Problem 85 - Chemistry Matter and Change

Question

Which are correct formulas for ionic compounds? For those that are not correct, give the correct formula and justify your answer. $$ \begin{array}{ll}{\text { a. AlCl }} & {\text { c. BaOH }_{2}} \\ {\text { b. } \mathrm{Na}_{3} \mathrm{SO}_{4}} & {\text { d. } \mathrm{Fe}_{2} \mathrm{O}}\end{array} $$

Step-by-Step Solution

Answer

Correct formulas are: a. AlCl₃, b. Na₂SO₄, c. Ba(OH)₂, d. FeO.

Step 1: Analyze Formula a, AlCl

The aluminum ion is \( \text{Al}^{3+} \) and chlorine is \( \text{Cl}^- \). To have a neutral compound, the charges must balance. Therefore, the correct formula is \( \text{AlCl}_3 \).

Step 2: Analyze Formula b, Na₃SO₄

The sodium ion is \( \text{Na}^+ \) and the sulfate ion is \( \text{SO}_4^{2-} \). For the charges to balance, we need two sodium ions for each sulfate ion. The correct chemical formula is \( \text{Na}_2\text{SO}_4 \).

Step 3: Check Formula c, Ba(OH)₂

The barium ion is \( \text{Ba}^{2+} \) and the hydroxide ion is \( \text{OH}^- \). The correct proportion is already given in the formula \( \text{Ba(OH)}_2 \), which balances the charges, so it is correct.

Step 4: Analyze Formula d, Fe₂O

The iron ion can be \( \text{Fe}^{2+} \) or \( \text{Fe}^{3+} \) and oxide ion is \( \text{O}^{2-} \). Given \( \text{Fe}_2 \text{O} \), assuming \( \text{Fe}^{2+} \), the correct formula is \( \text{FeO} \) as \( \text{Fe}^{3+} \) would require different balancing.

Key Concepts

Chemical Formulas

Chemical formulas represent the composition of a compound by showing the elements involved and the number of atoms of each element. For ionic compounds, the chemical formula is established based on the charges of the ions involved.
- Each chemical formula is unique to the specific elements and stoichiometric ratios that achieve charge balance.
Chemical formulas are important for communicating the exact structure of a compound using letters and numbers. By specifying how many atoms of each element are present with subscripts, it provides clarity on the compound's composition.
For instance, \( \text{Na}_2\text{SO}_4 \) indicates there are two sodium (Na) atoms and one sulfate group (SO₄) present in the formula. The subscripts ensure that both the qualitative and quantitative nature of the elements is clear.

Charge Balance

In ionic compounds, charge balance is a fundamental principle.
An ionic compound forms through the transfer of electrons between metal and non-metal ions, leading to positively charged cations and negatively charged anions. To form a stable ionic compound, the total positive charge from the cations must equal the total negative charge from the anions.
For example, the compound \( \text{AlCl}_3 \) demonstrates charge balance:

• Aluminum contributes a charge of \( +3 \)
• Each chloride ion has a charge of \( -1 \)

Thus, three chloride ions are required to counterbalance one aluminum ion, achieving charge neutrality. Charge balance ensures the compound is stable and electrically neutral.

Ions in Compounds

Ions are charged particles that form when atoms gain or lose electrons. In compounds, ions are key to the formation and characteristics of ionic bonds.
- Positive ions, or cations, are formed when an atom loses electrons. For instance, the sodium ion, \( \text{Na}^+ \), is a cation.
- Negative ions, or anions, occur when an atom gains electrons. For example, the chloride ion, \( \text{Cl}^- \), is an anion.
When forming compounds like \( \text{Na}_2\text{SO}_4 \), cations and anions bond by electrostatic attractions. The sodium cations partner with the sulfate anion to establish a stable compound, demonstrating how ions function within a compound.

Neutral Compounds

A neutral compound is one without an overall electric charge, achieved through the complete balance of charges between component ions. Every stable ionic compound must be neutral, reflecting effective charge balance.
Consider \( \text{Ba(OH)}_2 \) as an example:

• Barium (Ba) is a \( 2+ \) cation.
• The hydroxide (OH) is a \( 1- \) anion.

With two hydroxide ions, the total negative charge \( (2 \times -1) \) balances the positive charge provided by one barium ion, ensuring the compound is neutral. Neutrality in compounds is key for stability and reactivity in chemical environments.