Chapter 4: Problem 144
On standing, a concentrated nitric acid gradually turns yellow. Explain. (Hint: Nitric acid slowly decomposes. Nitrogen dioxide is a colored gas.)
Short Answer
Expert verified
Decomposition of nitric acid produces nitrogen dioxide, which colors the acid yellow.
Step by step solution
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Nitrogen dioxide
Nitrogen dioxide, represented by the chemical formula \(NO_2\), is an important component formed during the decomposition of nitric acid. It is well-known for its distinct reddish-brown color and plays a significant role in the chemical behavior of standing concentrated nitric acid. Over time, as nitric acid (HNO_3) decomposes, it releases \(NO_2\), which contributes to the yellow tint observed in nitric acid solutions. This color change is a visual indicator of the decomposition process occurring within the acid. Nitrogen dioxide is not only a byproduct; it also impacts the physical appearance of the acid, making it an essential aspect to consider when discussing the decomposition of nitric acid.
Redox reaction
A redox (reduction-oxidation) reaction involves the transfer of electrons between chemical species. In the case of nitric acid decomposition, it also functions as a redox reaction. During this process, nitric acid acts as an oxidizing agent, which means it is responsible for gaining electrons from other substances. As it decomposes, nitric acid reduces and releases nitrogen dioxide along with other byproducts. Understanding redox reactions is important because they help explain the chemical transformations that occur when nitric acid stands over time. These reactions involve complex electron exchanges that ultimately lead to the release of a visible gas, nitrogen dioxide, highlighting their importance in understanding this chemical process.
Chemical equation decomposition
Chemical equation decomposition is a crucial concept for representing the breakdown of compounds into simpler substances. The decomposition of nitric acid can be represented by the balanced chemical equation:\[4HNO_3 \rightarrow 4NO_2 + 2H_2O + O_2\]This equation shows that four molecules of nitric acid decompose to produce four molecules of nitrogen dioxide, two molecules of water, and one molecule of oxygen gas. Decomposition reactions like this involve the breakdown of a complex molecule into simpler products. By studying the balanced chemical equation, we can understand the stoichiometry of the reaction. This helps us observe the molar ratios in which products are formed, providing insight into the dynamics of the decomposition process.
Color change in chemicals
Color change in chemicals is a fascinating phenomenon often linked to various chemical reactions. In the decomposition of nitric acid, the color change is due to the formation of nitrogen dioxide. When \(NO_2\) is produced, it dissolves in the liquid phase of nitric acid, imparting a yellow hue to the solution. A color change serves as a visual cue indicating that a chemical reaction has taken place. In the case of nitric acid, this transition from a colorless to a yellow solution is subtly noticeable over time, demonstrating the ongoing decomposition process. Such observations are valuable because they enable chemists and students to infer the occurrence and progress of chemical reactions without the need for specialized equipment. Recognizing color changes enhances our understanding of the underlying chemical changes that are occurring.
