Question

Which nitrate will decompose to give \(\mathrm{NO}_{2}\) on heating? (a) \(\mathrm{NaNO}_{3}\) (b) \(\mathrm{KNO}_{3}\) (c) \(\mathrm{RbNO}_{3}\) (d) \(\mathrm{LiNO}_{3}\)

Short answer

(d) LiNO_3 will decompose to yield NO_2 upon heating.

Step-by-step solution

Understanding the Decomposition of Nitrates

When heated, nitrates of alkali metals decompose differently based on the size and reactivity of the metal cation. Small cations with high charge density (like Li) tend to decompose to form nitrites and oxygen. Larger cations such as Na, K, Rb result in a different decomposition product. However, all alkali metal nitrates, except lithium nitrate, decompose to form nitrites and oxygen.

Applying Decomposition Patterns to the Options

For this exercise, we need to identify the nitrate that decomposes to produce nitrogen dioxide (NO_2). We apply the knowledge that lithium nitrate (LiNO_3) is an exception among the alkali metal nitrates and decomposes to produce lithium oxide (Li_2O), NO_2, and oxygen (O_2), unlike the other options given.

Identifying the Correct Answer

Therefore, based on the decomposition pattern, the correct answer is (d) LiNO_3. It decomposes to give NO_2 and is the only nitrate among the given options that produces NO_2 as a decomposition product.

Key concepts

Chemical Decomposition

Chemical decomposition, also known as analysis or breakdown, is a fundamental chemical reaction where one substance is transformed into two or more different substances.
Imagine you have a piece of paper; tearing it apart results in smaller pieces of paper, but the paper's chemical structure hasn't changed. However, if you burn that paper, it chemically decomposes into ash, water vapor, and carbon dioxide.

In the context of nitrates, when they undergo thermal decomposition, they break down upon heating, resulting in various products depending on the type of nitrate and the conditions. For instance, the nitrates of alkali metals decompose differently. As part of this process, factors like the size of the metal cation and its charge density play a crucial role in determining the products of decomposition.
Undergraduate students typically explore these patterns in general chemistry laboratories to understand the principles of chemical reactivity and stability.

Alkali Metal Nitrates

Alkali metal nitrates are salts composed of an alkali metal cation and a nitrate anion. These substances are known for their solubility in water and their diverse reactivity when exposed to heat.

Common alkali metal nitrates include sodium nitrate (aNO_3), potassium nitrate (aNO_3), and lithium nitrate (aNO_3). Each of these has a distinct cation, which affects how it decomposes when heated. Those with larger cations such as sodium, potassium, and rubidium typically form nitrites (aNO_2) and oxygen (aO_2) as the main decomposition products.

However, lithium nitrate is unique among its peers. Due to lithium's small size and high charge density, it decomposes to produce lithium oxide (aLi_2O), nitrogen dioxide (aNO_2), and oxygen (aO_2), which is an exception to the typical decomposition pathway for nitrates of other alkali metals. This highlights the influence of ionic properties on chemical behavior, a topic often studied in courses on inorganic chemistry and materials science.

Nitrogen Dioxide Production

Nitrogen dioxide (aNO_2) is a reddish-brown gas that is a significant air pollutant and is involved in the formation of smog and acid rain.

In the laboratory, aNO_2 can be produced through the decomposition of nitrates. As mentioned earlier, lithium nitrate (aLiNO_3) decomposes upon heating to form aNO_2, along with lithium oxide (aLi_2O) and oxygen (aO_2). The production of aNO_2 in this reaction is an example of how specific conditions and reactants can lead to targeted chemical products.

• Lithium nitrate decomposes to produce aNO_2 because of lithium's small cation size.
• The decomposition process involves breaking the chemical bonds within the nitrate compound to form new substances.
• Understanding aNO_2 production has practical applications, such as controlling air pollution and studying atmospheric chemistry.

Nitrogen dioxide production from alkali metal nitrates' decomposition is a demonstration of applied inorganic chemistry and environmental science.