Problem 104 - Chemistry: The Central Science in SI Units, Global Edition

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

The likely identity of the metal is potassium (K), as the lilac-purple flame is associated with potassium. The balanced chemical equation for the reaction of the white substance (potassium superoxide) with water is: \[ 2 \mathrm{KO}_{2} (s) + 2 \mathrm{H}_{2}\mathrm{O} (l) \longrightarrow 2 \mathrm{KOH} (aq) + \mathrm{H}_{2}\mathrm{O}_{2} (aq) \]

Step 1: (a) Identify the alkali metal

We are given that the alkali metal reacts with oxygen to form a solid white substance. The presence of a lilac-purple flame is indicative of the alkali metal being potassium (K). This is because each alkali metal produces a characteristic flame color due to its atomic emission spectrum, and the lilac-purple flame is associated with potassium. Thus, the likely identity of the metal is potassium (K).

Step 2: (b) Write balanced chemical equation for the reaction of the white substance with water

We have established that the metal in question is potassium (K). When potassium reacts with oxygen, it forms potassium superoxide (KO₂), which is a solid white substance. The reaction of potassium superoxide with water produces potassium hydroxide (KOH) and hydrogen peroxide (H₂O₂). The balanced chemical equation for this reaction is: \[ 2 \mathrm{KO}_{2} (s) + 2 \mathrm{H}_{2}\mathrm{O} (l) \longrightarrow 2 \mathrm{KOH} (aq) + \mathrm{H}_{2}\mathrm{O}_{2} (aq) \] This equation represents the reaction between the potassium superoxide and water, resulting in the formation of potassium hydroxide and hydrogen peroxide.

Key Concepts

Alkali Metals

Alkali metals are a group of elements found in group 1 of the periodic table. This family of metals includes lithium, sodium, potassium, rubidium, cesium, and francium. They are known for being highly reactive, especially with water. This reactivity is due to their single valence electron, which they readily lose to form positive ions (cations).
Some characteristics of alkali metals include:

• They are soft and can be cut with a knife.
• They have low melting points compared to most other metals.
• They form ionic compounds such as metal hydroxides when reacting with water.

These metals also display unique colors when burned, due to their atomic emission spectrum. For instance, when potassium is burned, it exhibits a lilac or purple flame. This flame color helps in identifying the specific metal. Overall, understanding the properties of alkali metals is essential in predicting their reactions with various substances, such as oxygen and water.

Potassium Superoxide

Potassium superoxide ( KO₂) is a compound formed from the reaction of potassium with oxygen. It's interesting due to its role in generating oxygen and removing carbon dioxide when used in respiratory devices. In this context, however, it's important to focus on its white solid appearance and its reactivity with water.
When potassium superoxide reacts with water, it produces potassium hydroxide (KOH) and hydrogen peroxide ( H₂O₂). This reaction is quite fascinating because it involves the transformation of a superoxide into a hydroxide and an essential, yet unstable compound, hydrogen peroxide.
This process can be summarized in the chemical equation:

• KO₂ (s) + H₂O (l) → KOH (aq) + H₂O₂ (aq)

The behavior of potassium superoxide showcases the fascinating characteristics of the superoxide ion ( O₂⁻). Understanding this is crucial in applications where potassium superoxide is utilized, such as in the self-contained breathing apparatuses for producing oxygen.

Balancing Chemical Equations

Balancing chemical equations is a fundamental skill in chemistry that ensures the law of conservation of mass is upheld. This principle states that matter cannot be created or destroyed in a closed system. Hence, a balanced chemical equation has the same number of atoms of each element on both sides.
To balance equations, follow these steps:

• Write the unbalanced equation with correct formulas of reactants and products.
• Count and compare the number of atoms of each element on both sides of the equation.
• Add coefficients in front of the chemical formulas to equalize the number of atoms for each element.
• Continue adjusting the coefficients as needed, ensuring that you never change the subscripts in the formulas.
• Double-check to make sure the equation is balanced.

For instance, in the reaction given: 2KO₂ (s) + 2H₂O (l) → 2KOH (aq) + H₂O₂ (aq),
you see that the number of K, O, and H atoms is the same on both sides. This ensures that conservation of mass is strictly followed. A balanced equation allows for accurate predictions of reactants and products in any chemical reaction.