Problem 126 - General Chemistry: Principles and Modern Applications

Question

Two of the following, through a reaction occurring in a weakly acidic solution, produce the same gaseous product. They are (a) \(\mathrm{CaH}_{2}(\mathrm{s}) ;\) (b) \(\mathrm{Na}_{2} \mathrm{O}_{2}(\mathrm{s})\) (c) \(\mathrm{NaOH}(\mathrm{s}) ;(\mathrm{d}) \mathrm{Al}(\mathrm{s}) ;(\mathrm{e}) \mathrm{NaHCO}_{3}(\mathrm{s}) ;(\mathrm{f}) \mathrm{N}_{2} \mathrm{H}_{4}(1)\).

Step-by-Step Solution

Answer

The two substances that produce the same gaseous product (hydrogen gas) in a weakly acidic solution are (a) calcium hydride \(\mathrm{CaH}_{2}\) and (d) aluminum \(\mathrm{Al}\).

Step 1: Identify gaseous products for each substance

The reaction of each substance in a weakly acidic solution will produce a specific product. Let's identify these potential gaseous products. They are (a) calcium hydride \(\mathrm{CaH}_{2}\), (b) sodium peroxide \(\mathrm{Na}_{2} \mathrm{O}_{2}\), (c) sodium hydroxide \(\mathrm{NaOH}\), (d) aluminum \(\mathrm{Al}\), (e) sodium bicarbonate \(\mathrm{NaHCO}_{3}\), (f) hydrazine \(\mathrm{N}_{2} \mathrm{H}_{4}\).

Step 2: Determine possible gaseous product for each reaction

Now let's assume their reactions in a weak acidic solution. (a) \(\mathrm{CaH}_{2} + 2\mathrm{H}^{+} \rightarrow \mathrm{Ca}^{2+} + 2\mathrm{H}_{2}\) (hydrogen gas), (b) \(\mathrm{Na}_{2} \mathrm{O}_{2} + 2\mathrm{H}^{+} \rightarrow 2\mathrm{Na}^{+} + \mathrm{H}_{2}\mathrm{O}_{2}\) (oxygen gas), (c) \(\mathrm{NaOH} + \mathrm{H}^{+} \rightarrow \mathrm{Na}^{+} + \mathrm{H}_{2}\mathrm{O}\) (no gaseous product), (d) \(\mathrm{Al} + 3\mathrm{H}^{+} \rightarrow \mathrm{Al}^{3+} + 1.5\mathrm{H}_{2}\) (hydrogen gas), (e) \(\mathrm{NaHCO}_{3} + \mathrm{H}^{+} \rightarrow \mathrm{Na}^{+} + \mathrm{H}_{2}\mathrm{O} + \mathrm{CO}_{2}\) (carbon dioxide gas), (f) \(\mathrm{N}_{2} \mathrm{H}_{4} + 4\mathrm{H}^{+} \rightarrow 2\mathrm{N}_{2} + 4\mathrm{H}_{2}\) (nitrogen gas and hydrogen gas).

Step 3: Identify the substances that produce the same gaseous product

Looking at the reactions above, it is clear that substances (a) calcium hydride \(\mathrm{CaH}_{2}\) and (d) aluminum \(\mathrm{Al}\) both produce hydrogen gas when react with protons in a weakly acidic solution.

Key Concepts

Hydrogen Gas Production

Hydrogen gas is a fascinating product of chemical reactions, especially in a weakly acidic environment. When substances like calcium hydride \( \mathrm{CaH}_{2} \) and aluminum \( \mathrm{Al} \) react in this condition, they both produce hydrogen gas as a result. This happens because:

• Calcium hydride reacts with protons \( \mathrm{H}^+ \) to form calcium ions and hydrogen gas: \( \mathrm{CaH}_{2} + 2\mathrm{H}^{+} \rightarrow \mathrm{Ca}^{2+} + 2\mathrm{H}_{2} \).
• Aluminum, when reacting with protons, produces aluminum ions and hydrogen gas: \( \mathrm{Al} + 3\mathrm{H}^{+} \rightarrow \mathrm{Al}^{3+} + 1.5\mathrm{H}_{2} \).

The appearance of hydrogen is a common indicator that a reactive metal is interacting with acids. This is a classic experiment in laboratories where the bubbling of hydrogen gas is visibly noticed. Understanding this reaction helps in grasping the behavior of metals in acidic solutions and predicting the gaseous products formed.

Weak Acid Reactions

Weak acid solutions differ from strong acids because they do not completely dissociate in water, leading to interesting reactions. In the context of the provided problem, the interactions are between weak acids and various chemical substances.

• A weak acid environment provides just enough \( \mathrm{H}^+ \) ions to initiate reactions with substances like \( \mathrm{CaH}_{2} \) and \( \mathrm{Al} \).
• The reaction rates may differ as weak acids produce fewer \( \mathrm{H}^+ \) ions compared to strong acids, influencing the overall reaction dynamics.

This controlled reaction environment allows for the selective production of specific gases, as seen in the exercise. Reacting substances in weak acid solutions is pivotal in determining the type of gaseous and aqueous products due to the partial dissociation, unlike strong acids that often fully react with metals.

Chemical Reactions

Chemical reactions refer to the transformation of substances through the breaking and forming of bonds. In these reactions, the initial substances known as reactants change into products.

• Each chemical equation in the solution illustrates a unique transformation, such as converting aluminum into an aluminum ion and hydrogen gas.
• The conservation of mass and atoms is a fundamental principle, where atoms are rearranged but not lost.

The exercise emphasizes the reaction between reactants and hydrogen ion to yield gaseous products. By observing the chemical changes in the reaction equations, one can track the movement of electrons and the flow of energy, leading to the creation of new substances. This knowledge is crucial as it applies to real-world scenarios like energy production, metal corrosion, and environmental chemistry.