Problem 145 - Chemistry The Molecular Nature of Matter

Question

Classify each of the following as a strong electrolyte, weak electrolyte, or nonelectrolyte. (a) \(\mathrm{LiBr}\) (b) glucose (a sugar) (c) \(\mathrm{Ca}(\mathrm{OH})_{2}\) (d) \(\mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

The classification is as follows: (a) (LiBr) - strong electrolyte, (b) glucose - nonelectrolyte, (c) (Ca(OH)_{2}) - strong electrolyte, (d) (NH_{3}) - weak electrolyte.

Step 1: Definition of Electrolytes

Identify the type of compound (ionic or covalent) and predict its ability to conduct electricity when in solution. Strong electrolytes dissociate completely into ions, weak electrolytes partially dissociate, and nonelectrolytes do not dissociate into ions at all.

Step 2: Analyzing (LiBr)

Recognize that (LiBr) is an ionic compound since it is consisting of a metal ( Li+) and a halogen ( Br-). Ionic compounds are typically strong electrolytes because they dissociate completely into ions when dissolved in water.

Step 3: Classifying Glucose

Understand that glucose is a covalent compound (molecule) and does not dissociate into ions. Covalent compounds like sugars do not conduct electricity when dissolved in water, so they are considered nonelectrolytes.

Step 4: Analyzing (Ca(OH)_{2})

Note that (Ca(OH)_{2}) is an ionic compound (metal cation (Ca^{2+}) and hydroxide anions (OH^{-})). This compound is a strong base and dissociates completely into ions in water, classifying it as a strong electrolyte.

Step 5: Classifying (NH_{3})

Recognize that (NH_{3}) is a molecular compound with a covalent bond that does not dissociate into ions. However, it reacts with water to form (NH_{4}^{+}) and (OH^{-}) ions to a small extent, which makes it a weak electrolyte.

Key Concepts

Strong Electrolytes

Strong electrolytes are substances that completely dissociate into ions when dissolved in a solvent like water. For instance, table salt (NaCl) dissolves in water and dissociates completely into sodium (Na+) and chloride (Cl-) ions, allowing the solution to conduct electricity effectively. In the exercise, lithium bromide (LiBr) and calcium hydroxide (Ca(OH)₂) are examples of strong electrolytes. The complete dissociation of these ionic compounds in solution makes them excellent conductors of electricity.

It's crucial for students to recognize that all soluble salts, strong acids, and strong bases are categorized as strong electrolytes due to their full ionization in solution. Understanding this concept is fundamental to predicting the solubility and electrical conductivity of these compounds.

Weak Electrolytes

Weak electrolytes, in contrast, only partially dissociate into ions in a solution. Acetic acid (CH₃COOH), for example, only slightly ionizes in water to produce acetate ions (CH₃COO-) and hydrogen ions (H+). In the given exercise, ammonia (NH₃) is classified as a weak electrolyte. It reacts with water to form a small amount of ammonium ions (NH₄⁺) and hydroxide ions (OH-), which means it conducts electricity, but not as effectively as strong electrolytes.

Students sometimes struggle to differentiate between weak and strong electrolytes. The key is to understand that the extent of ionization is less for weak electrolytes, so their electric conductivity is lower. Substances like weak acids and bases fall in this category.

Nonelectrolytes

Nonelectrolytes are compounds that do not produce ions at all when dissolved in water. As a result, their solutions do not conduct electricity. The glucose (a sugar) in the exercise is an example of a nonelectrolyte. Because it is a covalent compound, it dissolves as intact molecules and does not form ions in solution.

Other common examples of nonelectrolytes include most organic compounds like ethanol or sucrose. It is important for students to note that nonelectrolytes may still dissolve well in water; however, their solutions lack the free-moving ions necessary for electrical conduction. This distinction helps in understanding the behavior of substances in water and their resultant electrical properties.

Ionic and Covalent Compounds

The type of bond—ionic or covalent—in a compound plays a significant role in determining its electrolyte strength. Ionic compounds, such as salts, contain ions held together by ionic bonds and readily dissociate into those ions in solution, making them typically strong electrolytes. On the other hand, covalent compounds consist of molecules formed by atoms sharing electrons and do not dissociate into ions when dissolved in water, often rendering them nonelectrolytes.

Understanding the chemical structure helps predict the behavior of substances in water. By identifying the type of bond in a compound, students can infer whether it will dissolve as ions or molecules, impacting its ability to conduct electricity.

Electric Conductivity in Solutions

Electric conductivity in solutions is a key property that depends on the presence of mobile, charged particles—ions. When an ionic compound dissolves in water, it separates into positive and negative ions that move freely in solution, thus enabling the flow of electricity. Strong electrolytes produce a high concentration of ions, leading to high conductivity, while weak electrolytes result in low conductivity due to fewer ions.

Solutions of nonelectrolytes do not conduct electricity as they lack ions. Conductivity measurements can therefore provide insights into the type of electrolyte and its ionization in solution. This principle is fundamental in fields ranging from chemistry labs to industrial processes where controlling electrical properties of solutions is vital.