4.6
Electrolyte and Nonelectrolyte Solutions
Substances that undergo either a physical or a chemical change in solution to yield ions that can conduct electricity are called electrolytes. If a su…
Water can dissolve many substances because of its chemical composition. Although its overall electrical charge is neutral, a water molecule has a partial negative charge at its oxygen end, and a partial positive charge at each of its hydrogen ends. This makes water a polar solvent.
When an ionic solute, such as a solid ionic compound, is added to water, the water molecules interact with the ions of the solute.
If the strong solute–solvent attractive forces overcome the solute–solute interactions holding the compound together, the solute ions are pulled apart.
The polar water molecules surround, or ‘hydrate’, the ions as they are separated. The hydrated ions are then dispersed uniformly throughout the solution.
When an electric potential is applied to an ionic solution, the ions become charge carriers that move towards the oppositely charged electrodes. This creates a current in the solution; in other words, it conducts electricity.
Substances that produce electrically conductive solutions when dissolved in polar solvents are called electrolytes. Sodium chloride is one such example.
However, when a molecular compound such as sucrose dissolves in water, its molecules remain intact as they become surrounded by water molecules. The solution contains only neutral sucrose molecules and therefore does not conduct electricity.
Substances that do not produce electrically conductive solutions when dissolved in polar solvents are called non-electrolytes.
How much of a compound dissociates into ions determines its strength as an electrolyte.
Nearly every molecule of a strong electrolyte, such as a strong acid like hydrochloric acid, will dissociate into ions in water, making the solution a good conductor of electricity.
On the other hand, only some molecules of a weak electrolyte, such as a weak acid like hydrofluoric acid, will dissociate into ions. Therefore, it can only conduct electricity to a small degree.
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Q1: Why does water dissolve ionic compounds?
Water dissolves ionic compounds because it is a polar solvent with partial negative charge at oxygen and partial positive charge at hydrogen. When ionic solutes are added, water molecules interact with the ions. If solute-solvent attractive forces overcome solute-solute interactions, the ions separate and become hydrated by surrounding water molecules, dispersing uniformly throughout the solution.
Q2: What is the difference between electrolytes and nonelectrolytes?
Electrolytes are substances that produce electrically conductive solutions when dissolved in polar solvents because they yield ions. Nonelectrolytes do not conduct electricity because they dissolve as neutral molecules. For example, sodium chloride is an electrolyte that dissociates into ions, while sucrose is a nonelectrolyte that remains intact in solution.
Q3: How do strong and weak electrolytes differ in their conductivity?
Strong electrolytes dissociate nearly completely into ions, making their solutions good conductors of electricity. Weak electrolytes only partially dissociate, so their solutions conduct electricity to a small degree. For example, hydrochloric acid is a strong electrolyte while hydrofluoric acid is a weak electrolyte, resulting in different levels of electrical conductivity in their aqueous solutions.
Q4: What role does ion-dipole attraction play in dissolving ionic compounds?
Ion-dipole attraction is the electrostatic attraction between an ion and a molecule with a dipole, such as water. When ionic compounds dissolve, water molecules surround and solvate the ions through ion-dipole interactions, reducing the strong electrostatic forces between them. This allows the ions to separate and disperse uniformly throughout the solution.
Q5: Why does hydrogen chloride gas conduct electricity when dissolved in water?
Pure hydrogen chloride gas contains covalent HCl molecules with no ions and does not conduct electricity. However, when dissolved in water, HCl molecules react chemically with water, transferring H+ ions to form hydronium ions and chloride ions. This reaction is nearly 100% complete, producing appreciable ion concentration that makes the aqueous solution a very good conductor.
Q6: How can you experimentally determine if a substance is a strong, weak, or nonelectrolyte?
You can measure electrical conductance by applying voltage to electrodes immersed in an aqueous solution. Quantitatively, measure electrical current flow; qualitatively, observe light bulb brightness in the circuit. Conductivity increases with ion concentration, allowing you to classify substances as strong electrolytes (high conductivity), weak electrolytes (low conductivity), or nonelectrolytes (no conductivity).
Q7: What happens when a weak acid dissolves in water?
When a weak acid like acetic acid dissolves in water, only some molecules dissociate into ions through a reversible reaction. This partial dissociation produces relatively low concentrations of ions, such as acetate ions and hydronium ions. Because ion concentration remains low, weak acids are classified as weak electrolytes and conduct electricity only to a limited degree.