Problem 130 - Chemistry: An Atoms-Focused Approach

Question

At high temperatures, such as those in the combustion chambers of automobile engines, nitrogen and oxygen form nitrogen monoxide: $$\mathrm{N}_{2}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}(g) \quad \Delta H_{\mathrm{comb}}^{\circ}=+180 \mathrm{kJ}$$ Any NO released into the environment may be oxidized to \(\mathrm{NO}_{2}:\) $$2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}_{2}(g) \quad \Delta H_{\mathrm{comb}}^{\circ}=-112 \mathrm{kJ}$$ Is the overall reaction, $$\mathrm{N}_{2}(g)+2 \mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}_{2}(g)$$ exothermic or endothermic? What is \(\Delta H_{\text {comb }}^{\circ}\) for this reaction?

Step-by-Step Solution

Answer

Answer: The overall reaction is endothermic, and its \(\Delta H_{\text {comb }}^{\circ}\) is \(68\mathrm{kJ}\).

Step 1: Write down the given reactions and their enthalpy values

We are given two reactions: 1. \(\mathrm{N}_{2}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}(g) \quad \Delta H_{\mathrm{comb}}^{\circ}=+180 \mathrm{kJ}\) 2. \(2\mathrm{NO}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}_{2}(g) \quad \Delta H_{\mathrm{comb}}^{\circ}=-112 \mathrm{kJ}\) Our task is to determine the overall reaction and its enthalpy value.

Step 2: Add the given reactions and their enthalpy values

By adding reaction 1 and reaction 2, we can find the overall reaction: $$\mathrm{N}_{2}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}(g)$$ $$2\mathrm{NO}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}_{2}(g)$$ ------------------------------------- $$\mathrm{N}_{2}(g)+2\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}_{2}(g)$$ Now, let's sum their enthalpy values: \(\Delta H_{\text {comb }}^{\circ} = +180\ \mathrm{kJ} -112\ \mathrm{kJ} = 68\ \mathrm{kJ}\)

Step 3: Determine the nature of the overall reaction and its enthalpy value

Since the overall reaction's \(\Delta H_{\text {comb }}^{\circ}\) is positive, it indicates that the reaction is endothermic (heat is absorbed). Therefore, the overall reaction is endothermic with \(\Delta H_{\text {comb }}^{\circ}=68\ \mathrm{kJ}\). So, the reaction $$\mathrm{N}_{2}(g)+2\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}_{2}(g)$$ is endothermic, and its \(\Delta H_{\text {comb }}^{\circ}\) is \(68\mathrm{kJ}\).

Key Concepts

Enthalpy Change

Enthalpy change is a fundamental concept in thermochemistry. It measures the heat change during a reaction at constant pressure.

• Enthalpy, symbolized as \( \Delta H \), reflects the total energy of a system.
• The change in enthalpy indicates whether a reaction absorbs or releases heat.

For the reactions involving nitrogen oxides, individual enthalpy changes for each step need to be calculated. By adding these changes, we find the enthalpy change for the overall reaction.
In the exercise, the enthalpy change of the overall reaction denotes the energy absorbed (\( \Delta H_{\text{comb}}^{\circ} = 68 \, \text{kJ} \)), showing that more energy is required to break bonds in the reactants than is released when new bonds form in the products.

Endothermic Reactions

An endothermic reaction is one that absorbs heat from its surroundings. These reactions have a crucial role in various atmospheric and industrial processes.

• Such reactions are characterized by a positive \( \Delta H \), as seen in the combined reaction of nitrogen and oxygen in the initial exercise.
• The example demonstrates that nitrogen and oxygen forming nitrogen dioxide require more energy input than they release.

This energy absorption leads to changes in temperature and need for additional energy sources. Understanding the endothermic nature of reactions aids in estimating energy requirements and environmental impacts when such reactions occur.

Reaction Mechanisms

Reaction mechanisms delve into the stepwise sequence of elementary reactions. They detail how reactants transform into products, helping us predict changes in energy and matter.

• In the described scenario, nitrogen monoxide forms in a two-step process first from nitrogen and oxygen, then transforming into nitrogen dioxide.
• Each step has its own enthalpy change, illustrating how mechanisms affect the overall energy profile of a reaction.

Exploring each step reveals insights into reaction rates and conditions. Understanding mechanisms is essential for controlling reactions in engines and reducing pollutants like nitrogen oxides.

Nitrogen Oxides

Nitrogen oxides, like NO and \( \text{NO}_2 \), are compounds formed during high-temperature processes such as combustion in car engines.

• They are significant because they contribute to atmospheric pollution, affecting environmental and human health.
• The formation from nitrogen and oxygen exemplifies their presence in exhaust systems, influencing air quality.

Managing nitrogen oxide levels is crucial in pollution control strategies, with processes adjusting combustion methods and introducing catalytic converters to limit emissions. Understanding how they form through energy changes can help mitigate their production.