The change is permanent and irreversible.

Going to Completion

When one of the products of a reaction is removed from the chemical equilibrium system as soon as it is produced, the reverse reaction cannot establish itself and equilibrium is never reached. Reactions such as these are said to go to completion. These processes are often referred to as non-reversible reactions. Reactions which go to completion tend to produce one of three types of products: (1) an insoluble precipitate, (2) a gas, (3) a molecular compound such as water. Examples of these reactions are shown below.

  1. Formation of a precipitate

\begin{align*}\text{AgNO}_3(aq)+\text{NaCl}(aq) \rightarrow \text{NaNO}_3(aq)+\text{AgCl}(s)\end{align*}

  1. Formation of a gas

\begin{align*}\text{Mg}(s)+2\text{HCl}(aq) \rightarrow \text{MgCl}_2(aq)+\text{H}_2(g)\end{align*}

  1. Formation of water

\begin{align*}\text{HCl}(aq)+\text{NaOH}(aq) \rightarrow \text{NaCl}(aq)+\text{H}_2\text{O}(l)\end{align*}

If we look at these reactions in more detail, we can see some things that are not apparent the way the equations are written. Looking at the first equation, we do not see a double arrow between reactants and products because the reaction is considered essentially irreversible. However if we consider the net ionic equation \begin{align*}\text{Ag}^+ +\text{Cl}^- \rightarrow \text{AgCl}\end{align*}, then the reverse reaction would be \begin{align*}\text{AgCl} \rightarrow \text{Ag}^+ + \text{Cl}^-\end{align*}. The \begin{align*}K_{eq}\end{align*} for the reverse reaction is 1.8 × 10-10. For all practical purposes, the reaction goes to completion.

Formation of a gas in an open system is essentially irreversible since the gas escapes into the atmosphere. Looking at the activity series we see that Mg is much higher in the series than hydrogen. So the reaction would be expected to go strongly in the indicated direction.

The third reaction gets a little more complicated. In solution, the reactants HCl and NaOH will be ionize completely as does the NaCl product. Water exists in an equilibrium with H+ and OH-, with the dissociation constant for water being 1 × 10-14. So, in the solution resulting from the reaction given here, the [H+] is 1 × 10-7 M, a very insignificant amount. For all practical purposes, this reaction can be said to go to completion.




Read the material at the link below and answer the following questions:


  1. How do we represent a reversible reaction?
  2. How do we represent an irreversible reaction?
  3. Why is the decomposition of potassium chlorate an irreversible reaction?
  4. Why is the reaction between barium chloride and sulfuric acid irreversible?



  1. List the three situations for non-reversible reactions.
  2. Silver nitrate also forms a precipitate with NaI. What would this precipitate be?
  3. Would the reaction between HBr and KOH also be considered irreversible?

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  1. [1]^ Credit: User:Awesomoman/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:Fire.JPG; License: CC BY-NC 3.0

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