Types of Aqueous Solutions

Electrolyte and Nonelectrolyte Solutions

Unlike nonelectrolytes, electrolytes contain dissolved ions that enable them to easily conduct electricity.

Electrolyte Solutions

An electrolyte is any salt or ionizable molecule that, when dissolved in solution, will give that solution the ability to conduct electricity. This is because when a salt dissolves, its dissociated ions can move freely in solution, allowing a charge to flow.

Electrolyte solutions are normally formed when a salt is placed into a solvent such as water. For example, when table salt, NaCl, is placed in water, the salt (a solid) dissolves into its component ions, according to the dissociation reaction:

NaCl(s) → Na⁺(aq) + Cl⁻(aq)

It is also possible for substances to react with water to yield ions in solution. For example, carbon dioxide gas, CO2, will dissolve in water to produce a solution that contains hydrogen ions, carbonate, and hydrogen carbonate ions:

2 CO₂(g)+ 2 H₂O(l) → 3 H⁺(aq) + CO₃^2-(aq) + HCO₃^-(aq)

The resulting solution will conduct electricity because it contains ions. It is important to keep in mind, however, that CO₂ is not an electrolyte, because CO₂ itself does not dissociate into ions. Only compounds that dissociate into their component ions in solution qualify as electrolytes.

Strong and Weak Electrolytes

As mentioned above, when an ionizable solute dissociates, the resulting solution can conduct electricity. Therefore, compounds that readily form ions in solution are known as strong electrolytes. (By this reasoning, all strong acids and strong bases are strong electrolytes.)

By contrast, if a compound dissociates to a small extent, the solution will be a weak conductor of electricity; a compound that only dissociates weakly, therefore, is known as a weak electrolyte.

A strong electrolyte will completely dissociate into its component ions in solution; a weak electrolyte, on the other hand, will remain mostly undissociated in solution. An example of a weak electrolyte is acetic acid, which is also a weak acid.

Nonelectrolyte Solutions

Nonelectrolytes are compounds that do not ionize at all in solution. As a result, solutions containing nonelectrolytes will not conduct electricity. Typically, nonelectrolytes are primarily held together by covalent rather than ionic bonds. A common example of a nonelectrolyte is glucose, or C₆H₁₂O₆. Glucose (sugar) readily dissolves in water, but because it does not dissociate into ions in solution, it is considered a nonelectrolyte; solutions containing glucose do not, therefore, conduct electricity.

Water’s Solvent Properties

Water's polarity makes it an excellent solvent for other polar molecules and ions.

Water's Solvent Properties

Water, which not only dissolves many compounds but also dissolves more substances than any other liquid, is considered the universal solvent. A polar molecule with partially-positive and negative charges, it readily dissolves ions and polar molecules. Water is therefore referred to as a solvent: a substance capable of dissolving other polar molecules and ionic compounds. The charges associated with these molecules form hydrogen bonds with water, surrounding the particle with water molecules. This is referred to as a sphere of hydration, or a hydration shell, and serves to keep the particles separated or dispersed in the water.

When ionic compounds are added to water, individual ions interact with the polar regions of the water molecules during the dissociation process, disrupting their ionic bonds. Dissociation occurs when atoms or groups of atoms break off from molecules and form ions. Consider table salt (NaCl, or sodium chloride): when NaCl crystals are added to water, the molecules of NaCl dissociate into Na⁺ and Cl^– ions, and spheres of hydration form around the ions. The positively-charged sodium ion is surrounded by the partially-negative charge of the water molecule's oxygen; the negatively-charged chloride ion is surrounded by the partially-positive charge of the hydrogen in the water molecule.

