Chapter 4: Chemical Reactions in Aqueous Solutions – Properties of Aqueous Solutions and Electrolytes

Chemical Properties of Aqueous Solutions

Introduction to Aqueous Solutions

Aqueous solutions are mixtures where water acts as the solvent. Many chemical reactions in general chemistry occur in aqueous solutions, making it essential to understand their properties and the behavior of solutes within them.

• Solute: The substance dissolved in a solvent.

• Solvent: The substance (often water) that dissolves the solute.

• Aqueous solution: A solution where water is the solvent.

Classification of Solutes: Electrolytes vs. Nonelectrolytes

Definitions and Properties

Solutes in aqueous solutions can be classified based on their ability to conduct electricity, which depends on the presence of ions.

• Electrolyte: A substance that, when dissolved in water, conducts electricity due to the presence of ions. Example: Sodium chloride (NaCl) dissociates into Na+ and Cl- ions.

• Nonelectrolyte: A substance that, when dissolved in water, does not conduct electricity because it does not produce ions. Example: Sucrose (C12H22O11) dissolves as molecules, not ions.

Electrolytes are typically ionic compounds (salts, bases, acids), while nonelectrolytes are usually covalent compounds.

Visual Representation

• Strong electrolyte: Light bulb glows brightly (many ions present).

• Weak electrolyte: Light bulb glows dimly (few ions present).

• Nonelectrolyte: Light bulb does not glow (no ions present).

Strong vs. Weak Electrolytes

Complete vs. Partial Dissociation

The strength of an electrolyte depends on the extent to which it dissociates into ions in water.

• Strong electrolytes: Substances that completely dissociate into ions in solution. All water-soluble ionic compounds are strong electrolytes.

• Weak electrolytes: Substances that partially dissociate into ions. Most molecular compounds are weak electrolytes or nonelectrolytes. Weak electrolytes produce some ions but exist mostly as molecules that are not ionized.

There are only seven molecular compounds (all acids) that are considered strong electrolytes:

• Hydrochloric acid: HCl

• Hydrobromic acid: HBr

• Hydroiodic acid: HI

• Nitric acid: HNO3

• Chloric acid: HClO3

• Perchloric acid: HClO4

• Sulfuric acid: H2SO4

Examples of Dissociation

• Strong acid: $\mathrm{HCl}(g) \rightarrow \mathrm{H}^+(aq) + \mathrm{Cl}^-(aq)$

• Weak acid: $\mathrm{HC_2H_3O_2}(l) \rightleftharpoons \mathrm{H}^+(aq) + \mathrm{C_2H_3O_2}^-(aq)$

• Weak base: $\mathrm{NH_3}(g) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{NH_4}^+(aq) + \mathrm{OH}^-(aq)$

Classification Table: Solutes in Aqueous Solution

Comparison of Strong Electrolytes, Weak Electrolytes, and Nonelectrolytes

Strong Electrolyte	Weak Electrolyte	Nonelectrolyte
NaCl	CH3COOH	(NH2)2CO (urea)
KNO3	HF	CH3OH (methanol)
HClO4	HNO2	C2H5OH (ethanol)
H2SO4 *	NH3	C6H12O6 (glucose)
NaOH	H2O†	C12H22O11 (sucrose)
Ba(OH)2
Ionic compounds		Covalent compounds

*H2SO4 has two ionizable H+ ions, but only one is totally ionized in water.

†Water is an extremely weak electrolyte.

Practice Example

Which compound is a weak electrolyte when dissolved in water?

• HNO3

• CH3OH

• NH3

• NaNO3

Answer: NH3 is a weak electrolyte.

Additional info:

• Strong electrolytes are important in conducting electricity in solutions, which is essential for many chemical and biological processes.

• Weak electrolytes are often involved in equilibrium reactions, such as acid-base equilibria.

• Nonelectrolytes do not affect the electrical conductivity of water and are common in organic chemistry.