Problem 51 - Chemistry and Chemical Reactivity

Question

Give the formula for each of the following ionic compounds: (a) ammonium carbonate (b) calcium iodide (c) copper(II) bromide (d) aluminum phosphate (e) silver( I) acetate

Step-by-Step Solution

Answer

(a) (NH4)2CO3 (b) CaI2 (c) CuBr2 (d) AlPO4 (e) AgC2H3O2

Step 1: Identify Ions in Ammonium Carbonate

The chemical formula for ammonium is \( \text{NH}_4^+ \) and the formula for carbonate is \( \text{CO}_3^{2-} \). These are the ions present in ammonium carbonate.

Step 2: Balance Charges in Ammonium Carbonate

To balance the charges, two ammonium ions are needed to pair with one carbonate ion. Thus, the formula for ammonium carbonate is \( \text{(NH}_4)_2\text{CO}_3 \).

Step 3: Identify Ions in Calcium Iodide

Calcium has a charge of \( +2 \) (\( \text{Ca}^{2+} \)) and iodide has a charge of \( -1 \) (\( \text{I}^- \)). These are the ions in calcium iodide.

Step 4: Balance Charges in Calcium Iodide

Two iodide ions are required to neutralize the charge on one calcium ion, resulting in the formula \( \text{CaI}_2 \).

Step 5: Identify Ions in Copper(II) Bromide

Copper(II) indicates a charge of \( +2 \) on the copper ion (\( \text{Cu}^{2+} \)), and bromide has a charge of \( -1 \) (\( \text{Br}^- \)). These are the ions in copper(II) bromide.

Step 6: Balance Charges in Copper(II) Bromide

Two bromide ions are needed to balance the charge of one copper ion, so the formula is \( \text{CuBr}_2 \).

Step 7: Identify Ions in Aluminum Phosphate

Aluminum has a charge of \( +3 \) (\( \text{Al}^{3+} \)), and phosphate has a charge of \( -3 \) (\( \text{PO}_4^{3-} \)). These are the ions in aluminum phosphate.

Step 8: Balance Charges in Aluminum Phosphate

Since the charges are already balanced, the formula for aluminum phosphate is \( \text{AlPO}_4 \).

Step 9: Identify Ions in Silver(I) Acetate

Silver(I) indicates a charge of \( +1 \) (\( \text{Ag}^+ \)), and acetate has a charge of \( -1 \) (\( \text{C}_2\text{H}_3\text{O}_2^- \)). These are the ions in silver(I) acetate.

Step 10: Balance Charges in Silver(I) Acetate

The charges are naturally balanced with one silver ion and one acetate ion, so the formula is \( \text{AgC}_2\text{H}_3\text{O}_2 \).

Key Concepts

Chemical Formula

Formulating the chemical formula of an ionic compound involves putting together symbols and numbers that represent the elements and the number of atoms of each element. A chemical formula is crucial as it provides a quick insight into the compound's structure and properties. For ionic compounds, this involves indicating the ions present and their respective amounts to ensure that the charges are balanced.
To write a chemical formula, identify the symbols of the cation (positive ion) and anion (negative ion). Next, the subscripts are used to show the necessary number of each ion needed for charge neutrality. This neutrality means the sum of the positive charges equals the sum of the negative charges.

• For instance, ammonium carbonate is composed of ammonium ( H_4^+ ) and carbonate ( CO_3^{2-} ). The balancing of their charges is shown with the chemical formula (NH_4)_2CO_3 .
• In another example, calcium iodide consists of one calcium ion ( Ca^{2+} ) and two iodide ions ( I^- ), resulting in the formula CaI_2 .

Understanding how to correctly write a chemical formula is vital for effectively communicating chemical information.

Ion Identification

Identifying ions is the first step in understanding ionic compounds. Each ion in a compound has specific symbol and charge that is typically derived from the number of electrons lost or gained by an atom. Cations, such as Ca^{2+} and NH_4^+ , carry a positive charge as they lose electrons. Anions, such as I^- or CO_3^{2-} , carry a negative charge as they gain electrons.
When identifying the ions in a compound:

• Check the periodic table or known polyatomic ions to find the charge of the atoms or groups of atoms forming ions.
• For transition metals like copper(II) ( Cu^{2+} ), care is needed as they can exhibit multiple oxidation states. Roman numerals in their names indicate the charge.

Knowing the correct charges of the ions is critical to forming accurate chemical formulas.

Charge Balancing

Balancing the charges in ionic compounds is how we ensure the electrical neutrality of the compound. This is done by adjusting the ratio of cations to anions until the positive and negative charges cancel each other out.
For every ionic compound:

• The total positive charge must equal the total negative charge.
• Adjust the subscripts in the formula to get the lowest whole number ratio.

For instance, in copper(II) bromide ( CuBr_2 ), two bromide ions ( Br^- ) are needed for each copper(II) ion ( Cu^{2+} ) to equalize the charges. Likewise, aluminum phosphate ( AlPO_4 ) requires no additional ions since the charges ( Al^{3+} and PO_4^{3-} ) naturally balance. Charge balancing is essential for writing valid chemical formulas and understanding reactions.

Naming Ionic Compounds

Naming ionic compounds involves stating the cation first and the anion second. Naming conventions are crucial for unambiguously identifying compounds based on their composition.
When naming ionic compounds, follow these rules:

• The cation's name comes first, followed directly by the anion's. For instance, calcium iodide is formed from calcium and iodide ions.
• For transition metals and elements with multiple oxidation states, use Roman numerals to denote the ionic charge, such as copper(II) bromide.
• Polyatomic ions, like ammonium ( NH_4^+ ) or carbonate ( CO_3^{2-} ), retain their specific names in the compound.

Naming each component of the compound correctly ensures clear communication, especially in scientific and educational settings.