Problem 79 - Chemistry: The Central Science

Question

The oxidation of \(\mathrm{SO}_{2}\) to \(\mathrm{SO}_{3}\) is accelerated by \(\mathrm{NO}_{2}\). The reaction proceeds according to: $$ \begin{aligned} &\mathrm{NO}_{2}(g)+\mathrm{SO}_{2}(g) \longrightarrow \mathrm{NO}(g)+\mathrm{SO}_{3}(g) \\ &2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g) \end{aligned} $$ (a) Show that, with appropriate coefficients, the two reactions can be summed to give the overall oxidation of \(\mathrm{SO}_{2}\) by \(\mathrm{O}_{2}\) to give \(\mathrm{SO}_{3}\). (b) \(\mathrm{Do}\) we consider \(\mathrm{NO}_{2}\) a catalyst or an intermediate in this reaction? (c) Is this an example of homogeneous catalysis or heterogeneous catalysis?

Step-by-Step Solution

Answer

The overall oxidation reaction of SO₂ by O₂ is: \(SO_2 + O_2 \rightarrow SO_3\). NO₂ acts as a catalyst in this reaction, and this is an example of homogeneous catalysis.

Step 1: : First, let's identify the number of molecules of each species involved in each of the given reactions. Reaction 1: \(NO_2 + SO_2 \rightarrow NO + SO_3\) Reaction 2: \(2NO + O_2 \rightarrow 2NO_2\) Now let's sum the reactions: Reaction 1 + Reaction 2: \(NO_2 + SO_2 + 2NO + O_2 \rightarrow NO + SO_3 + 2NO_2\) We can see that both sides of the equation have the same species of NO and NO₂. We can eliminate them from the equation. #a_Title#: Overall Oxidation Reaction

: By eliminating the equal species on both sides, we get the overall oxidation reaction: \(SO_2 + O_2 \rightarrow SO_3\) So the complete reaction of the oxidation of SO₂ by O₂ to produce SO₃ is the sum of the two given reactions. #b_Title#: Identifying NO₂ as a Catalyst or Intermediate

Step 2: : A catalyst is a substance that facilitates a reaction without being consumed in the process, while an intermediate is a substance that is produced during the reaction and then consumed later on in the process. In order to determine whether NO₂ is a catalyst or an intermediate in this reaction, let's examine its role more closely. In the first reaction, NO₂ reacts with SO₂ to form NO and SO₃. In the second reaction, NO reacts with O₂ to regenerate NO₂. Since NO₂ is both consumed in the first reaction and regenerated in the second reaction, it is not being ultimately consumed in the overall process. Therefore, NO₂ is considered a catalyst in this reaction. #c_Title#: Identifying the Type of Catalysis

: Homogeneous catalysis refers to a reaction in which the catalyst and the reactants are in the same phase, while heterogeneous catalysis refers to a reaction in which the catalyst and the reactants are in different phases. In this case, all reactants and the catalyst are in the gaseous phase (indicated by the (g) symbol in the equations). Hence, this is an example of homogeneous catalysis. To summarize: a) The overall oxidation reaction of SO₂ by O₂ is: \(SO_2 + O_2 \rightarrow SO_3\) b) NO₂ is a catalyst in this reaction. c) This is an example of homogeneous catalysis.

Key Concepts

Chemical Reaction Balancing

Understanding how to balance a chemical reaction is fundamental in chemistry. It ensures the law of conservation of mass is upheld, meaning the number of atoms of each element must be the same on both sides of the equation. In the context of the oxidation of \textbf{SO\(_2\)} to \textbf{SO\(_3\)}, the exercise showcased the need to align the number of molecules for a complete and balanced overall equation.

Initial individual reactions involve different substances that appear and disappear. When we look at the summed equation:
\textbf{SO\(_2\)} + \textbf{O\(_2\)} \rightarrow \textbf{SO\(_3\)},
it illustrates a balanced reaction showing the conservation of sulfur and oxygen atoms. Notice how nitrogen oxides, \textbf{NO} and \textbf{NO\(_2\)}, were not included in the final equation as they cancel each other out, indicating their role beyond simple reactants or products.

Role of Catalysts

Catalysts are pivotal in chemistry for increasing the rate of reactions without undergoing permanent chemical changes themselves. In the provided exercise, \textbf{NO\(_2\)} demonstrates the classic behavior of a catalyst. It engages in the initial steps of a reaction to facilitate the process, but it's eventually regenerated to be used again.

From the equations:
\textbf{NO\(_2\)} + \textbf{SO\(_2\)} \rightarrow \textbf{NO} + \textbf{SO\(_3\)} and
2\textbf{NO} + \textbf{O\(_2\)} \rightarrow 2\textbf{NO\(_2\)},
we deduce that \textbf{NO\(_2\)}, despite temporarily reacting, is not consumed as it's reproduced in the cycle. This characteristic is what classifies \textbf{NO\(_2\)} as a catalyst, playing a non-permanent, yet crucial, role.

Homogeneous Catalysis

Homogeneous catalysis occurs when the catalyst and reactants are in the same phase—solid, liquid, or gas. This allows for efficient mixing and interaction among the molecules, which is essential for the catalyst to work effectively.

In the given exercise, \textbf{NO\(_2\)} acts as a catalyst in the gas phase, just like the reactants \textbf{SO\(_2\)} and \textbf{O\(_2\)}, and the products \textbf{NO} and \textbf{SO\(_3\)}. This shared phase facilitates the catalytic action of \textbf{NO\(_2\)} and qualifies the process as homogeneous catalysis. Such homogeneity leads to an efficient and uniformly distributed interaction between the catalyst and the substances involved, allowing for the rapid and smooth progress of the reaction.