Problem 118 - Chemistry and Chemical Reactivity

Question

When \(1.00 \mathrm{g}\) of a white solid \(\mathrm{A}\) is strongly heated, you obtain another white solid, \(\mathbf{B},\) and a gas. An experiment is carried out on the gas, showing that it exerts a pressure of \(209 \mathrm{mm}\) Hg in a \(450-\mathrm{mL}\) flask at \(25^{\circ} \mathrm{C} .\) Bubbling the gas into a solution of \(\mathrm{Ca}(\mathrm{OH})_{2}\) gives another white solid, \(\mathrm{C}\). 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 \(\mathbf{B}\) and evaporation of the resulting solution to dryness yield 1.055 g of a white solid D. When \(\mathbf{D}\) is placed in a Bunsen burner flame, it colors the flame green. Finally, if the aqueous solution of \(\mathbf{B}\) is treated with sulfuric acid, a white precipitate, \(\mathbf{E},\) forms. Identify the reaction scheme. (IMAGE CAN'T COPY)

Step-by-Step Solution

Answer

The reactions involve calcium compounds: A is CaCO₃, B is CaO, C is CaCO₃, D is CaCl₂, and E is CaSO₄.

Step 1: Determine the Gas Identity

The gas exerts a pressure of 209 mm Hg in a 450-mL flask at 25°C. This points to the possible formation of CO₂. When CO₂ is bubbled into a Ca(OH)₂ solution, it forms calcium carbonate (CaCO₃) which aligns with the creation of a white solid, C.

Step 2: Analyze the Characteristics of Solid B

Solid B, when dissolved in water, turns red litmus blue. This indicates that substance B is basic in nature. Alkaline earth metal oxides such as CaO can have this property, suggesting that B could be CaO.

Step 3: Determine the Composition of Solid A

Since heating solid A yields a gas (CO₂) and a solid (B, which was determined to be CaO), solid A is likely calcium carbonate (CaCO₃). The decomposition reaction can be represented as: \[ \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \]

Step 4: Identify Solid D

When aqueous HCl is added to B (CaO), CaCl₂ forms, which is D. On heating, it produces a green flame, indicating that D is likely calcium chloride (CaCl₂) as calcium compounds generally produce a green flame.

Step 5: Identify the Precipitate E

Addition of sulfuric acid to the aqueous solution of solid B (CaO) produces a white precipitate (E), which is likely calcium sulfate (CaSO₄). This confirms that B is an oxide of calcium, supporting the identity of B as CaO.

Key Concepts

Gas Identification

Identifying a gas generated in a chemical reaction involves understanding its properties and knowing how it interacts with other substances. In this exercise, heating a solid led to the formation of a gas which was identified by its behavior and effect. This gas exerted a pressure of 209 mm Hg in a 450 mL flask at 25°C. Such conditions are typical for carbon dioxide ( CO_2 ). Additionally, carbon dioxide creates a white precipitate of calcium carbonate ( CaCO_3 ) when bubbled through a Ca(OH)_2 solution. This reaction aligns with the initial information, confirming that the gas involved is indeed CO_2 . Understanding how gases interact with solutions is key for their identification in chemical experiments.

Precipitate Formation

Precipitate formation is a common occurrence in chemical reactions where an insoluble solid is formed from two soluble substances. In the exercise, the gas ( CO_2 ) bubbling through a Ca(OH)_2 solution results in the formation of a white solid, which is calcium carbonate ( CaCO_3 ). Similarly, when CaO (solid B) in solution is treated with sulfuric acid, a white precipitate ( CaSO_4 ) forms, further strengthening its identity as calcium oxide. Precipitates are useful as they often indicate the completion of a reaction, making them important tools for chemists.

Acid-Base Reactions

Acid-base reactions are characterized by the transfer of hydrogen ions (H+). In the exercise, the identification of an alkaline substance occurs when solid B is added to water, resulting in a solution that turns red litmus paper blue. This indicates the presence of a base, suggesting that B is most likely CaO , which in water forms calcium hydroxide ( Ca(OH)_2 ), a basic solution. Understanding how acids and bases react—and how indicators like litmus paper function—is crucial when analyzing chemical reactions.

Flame Test

The flame test is a simple yet effective method for identifying metal ions based on the color the flame turns when a sample is heated. In this scenario, solid D, upon heating, imparts a green color to a Bunsen burner flame. This coloration is typical of calcium ions, verifying that solid D is calcium chloride ( CaCl_2 ). Flame tests are valuable because they provide a quick and distinct means of confirming the presence of specific elements, especially metal ions, in a sample.