Problem 54 - Chemistry and Chemical Reactivity

Question

Write the formulas for the four ionic compounds that can be made by combining the cations \(\mathrm{Mg}^{2+}\) and \(\mathrm{Fe}^{3+}\) with the anions \(\mathrm{PO}_{4}^{3-}\) and \(\mathrm{NO}_{3}^{-} .\) Name each compound formed.

Step-by-Step Solution

Answer

The compounds are magnesium phosphate, magnesium nitrate, iron(III) phosphate, and iron(III) nitrate.

Step 1: Identify Possible Combinations

Identify the cations and anions provided in the exercise: the cations are \(\mathrm{Mg}^{2+}\) and \(\mathrm{Fe}^{3+}\), and the anions are \(\mathrm{PO}_{4}^{3-}\) and \(\mathrm{NO}_{3}^{-}\). Each combination of cation and anion forms a different ionic compound.

Step 2: Determine Compound Formulas

Combine each cation with each anion, balancing the charges to ensure that the compound is electrically neutral. For \(\mathrm{Mg}^{2+}\), - With \(\mathrm{PO}_{4}^{3-}\): Use a 3:2 ratio (\(\mathrm{Mg}_3(\mathrm{PO}_4)_2\)) because three \(\mathrm{Mg}^{2+}\) (total +6 charge) will balance with two \(\mathrm{PO}_{4}^{3-}\) (total -6 charge).- With \(\mathrm{NO}_{3}^{-}\): Use a 1:2 ratio (\(\mathrm{Mg}(\mathrm{NO}_3)_2\)) to balance the charges.For \(\mathrm{Fe}^{3+}\), - With \(\mathrm{PO}_{4}^{3-}\): Use a 1:1 ratio (\(\mathrm{FePO}_4\)) because the end charges are balanced.- With \(\mathrm{NO}_{3}^{-}\): Use a 1:3 ratio (\(\mathrm{Fe}(\mathrm{NO}_3)_3\)).

Step 3: Name Each Compound

Name each compound formed based on ionic naming conventions:- \(\mathrm{Mg}_3(\mathrm{PO}_4)_2\) becomes magnesium phosphate.- \(\mathrm{Mg}(\mathrm{NO}_3)_2\) becomes magnesium nitrate.- \(\mathrm{FePO}_4\) becomes iron(III) phosphate.- \(\mathrm{Fe}(\mathrm{NO}_3)_3\) becomes iron(III) nitrate.

Key Concepts

Cations

Cations are positively charged ions that have lost one or more electrons. They are typically formed from metal atoms. In ionic compounds, cations are paired with anions, which carry the opposite charge. Cations are denoted by a positive superscript, indicating the number of electrons lost. For example, the magnesium ion, \(\mathrm{Mg}^{2+}\), has lost two electrons.
The charge of a cation plays a crucial role in determining how it combines with anions to form stable ionic compounds. The positive charge needs to balance with the negative charge of anions so that the resulting compound is neutral. List of important points about cations:

• Formed by loss of electrons.
• Typically metal ions.
• Always have a positive charge.

Understanding cations is important when writing chemical formulas for ionic compounds, as the charge determines their combination with anions.

Anions

Anions are negatively charged ions, formed by the gain of electrons. These ions typically originate from non-metal elements. Anions, just like cations, are symbolized with a superscript indicating the charge, but with a negative sign. For instance, \(\mathrm{PO}_{4}^{3-}\) is the phosphate ion with three extra electrons.
The balance between the positive charges from cations and the negative charges from anions ensures that the resultant compound is electrically neutral. This balance is critical during the formulation of ionic compounds.Key notes on anions include:

• Formed by gain of electrons.
• Usually composed of non-metals.
• Always carry a negative charge.

The role of anions is pivotal in securing the integrity of ionic compounds by stabilizing cationic charges.

Chemical Formulas

Chemical formulas represent the composition of chemical compounds. In the context of ionic compounds, these formulas indicate the ratio of ions required for electrical neutrality. Constructing a chemical formula involves balancing the total positive and negative charges of the cations and anions.
Consider \(\mathrm{Mg}_3(\mathrm{PO}_4)_2\): this formula reveals that three \(\mathrm{Mg}^{2+}\) ions and two \(\mathrm{PO}_{4}^{3-}\) ions combine, resulting in a neutral compound. Understanding these formulas involves recognizing the necessary stoichiometry, which ensures balanced charges.Noteworthy aspects of chemical formulas:

• Indicate the ion ratio needed for neutrality.
• Based on charge balance between cations and anions.
• Reveals the composition of ionic compounds.

Mastering chemical formulas strengthens one’s ability to visualize the actual atomic ratios within the compound.

Chemical Naming Conventions

Chemical naming conventions for ionic compounds involve systematically naming the involved ions. The cation is named first, followed by the anion. These names often reveal the charge of the ions, especially for transition metals that can have multiple oxidation states. If a transition metal like iron is involved, the charge is specified in parentheses using Roman numerals.
For example, \(\mathrm{FePO}_4\) is named iron(III) phosphate, indicating that the iron ion has a +3 charge. Anion names typically end with "-ide," unless they are polyatomic ions like \(\mathrm{NO}_3^{-}\) (nitrate) or \(\mathrm{PO}_{4}^{3-}\) (phosphate).Key points about naming ionic compounds:

• Cation name is listed first.
• Anion name follows, often ending in "-ide."
• Charges of transition metals are indicated.

Proper naming is essential for clear communication in chemistry, ensuring that compounds are accurately and consistently identified.