# 15.9: Strong and Weak Electrolytes

Car batteries are used around the world to provide the power to start car engines. One essential component of car batteries is the strong electrolyte sulfuric acid. In the battery, this material ionizes into hydrogen ions and sulfate ions. As the battery is used, the concentrations of these ions decrease. Older batteries had openings in the top where new sulfuric acid could be added to replenish the supply. Today, batteries are sealed to prevent leakage of the hazardous sulfuric acid.

## Strong and Weak Electrolytes

Some polar molecular compounds are nonelectrolytes when they are in their pure state, but become electrolytes when they are dissolved in water. Hydrogen chloride $$\left( \ce{HCl} \right)$$ is a gas in its pure molecular state and is a nonelectrolyte. However, when $$\ce{HCl}$$ is dissolved in water, it conducts a current well because the $$\ce{HCl}$$ molecule ionizes into hydrogen and chloride ions.

$\ce{HCl} \left( g \right) \rightarrow \ce{H^+} \left( aq \right) + \ce{Cl^-} \left( aq \right)$

When $$\ce{HCl}$$ is dissolved, into water, it is called hydrochloric acid. Ionic compounds and some polar compounds are completely broken apart into ions and thus conduct a current very well. A strong electrolyte is a solution in which a large fraction of the dissolved solute exists as ions.

Some other polar molecular compounds become electrolytes upon being dissolved into water, but do not ionize to very great extent. Gaseous nitrous acid ionizes in solution to hydrogen ions and nitrite ions, but does so very weakly. Aqueous nitrous acid is composed of only about $$5\%$$ ions and $$95\%$$ intact nitrous acid molecules. A weak electrolyte is a solution in which only a small fraction of the dissolved solute exists as ions. The equation showing the ionization of a weak electrolyte utilizes a double arrow indicating an equilibrium between the reactants and products.

$\ce{HNO_2} \left( g \right) \rightleftharpoons \ce{H^+} \left( aq \right) + \ce{NO_2^-} \left( aq \right)$

## Summary

• A strong electrolyte exists mainly as ions in solution.
• A weak electrolyte has only a small amount of ionization in solution.

## Contributors

• CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.