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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 its original state. 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.
- Formation of a precipitate:
\[\ce{AgNO_3} \left( aq \right) + \ce{NaCl} \left( aq \right) \rightarrow \ce{NaNO_3} \left( aq \right) + \ce{AgCl} \left( s \right)\nonumber \] - Formation of a gas:
\[\ce{Mg} \left( s \right) + 2 \ce{HCl} \left( aq \right) \rightarrow \ce{MgCl_2} \left( aq \right) + \ce{H_2} \left( g \right)\nonumber \] - Formation of water:
\[\ce{HCl} \left( aq \right) + \ce{NaOH} \left( aq \right) \rightarrow \ce{NaCl} \left( aq \right) + \ce{H_2O} \left( l \right)\nonumber \]
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
\[\ce{Ag^+} + \ce{Cl^-} \rightarrow \ce{AgCl}\nonumber \]
then the reverse reaction would be:
\[\ce{AgCl} \rightarrow \ce{Ag^+} + \ce{Cl^-}\nonumber \]
The \(K_\text{eq}\) for the reverse reaction is \(1.8 \times 10^{-10}\). 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 \(\ce{Mg}\) 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 \(\ce{HCl}\) and \(\ce{NaOH}\) will be ionized completely, as does the \(\ce{NaCl}\) product. Water exists in an equilibrium with \(\ce{H^+}\) and \(\ce{OH^-}\), with the dissociation constant for water being \(1 \times 10^{-14}\). So, in the solution resulting from the reaction given here, the \(\left[ \ce{H^+} \right]\) is \(1 \times 10^{-7} \: \text{M}\), a very insignificant amount. For all practical purposes, this reaction can be said to go to completion.
