Problem 92 - Chemistry and Chemical Reactivity

Question

When \(1.00 \mathrm{g}\) of a white solid \(\mathrm{A}\) is strongly heated, you obtain another white solid, \(\mathrm{B},\) and a gas. An experiment is carried out on the gas, showing that it exerts a pressure of \(209 \mathrm{mm}\) Hg in a 450 -mL flask at \(25^{\circ} \mathrm{C}\) Bubbling the gas into a solution of \(\mathrm{Ca}(\mathrm{OH})_{2}\) gives another white solid, C. If the white solid B is added to water, the resulting solution turns red litmus paper blue. Addition of aqueous HCl to the solution of \(B\) and evaporation of the resulting solution to dryness yield 1.055 g of a white solid D. When \(\mathrm{D}\) is placed in a Bunsen burner flame, it colors the flame green. Finally, if the aqueous solution of \(B\) is treated with sulfuric acid, a white precipitate, \(\mathbf{E},\) forms. Identify the lettered compounds in the reaction scheme.

Step-by-Step Solution

Answer

A = BaCO3, B = BaO, C = CaCO3, D = BaCl2, E = BaSO4.

Step 1: Identify Gas Produced

The gas obtained from heating solid A can produce a white precipitate when bubbled through a solution of \( \text{Ca(OH)}_2 \), suggesting it is CO2, as CO2 reacts with \( \text{Ca(OH)}_2 \) to form calcium carbonate, a white solid.

Step 2: Identify Solid B

Solid B turns red litmus paper blue, indicating it is a base. The formation of a solid D upon reaction with aqueous HCl suggests solid B might be a metal oxide or hydroxide. Given the observation, solid B is likely sodium carbonate \( \text{Na}_2\text{CO}_3 \).

Step 3: Identify Solid D

The white compound D, formed after reaction and evaporation, colors a flame green, indicating the presence of barium. Solid D in this case is likely barium chloride \( \text{BaCl}_2 \).

Step 4: Identify Solid E

The substance E precipitates when sulfuric acid is added to the aqueous solution of B. This suggests E is barium sulfate \( \text{BaSO}_4 \), due to the reaction of Ba ions with \( \text{SO}_4^{2-} \).

Step 5: Final Identification of Solid A

Compiling all evidence, solid A is likely to be barium carbonate \( \text{BaCO}_3 \), since it decomposes to form \( \text{BaO} \) (which is part of \( \text{B} \) that turns litmus blue) and CO2 gas, and upon reactions, leads to the formation of \( \text{BaCl}_2 \) and \( \text{BaSO}_4 \).

Key Concepts

Gas Identification

Identifying gases is an important part of chemistry, often necessary for understanding reactions better. In the exercise, heating the compound labeled 'A' releases a gas. This gas reacts with a solution of calcium hydroxide \[ \text{Ca(OH)}_2 \] which causes the formation of a white solid. This is indicative of carbon dioxide \( \text{CO}_2 \) because when \( \text{CO}_2 \) is bubbled through \( \text{Ca(OH)}_2 \), it forms a precipitate of calcium carbonate \( \text{CaCO}_3 \). Here are a few points to remember about identifying gases in reactions:

• Always consider the conditions and the products formed.
• Conduct specific tests, like bubbling the gas through a solution, to confirm its identity.
• Use chemical knowledge and reactions that are commonly associated with certain gases.

These steps can be crucial for determining what gas is involved in a chemical process, much like identifying carbon dioxide in this exercise.

Metal Oxides

Metal oxides are compounds composed of metal ions combined with oxygen. These chemicals often result from the heating of metal carbonates, which decompose to form the respective metal oxide and release gas, such as carbon dioxide. In this problem, compound 'A' likely decomposes to a metal oxide and carbon dioxide gas.

Understanding the properties of metal oxides can help identify them in reactions. Metal oxides generally exhibit:

• Basic properties: They react with acids to form salts and water.
• High melting points: Involves strong ionic bonding.
• Forms: Could exist as simple or complex oxides depending on conditions.

Metal oxides play a significant role in various industrial processes and are often involved in important reactions like acid-base chemistry.

Flame Test

The flame test is a simple yet effective method of identifying metal ions based on the color they produce in a flame. In this scenario, compound 'D' provides a green flame, pointing to the presence of barium ions.

Here's how the flame test works:

• Place a small sample of the compound in a flame.
• Observe the color emitted, which is characteristic of specific elements due to their electronic transitions.
• Compare the color to known standards for identification.

Some common flame colors for metal ions include:

• Barium: Green
• Sodium: Yellow
• Calcium: Orange-red

The flame test is a quick and practical approach to confirm the presence of specific metal ions within a sample.

Precipitation Reaction

Precipitation reactions occur when two solutions are combined, resulting in the formation of an insoluble solid, known as a precipitate. In the exercise, the formation of compound 'E' upon adding sulfuric acid to the solution suggests a classic precipitation reaction.

To understand precipitation reactions better, consider these points:

• Occurs due to the exchange of ions between the two reactant solutions.
• The precipitate appears as a solid portion that separates from the solution.
• Solubility rules help predict whether a precipitate will form.

For instance, the reaction of barium ions with sulfate ions produces barium sulfate \( \text{BaSO}_4 \), a common precipitate in chemical reactions. Recognizing these reactions is key in identifying the components in unknown samples.