Problem 48 - Chemistry and Chemical Reactivity

Question

Which of the following are correct formulas for ionic compounds? For those that are not, give the correct formula. (a) \(\mathrm{Ca}_{2} \mathrm{O}\) (b) \(\operatorname{SrBr}_{2}\) (c) \(\mathrm{Fe}_{2} \mathrm{O}_{5}\) (d) \(\mathrm{Li}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

Correct formulas: (a) CaO, (b) SrBr₂, (d) Li₂O; (c) should be Fe₂O₃ or FeO.

Step 1: Validate calcium oxide formula

The compound \(\text{Ca}_2\text{O}\) suggests a combination of calcium ions \(\text{Ca}^{2+}\) and oxide ions \(\text{O}^{2-}\). One calcium ion combines with one oxide ion because their charges have the same magnitude and opposite signs, neutralizing each other. Thus, the correct formula is \(\text{CaO}\).

Step 2: Validate strontium bromide formula

The given formula \(\text{SrBr}_2\) consists of strontium ions \(\text{Sr}^{2+}\) and bromide ions \(\text{Br}^-\). Since strontium has a 2+ charge and each bromide ion has a 1- charge, two bromide ions are required to balance one strontium ion, making the formula \(\text{SrBr}_2\) correct.

Step 3: Validate iron(II) oxide formula

The formula \(\text{Fe}_2\text{O}_5\) is proposed but incorrect because iron forms common ions of \(\text{Fe}^{2+}\) or \(\text{Fe}^{3+}\), while oxide ions are \(\text{O}^{2-}\). To balance 3+ ions, \(\text{Fe}_2\text{O}_3\) is often formed when using iron (III), or \(\text{FeO}\) in case of iron (II) oxide based on 2+ charge.

Step 4: Validate lithium oxide formula

The compound \(\text{Li}_2\text{O}\) pairs lithium ions \(\text{Li}^+\) with oxide ions \(\text{O}^{2-}\). Two lithium ions balance the 2- charge of one oxide ion. Thus, \(\text{Li}_2\text{O}\) is the correct formula.

Key Concepts

calcium oxide

Calcium oxide is a simple yet important ionic compound. An ionic bond forms between the calcium ion, Ca\(^{2+}\), and the oxide ion, O\(^{2-}\). Due to their matching charges in magnitude but opposite in sign, only one calcium ion is needed to balance one oxide ion's charge. This results in a neutral compound, adding up to the formula CaO.

• Calcium ions carry a 2+ charge, indicating a loss of two electrons from a neutral calcium atom.
• Oxide ions, on the other hand, carry a 2- charge, formed when an oxygen atom gains two electrons.

When a compound like Ca\(_2\)O is proposed, it suggests an unnecessary extra calcium ion. Remember, in ionic compounds, the total positive charge must equal the total negative charge. This ensures the compound is electrically neutral. Hence, CaO is the correct formula.

strontium bromide

Strontium bromide is another type of ionic compound that forms through the interaction between strontium ions (Sr\(^{2+}\)) and bromide ions (Br\(^-\)). The resulting compound is correctly noted as SrBr\(_2\).

• The strontium ion has a charge of 2+, indicating it loses two electrons.
• Bromide ions each have a 1- charge, gaining one electron to form a stable ion.

Two bromide ions are required to balance the 2+ charge of one strontium ion. This results in a net neutral charge, which is essential for the formation of stable ionic compounds. Therefore, SrBr\(_2\) is indeed the correct formula, showing the proper ratio of the ions involved in this compound.

iron oxide

Iron oxides can be a bit tricky due to the multiple oxidation states iron can exhibit. Commonly, you might encounter either iron(II) oxide (FeO) or iron(III) oxide (Fe\(_2\)O\(_3\)).

• Iron(II) oxide involves Fe\(^{2+}\) ions combining with O\(^{2-}\) ions. A one-to-one ratio of these ions forms FeO.
• Iron(III) oxide uses Fe\(^{3+}\) ions, requiring two iron ions to balance three oxide ions. This results in the compound Fe\(_2\)O\(_3\).

A suggested formula like Fe\(_2\)O\(_5\) is incorrect because it does not correspond to the charge balance created with common iron states. By understanding the charges that common iron ions possess, we can easily predict and write correct formulas for its oxides.

lithium oxide

Lithium oxide showcases a more straightforward ionic compound formation. It involves lithium ions, Li\(^+\), bonding with oxide ions, O\(^{2-}\). To achieve charge neutrality in lithium oxide, two Li\(^+\) ions are needed to balance the charge of one O\(^{2-}\) ion, leading to the formula Li\(_2\)O.

• Lithium ions are derived from lithium atoms that have lost one electron each, resulting in a 1+ charge for each ion.
• The oxide ion has a 2- charge, needing two positively charged lithium ions to fully neutralize it.

This balance of charges results in a stable ionic compound, confirming that Li\(_2\)O is the correct chemical formula for lithium oxide. This simple 2:1 ratio highlights how fundamental charge balance rules govern the composition of ionic compounds.