Problem 37 - Chemistry The Molecular Science

Question

Which of these are the correct formulas of compounds? For those that are not, give the correct formula. (a) \(\mathrm{Ca}_{2} \mathrm{O}\) (b) \(\mathrm{SrCl}_{2}\) (c) \(\mathrm{Fe}_{2} \mathrm{O}_{5}\) (d) \(\mathrm{K}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

(a) CaO, (b) Correct, (c) FeO or Fe2O3, (d) Correct.

Step 1: Check the Valency of Calcium and Oxygen

Calcium (Ca) has a valency of +2, and oxygen (O) has a valency of -2. The correct formula should balance the charges to zero.

Step 2: Correct Formula for Calcium Oxide

With both elements having a valency of 2, the correct formula is \( \mathrm{CaO} \). The charges are balanced.

Step 3: Check the Valency of Strontium and Chlorine

Strontium (Sr) has a valency of +2, while chlorine (Cl) has a valency of -1. To balance the charges, there should be two chlorine ions.

Step 4: Verification of Strontium Chloride Formula

The formula \( \mathrm{SrCl}_{2} \) correctly balances the charges, so it is correct.

Step 5: Check the Valency of Iron and Oxygen in \( \mathrm{Fe}_{2} \mathrm{O}_{5} \)

Iron can have multiple valencies, commonly +2 (ferrous) and +3 (ferric). Oxygen has a valency of -2. We need to find a matching compound.

Step 6: Correct Formula for Iron Oxide

For common iron oxides, the correct formulas are \( \mathrm{FeO} \) or \( \mathrm{Fe}_{2} \mathrm{O}_{3} \), balancing the +2 or +3 valencies of iron. \( \mathrm{Fe}_{2} \mathrm{O}_{5} \) is incorrect.

Step 7: Check the Valency of Potassium and Oxygen

Potassium (K) has a valency of +1, and oxygen (O) has a valency of -2. Two potassium ions are needed to balance one oxygen ion.

Step 8: Verification of Potassium Oxide Formula

The formula \( \mathrm{K}_{2} \mathrm{O} \) correctly balances the charges, so it is correct.

Key Concepts

Valency

Understanding valency is crucial when working with chemical formulas. Valency refers to the combining power of an element, typically based on the number of electrons that can be lost, gained, or shared to achieve a full outer shell. Each element has a specific valency which determines how it will bond with other elements.
For example:

• Calcium (Ca) has a valency of +2 because it loses two electrons.
• Oxygen (O) has a valency of -2 because it gains two electrons.
• Chlorine (Cl) has a valency of -1 as it gains one electron.
• Potassium (K) has a valency of +1 as it loses one electron.

The valency helps in predicting how elements combine to form compounds. By knowing the valencies, one can determine the correct ratios of elements that form neutral compounds.

Ionic Compounds

Ionic compounds consist of positively and negatively charged ions. These are formed when atoms transfer electrons to achieve stability.
A key feature of ionic compounds is that they are neutral, meaning the total positive charge must balance the total negative charge. This balance is the reason behind the specific formulas for ionic compounds.
Common examples include:

• Calcium Oxide (\( \mathrm{CaO} \)): Calcium loses two electrons to oxygen, forming a bond.
• Strontium Chloride (\( \mathrm{SrCl}_{2} \)): Strontium loses two electrons, each gained by chlorine ions.

The ionic bond has a great impact on the properties of these compounds, making them solid at room temperature with high melting points.

Charge Balancing

Balancing the charge is essential in writing correct chemical formulas for ionic compounds. Each compound must have a net charge of zero, meaning the total positive charges and negative charges must equal each other.
This requires identifying the valency of each ion and calculating how many ions are needed to balance the overall charge. For example:

• In \( \mathrm{CaO} \), the +2 charge of calcium perfectly balances the -2 charge of oxygen.
• In \( \mathrm{Fe}_{2} \mathrm{O}_{3} \), two iron atoms (each with a +3 charge) balance three oxygen atoms (each with a -2 charge), resulting in zero net charge.
• K in \( \mathrm{K}_{2} \mathrm{O} \) has a +1 charge, thus two potassium ions are needed to balance the -2 charge of one oxygen ion.

Correctly balancing these charges ensures a neutral compound, crucial for determining the chemical behavior and properties of the compound.