Problem 87 - Introductory Chemistry Atoms First

Question

Classify each compound as acid, base, or salt: (a) \(\mathrm{KCl}\) (b) \(\mathrm{CH}_{3} \mathrm{COOH}\) (c) \(\mathrm{Al}(\mathrm{OH})_{3}\) (d) \(\mathrm{H}_{3} \mathrm{BO}_{3}\) (e) \(\mathrm{LiC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{KCl}\): salt (b) \(\mathrm{CH}_{3} \mathrm{COOH}\): acid (c) \(\mathrm{Al}(\mathrm{OH})_{3}\): base (d) \(\mathrm{H}_{3} \mathrm{BO}_{3}\): acid (e) \(\mathrm{LiC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\): salt

Step 1: Determine the type of compound a: \(\mathrm{KCl}\)

We see that \(\mathrm{KCl}\) does not contain an \(\mathrm{H}\) in the front of the formula or an \(\mathrm{OH^-}\) group. It is, therefore, classified as a salt.

Step 2: Determine the type of compound b: \(\mathrm{CH}_{3} \mathrm{COOH}\)

The chemical formula \(\mathrm{CH}_{3} \mathrm{COOH}\) has an \(\mathrm{H}\) in the front, but it does not contain a hydroxide group \(\mathrm{(OH^-)}\). Therefore, it is classified as an acid.

Step 3: Determine the type of compound c: \(\mathrm{Al}(\mathrm{OH})_{3}\)

The chemical formula \(\mathrm{Al}(\mathrm{OH})_{3}\) contains the hydroxide group \(\mathrm{(OH^-)}\), which indicates that it is a base.

Step 4: Determine the type of compound d: \(\mathrm{H}_{3} \mathrm{BO}_{3}\)

The chemical formula \(\mathrm{H}_{3} \mathrm{BO}_{3}\) has an \(\mathrm{H}\) in the front, but it does not contain a hydroxide group \(\mathrm{(OH^-)}\). Therefore, it is classified as an acid.

Step 5: Determine the type of compound e: \(\mathrm{LiC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\)

The chemical formula of \(\mathrm{LiC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\) neither starts with an \(\mathrm{H}\) nor contains a hydroxide group \(\mathrm{(OH^-)}\). Therefore, it is classified as a salt.

Key Concepts

Chemical Compounds

Chemical compounds are substances formed by the combination of two or more elements. These elements are held together by chemical bonds. Such compounds can be classified into different categories based on their properties and composition. Common types include acids, bases, and salts. Understanding the nature of each of these compounds helps in predicting their behavior in reactions.

In a chemical compound, the elements are always present in fixed ratios by mass. For example, water (\(\mathrm{H}_2\mathrm{O}\)) consists of two hydrogen atoms for every oxygen atom. Chemical formulas denote the composition of the compound and provide information on the quantity and types of atoms within it. This is fundamental for their identification and classification.

Classifying compounds is crucial, especially in the context of acids, bases, and salts, as it helps chemists understand the potential reactions and uses for these substances in various applications.

Acid Identification

Acids are substances that can donate a proton (\(\mathrm{H}^+\)) to another substance in a chemical reaction. A common way to recognize acids is by their chemical formula, often starting with hydrogen (\(\mathrm{H}\)). For instance, \(\mathrm{CH}_3\mathrm{COOH}\) (acetic acid) and \(\mathrm{H}_3\mathrm{BO}_3\) (boric acid) both start with hydrogen, indicating they are acids.

Acids have distinct properties:

• They have a sour taste.
• They turn blue litmus paper red.
• They react with bases to form salts and water.

The strength of an acid refers to its ability to dissociate and donate \(\mathrm{H}^+\) ions in a solution. This is usually measured by the pH scale, where strong acids will have lower pH values.

Base Identification

Bases are substances that can accept hydrogen ions or donate a hydroxide ion (\(\mathrm{OH}^-\)). These compounds often contain the hydroxide group (\(\mathrm{OH}^-\)) in their formula. For example, \(\mathrm{Al(OH)_3}\) (aluminum hydroxide) is a base as it includes the \(\mathrm{OH}^-\) group.

Key characteristics of bases include:

• A bitter taste.
• They turn red litmus paper blue.
• They feel slippery to the touch due to their saponification properties.

Bases can be strong or weak, depending on their ability to dissociate and produce \(\mathrm{OH}^-\) ions in solution. Like acids, this is often measured by their pH, with stronger bases having higher pH values.

Salt Identification

Salts are ionic compounds that result from the neutralization reaction between an acid and a base. They consist of positive ions (cations) from the base and negative ions (anions) from the acid. Examples include \(\mathrm{KCl}\) (potassium chloride) and \(\mathrm{LiC}_2\mathrm{H}_3\mathrm{O}_2\) (lithium acetate).

Salts have several common properties:

• They generally have a crystalline form.
• They are often soluble in water, though solubility can vary.
• Salts conduct electricity when dissolved in water due to the presence of ions.

Identifying whether a compound is a salt involves checking if it contains neither a prefix hydrogen like acids nor the hydroxide group like bases. Salts are important in various industrial, domestic, and biological applications.