Question

(a) \(\mathrm{A}\) group 2 metal, \(\mathrm{M}\), dissolves in liquid \(\mathrm{NH}_{3}\), and from the solution, compound \(\mathbf{A}\) can be isolated. A slowly decomposes to B with liberation of \(\mathrm{NH}_{3}\) and a gas \(\mathrm{C}\) Metal \(\mathbf{M}\) gives a crimson flame test; through blue glass, the flame appears pale purple. Suggest identities for \(\mathbf{M}, \mathbf{A}, \mathbf{B}\) and \(\mathbf{C}\) (b) The group 2 metal \(\mathbf{X}\) occurs naturally in great abundance as the carbonate. Metal X reacts with cold water, forming compound \(\mathbf{D}\), which is a strong base. Aqueous solutions of \(\mathbf{D}\) are used in qualitative tests for \(\mathrm{CO}_{2} .\) X combines with \(\mathrm{H}_{2}\) to give a saline hydride that is used as a drying agent. Identify \(\mathbf{X}\) and \(\mathbf{D}\). Write equations for the reaction of \(\mathbf{X}\) with \(\mathrm{H}_{2} \mathrm{O}\) and of the hydride of \(\mathbf{X}\) with \(\mathrm{H}_{2} \mathrm{O}\). Explain how you would carry out a qualitative test for \(\mathrm{CO}_{2}\) using an aqueous solution of \(\mathbf{D}\)

Short answer

M is strontium, A is an ammonia complex or amide, B is strontium nitride, C is hydrogen gas. X is calcium, D is calcium hydroxide. Reactions: Ca + 2H₂O → Ca(OH)₂ + H₂; CaH₂ + 2H₂O → Ca(OH)₂ + 2H₂. Test for CO₂ with lime water turning milky.

Step-by-step solution

Analyze Metal M from Flame Test

The metal M gives a crimson flame test, and through blue glass, the flame appears pale purple. This characteristic matches that of strontium (Sr), a group 2 metal. Strontium is known for its red/crimson flame color.

Identify Compound A & B

The metal M (strontium) dissolves in liquid ammonia (NH₃) to form a possible amide or complex, which starts decomposing to form the metal nitride or hydride - typical decomposition process involving ammonia. Therefore, compound A could be Sr(NH₃)_x or Sr(NH₂)_x, and upon decomposition, compound B is likely Sr₃N₂ (strontium nitride) with the liberation of NH₃ and hydrogen gas (H₂) as compound C.

Analyze Metal X and Compound D

Group 2 metal X reacts with water, forming a strong base as compound D. Since it reacts with water and produces a base, it is most likely calcium (Ca), which is abundant as a carbonate in nature. The strong base D formed is calcium hydroxide (Ca(OH)₂).

Identify Reaction Equations

Calcium (Ca) reacts with water to produce calcium hydroxide and hydrogen gas: \[ \text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2 \]Calcium reacts with hydrogen to form calcium hydride (CaH₂). This hydride reacts with water as follows: \[ \text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + 2\text{H}_2 \]

Qualitative Test for CO₂

To test for carbon dioxide using an aqueous solution of calcium hydroxide (Ca(OH)₂), known as lime water, introduce the carbon dioxide gas into the solution. The reaction \( \text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \) will occur, forming calcium carbonate, which precipitates, turning the solution milky.

Key concepts

Understanding Flame Tests for Metals

A flame test is a simple and informative qualitative analysis technique used to identify certain metal ions based on the characteristic color they emit when heated in a flame. The process involves cleaning a wire loop or other tool, dipping it into a solution or powder of the metal compound, and placing it in the flame. When the metal atoms in the compound are excited by the heat of the flame, they emit light at specific wavelengths. The color of the flame can be directly correlated to specific elements, and consequently, specific metals. For example, in the case of group 2 metal strontium (Sr), the flame appears crimson. When viewed through a blue glass, the crimson flame turns pale purple, due to filtering out certain wavelengths. This specificity makes flame tests a useful initial step in the identification of unknown metal ions.

The Role of Metal Compounds in Chemical Reactions

Metal compounds play a crucial part in countless chemical reactions. These compounds often contain metal cations paired with various anions, forming stable structures that can participate in reactions. In the original exercise, metal M is involved in forming compound A, when dissolved in liquid ammonia. This implies that metal compounds can form complexes or amides, transforming the metal's state and properties. As compounds participate in chemical reactions, they may undergo decomposition, recombination, or other transformation processes. For instance, upon decomposition, compound A evolves into compound B and liberates compound C. This exemplifies how metal compounds are dynamic participants in chemical transformations, continuously altering their form and function.

Grasping Chemical Reactions with Metals

Chemical reactions involving metals, especially those in group 2, can demonstrate a wide range of transformations. When these metals react with other elements or compounds, they often form stable products or evolve gases.For instance, group 2 metal X, identified as calcium, reacts with water to form calcium hydroxide, a strong base. The chemical equation for this reaction is:\[\text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2\]This equation not only shows the product, Ca(OH)₂, but also highlights the release of hydrogen gas, a common outcome in reactions involving metal in water.Additionally, the reaction of calcium hydride with water is another crucial insight:\[\text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + 2\text{H}_2\]Understanding these reactions deepens our comprehension of the role metals play in converging into other chemical elements.

Performing Qualitative Tests with Metal Compounds

Qualitative tests are techniques used to identify or confirm the presence of specific chemical elements or compounds. They often rely on observable changes such as color, precipitate formation, or gas evolution to draw conclusions.In the context of the original exercise, a qualitative test is described for identifying carbon dioxide (CO₂) using the aqueous solution of calcium hydroxide, commonly known as lime water.When CO₂ gas is bubbled through lime water, the chemical reaction that occurs is:\[\text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}\]Calcium carbonate (CaCO₃) forms as a precipitate, making the solution turn milky. This observable change confirms the presence of carbon dioxide. Through such qualitative tests, we can gain valuable insight into chemical compositions and reactions, largely relying on the predictable behaviors of metal compounds.