Problem 20 - Chemistry and Chemical Reactivity

Question

Name each of the following ionic compounds: (a) \(\mathrm{Ca}\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2}\) (c) \(\mathrm{Al}(\mathrm{OH})_{3}\) (b) \(\mathrm{Ni}_{3}\left(\mathrm{PO}_{4}\right)_{2}\) (d) \(\mathrm{KH}_{2} \mathrm{PO}_{4}\)

Step-by-Step Solution

Answer

(a) Calcium Acetate, (b) Nickel(II) Phosphate, (c) Aluminum Hydroxide, (d) Potassium Dihydrogen Phosphate.

Step 1: Identify the Cation and Anion for Compound (a)

The chemical formula given is \( \mathrm{Ca(CHO_3CO_2)_2} \). The cation is Ca, which is calcium. The anion is \( CH_3CO_2^- \), which is acetate. Therefore, the compound is composed of calcium and acetate ions.

Step 2: Name the Compound (a)

Calcium forms a 2+ cation, and acetate is a 1- anion. The formula correctly balances two acetate ions with one calcium ion. Thus, the name of the compound (a) is Calcium Acetate.

Step 3: Identify the Cation and Anion for Compound (b)

The chemical formula given is \( \mathrm{Ni}_3(\mathrm{PO_4})_2 \). The cation is \( \mathrm{Ni} \), which is nickel. The anion is \( \mathrm{PO_4}^{3-} \), which is phosphate. Nickel commonly forms a \( 2+ \) cation in this compound.

Step 4: Name the Compound (b)

With nickel typically having a \( 2+ \) charge, two phosphate ions \((3-\) charge each) are needed to balance three nickel ions. Consequently, the name of the compound (b) is Nickel(II) Phosphate.

Step 5: Identify the Cation and Anion for Compound (c)

The chemical formula given is \( \mathrm{Al(OH)_3} \). The cation is \( \mathrm{Al} \), which is aluminum. The anion is \( \mathrm{OH}^- \), which is the hydroxide ion.

Step 6: Name the Compound (c)

Aluminum always forms a \( 3+ \) cation, and three hydroxide ions \((1-\) each) balance one aluminum ion. Therefore, the name of the compound (c) is Aluminum Hydroxide.

Step 7: Identify the Cation and Anion for Compound (d)

The chemical formula given is \( \mathrm{KH_2PO_4} \). The cation is \( \mathrm{K} \), which is potassium. The anion is \( \mathrm{H_2PO_4}^- \), known as the dihydrogen phosphate ion.

Step 8: Name the Compound (d)

Potassium forms a \( 1+ \) cation. The single \( \mathrm{H_2PO_4}^- \) balances one potassium cation. Thus, the name of the compound (d) is Potassium Dihydrogen Phosphate.

Key Concepts

Chemical Nomenclature

In chemistry, nomenclature refers to the system used for naming chemical substances. With over millions of compounds, chemical nomenclature aims to provide each one a unique and interpretative name. For ionic compounds in particular, the naming process is systematic, focusing first on the cation and then the anion.
When naming an ionic compound:

• The cation (positively charged ion) is named first, and it retains the name of the element.
• The anion (negatively charged ion) is named second, typically ending with the suffix “-ide.” However, when dealing with polyatomic ions like sulfate ( SO_4^{2-}) or phosphate ( PO_4^{3-}), the names of these ions are retained.

Following these rules ensures consistency and clarity in communication, allowing chemists worldwide to understand each compound's composition at a glance. These conventions are essential not just for writing, but also for speaking about compounds in fields like pharmacy, biochemistry, and materials science.

Cations and Anions

Ions are charged particles and can either be cations or anions. These are the building blocks of ionic compounds. A cation is a positively charged ion, formed when an atom loses one or more electrons. For instance, calcium ( Ca^{2+}) becomes a cation by losing two electrons.
In contrast, an anion is a negatively charged ion, formed when an atom gains electrons. An example is the acetate ion ( CH_3CO_2^-), which forms when acetic acid loses a proton and gains a negative charge.

• Cations are typically metals such as sodium ( Na^{+}) or potassium ( K^{+}).
• Anions generally stem from nonmetals or polyatomic ions, like chloride ( Cl^{-}) or hydroxide ( OH^{-}).

Cations and anions are the contributors to the overall charge balance in ionic compounds. Their charges must sum to zero in a chemical formula, a rule that guides the basis for writing chemical formulas. Understanding these ions and their interactions is key to mastering ionic chemistry.

Balancing Chemical Formulas

Balancing chemical formulas is crucial for representing the correct proportions of ions in a compound. Since compounds must have a neutral overall charge, the total positive charge from cations must balance the total negative charge from anions. This is often achieved by adjusting the subscripts in the chemical formula.
For example, to balance the formula for calcium acetate ( Ca(CH_3CO_2)_2), we recognize:

• Calcium ( Ca^{2+}) contributes a +2 charge.
• Each acetate ion ( CH_3CO_2^{-}) contributes a -1 charge.

Two acetate ions are required to balance the +2 charge of calcium, resulting in the formula Ca(CH_3CO_2)_2. Similarly, in aluminum hydroxide ( Al(OH)_3), aluminum ( Al^{3+}) pairs with three hydroxide ions ( OH^{-}) to balance its tripositive charge.
Balancing these relationships accurately is a critical skill in chemistry, not only for naming compounds, but also for predictions in chemical reactions and stoichiometry.