Problem 104 - Chemistry The Central Science

Question

(a) One of the alkali metals reacts with oxygen to form a solid white substance. When this substance is dissolved in water, the solution gives a positive test for hydrogen peroxide, \(\mathrm{H}_{2} \mathrm{O}_{2}.\) When the solution is tested in a burner flame, a lilac-purple flame is produced. What is the likely identity of the metal? (b) Write a balanced chemical equation for the reaction of the white substance with water.

Step-by-Step Solution

Answer

(a) The likely identity of the alkali metal is potassium (K), and the white substance formed is potassium peroxide (\(\mathrm{K}_2\mathrm{O}_2\)). (b) The balanced chemical equation for the reaction of potassium peroxide with water is: \[ \mathrm{K}_2\mathrm{O}_2 (s) + 2 \mathrm{H}_2\mathrm{O}(l) \rightarrow 2\mathrm{KOH}(aq) + \mathrm{H}_2\mathrm{O}_2 (aq)\]

Step 1: Identify the alkali metal based on the given information

The given properties of the alkali metal are: 1. Reacts with oxygen to form a solid white substance. 2. Solution gives a positive test for hydrogen peroxide, \(\mathrm{H}_{2} \mathrm{O}_{2}\), when the white substance is dissolved in water. 3. When the solution is tested in a burner flame, a lilac-purple color is produced. From the given information, we can infer that the alkali metal most likely reacts with oxygen to form a peroxide (\(\mathrm{M}_2\mathrm{O}_2\)). Additionally, we know that the alkali metals have characteristic colors when they burn in a flame. Based on the lilac-purple flame, the likely alkali metal is potassium (K). Thus, the white substance formed is potassium peroxide (\(\mathrm{K}_2\mathrm{O}_2\)).

Step 2: Write a balanced chemical equation for the reaction of potassium peroxide with water

To write the balanced chemical equation, first break down the reaction: 1. Potassium peroxide (\(\mathrm{K}_2\mathrm{O}_2\)) reacts with water (\(\mathrm{H}_2\mathrm{O}\)). 2. Hydrogen peroxide (\(\mathrm{H}_2\mathrm{O}_2\)) is produced, as mentioned in the question. We can now write the balanced chemical equation as: \[ \mathrm{K}_2\mathrm{O}_2 (s) + 2 \mathrm{H}_2\mathrm{O}(l) \rightarrow 2\mathrm{KOH}(aq) + \mathrm{H}_2\mathrm{O}_2 (aq)\] Here, we have potassium peroxide reacting with water to produce potassium hydroxide and hydrogen peroxide. The equation is balanced, with equal numbers of each element on both sides.

Key Concepts

Potassium Peroxide

Potassium peroxide is a chemical compound with the formula \( \mathrm{K}_2\mathrm{O}_2 \). It's an example of an inorganic peroxide, and when it comes into contact with moisture, it releases oxygen. This compound, typically a solid white substance, is one of several peroxides associated with alkali metals.

The production of potassium peroxide occurs when potassium, the alkali metal, reacts with oxygen in the air. In laboratories and various industrial processes, it can be used as an oxygen source due to its ability to produce oxygen when heated. It's also valuable in environmental chemistry for removing carbon dioxide \( \mathrm{CO}_2 \) from the air, which highlights its role in potential life support systems, such as those used in space travel.

Chemical Reaction Equation

Chemical reaction equations are symbolic representations of chemical reactions, showing the starting substances, or reactants, converting to the resulting substances, or products. Writing a balanced chemical equation involves ensuring that the number of atoms for each element is the same on both sides of the equation, which complies with the law of conservation of mass.

For instance, in the reaction equation of potassium peroxide and water, \[ \mathrm{K}_2\mathrm{O}_2 (s) + 2 \mathrm{H}_2\mathrm{O}(l) \rightarrow 2\mathrm{KOH}(aq) + \mathrm{H}_2\mathrm{O}_2 (aq) \], the number of potassium, oxygen, and hydrogen atoms are equal on both sides. This is crucial for students to understand as it is a fundamental concept in chemical stoichiometry and helps in predicting the amounts of reactants needed or products formed in a chemical reaction.

Hydrogen Peroxide Detection

Hydrogen peroxide \( \mathrm{H}_2\mathrm{O}_2 \) can be identified using a variety of detection methods due to its oxidizing properties. In laboratories, it's often detected through a simple test where it's added to a solution containing potassium iodide \( \mathrm{KI} \). The \( \mathrm{H}_2\mathrm{O}_2 \) oxidizes the iodide ions into iodine, which can be observed as a yellow-brown coloration in the solution.

For the scenario presented in the textbook problem, when potassium peroxide dissolves in water, it yields hydrogen peroxide, which is detected by a positive test. This could imply testing for the typical reactions associated with \( \mathrm{H}_2\mathrm{O}_2 \), such as the oxidation of iodide ions, or using more sophisticated equipment like spectrophotometers that can measure the unique absorbance of this chemical at specific wavelengths. Regardless of the method, the ability to detect \( \mathrm{H}_2\mathrm{O}_2 \) is important in fields ranging from clinical diagnostics to environmental monitoring.

Flame Colors of Alkali Metals

The flame test is a qualitative analysis technique employed to detect the presence of certain metal ions based on the characteristic colors they emit when heated to high temperatures. Alkali metals are well known for their distinct flame colors. Sodium burns with a bright yellow flame, lithium yields a crimson color, while cesium and rubidium produce shades of violet and red-violet respectively.

The exercise points out that the unknown alkali metal emits a lilac-purple flame, which is characteristic for potassium. Such flame tests are fundamental experiments in analytical chemistry and often serve as a handy tool for a quick identification of metal ions, especially in educational settings. The vivid flame colors result from the excitement of electrons in the metal ion when exposed to heat. As the electrons return to their original energy state, they release photons of light, the color of which is specific to the energy levels involved, thereby serving as a 'fingerprint' for the element.