Since many biomolecules are either polar or charged, water readily dissolves these hydrophilic compounds. Water is a poor solvent, however, for hydrophobic molecules such as lipids. Nonpolar molecules experience hydrophobic interactions in water: the water changes its hydrogen bonding patterns around the hydrophobic molecules to produce a cage-like structure called a clathrate. This change in the hydrogen-bonding pattern of the water solvent causes the system's overall entropy to greatly decrease, as the molecules become more ordered than in liquid water. Thermodynamically, such a large decrease in entropy is not spontaneous, and the hydrophobic molecule will not dissolve.

Electrolytic Properties

When electrodes are placed in an electrolyte solution and a voltage is applied, the electrolyte will conduct electricity.

Electrolytic Properties

When electrodes are placed in an electrolyte solution and a voltage is applied, the electrolyte will conduct electricity. Lone electrons cannot usually pass through the electrolyte; instead, a chemical reaction occurs at the cathode that consumes electrons from the anode. Another reaction occurs at the anode, producing electrons that are eventually transferred to the cathode. As a result, a negative charge cloud develops in the electrolyte around the cathode, and a positive charge develops around the anode. The ions in the electrolyte neutralize these charges, enabling the electrons to keep flowing and the reactions to continue.

For example, in a solution of ordinary table salt (sodium chloride, NaCl) in water, the cathode reaction will be:



and hydrogen gas will bubble up. The anode reaction is:



and chlorine gas will be liberated. The positively-charged sodium ions Na⁺ will react toward the cathode, neutralizing the negative charge of OH⁻ there; the negatively-charged hydroxide ions OH⁻ will react toward the anode, neutralizing the positive charge of Na⁺ there. Without the ions from the electrolyte, the charges around the electrode slow continued electron flow; diffusion of H⁺ and OH⁻ through water to the other electrode takes longer than movement of the much more prevalent salt ions.

In other systems, the electrode reactions can involve electrode metal as well as electrolyte ions. In batteries for example, two materials with different electron affinities are used as electrodes: outside the battery, electrons flow from one electrode to the other; inside, the circuit is closed by the electrolyte's ions. Here, the electrode reactions convert chemical energy to electrical energy.

Oxidation and Reduction at the Electrodes

Oxidation of ions or neutral molecules occurs at the anode, and the reduction of ions or neutral molecules occurs at the cathode. Two mnemonics for remembering that reduction happens at the cathode and oxidation at the anode are: "Red Cat" (reduction - cathode) and "An Ox" (anode - oxidation). The mnemonic "LeO said GeR" is useful for remembering "lose an electron in oxidation" and "gain an electron in reduction."

It is possible to oxidize ferrous ions to ferric ions at the anode. For example:



Neutral molecules can also react at either electrode. For example, p-Benzoquinone can be reduced to hydroquinone at the cathode:



In the last example, H⁺ ions (hydrogen ions) also take part in the reaction, and are provided by an acid in the solution or by the solvent itself (water, methanol, etc.). Electrolysis reactions involving H⁺ ions are fairly common in acidic solutions, while reactions involving OH- (hydroxide ions) are common in alkaline water solutions.

The oxidized or reduced substances can also be the solvent (usually water) or electrodes. It is possible to have electrolysis involving gases.

In order to determine which species in solution will be oxidized and which will be reduced, the standard electrode potential of each species may be obtained from a table of standard reduction potentials, a small sampling of which is shown here:

Historically, oxidation potentials were tabulated and used in calculations, but the current standard is to only record the reduction potential in tables. If a problem demands use of oxidation potential, it may be interpreted as the negative of the recorded reduction potential. For example, referring to the data in the table above, the oxidation of elemental sodium (Na(s)) is a highly favorable process with a value of = + 2.71 V; this makes intuitive sense because the loss of one electron from a sodium atom produces a sodium cation, which has the same electron configuration as neon, a noble gas. The production of this low-energy and stable electron configuration is clearly a favorable process. Chlorine gas on the other hand is much more likely to be reduced under normal conditions, as can be inferred from the value of = +1.36 V in the table. Recall that a more positive potential always means that that reaction will be favored; this will have consequences concerning redox reactions.