Question

Which of the following reactions of hydrogen with non-metals represents Haber's process? (a) \(2 \mathrm{H}_{2}+\mathrm{O}_{2} \stackrel{\mathrm{hen}}{\longrightarrow} 2 \mathrm{H}_{2} \mathrm{O} ; \Delta H=-285.9 \mathrm{k} \mathrm{J} \mathrm{mol}^{-1}\) (b) \(3 \mathrm{H}_{2}+\mathrm{N}_{2} \frac{673 \mathrm{~K}, \mathrm{Fe}}{200 \mathrm{~atm}} 2 \mathrm{NH}_{3} ; \Delta H=-92.6 \mathrm{~kJ} \mathrm{~mol}^{-1}\) (c) \(\mathrm{H}_{2}+\mathrm{Cl}_{2} \stackrel{\mathrm{hv}}{\longrightarrow} 2 \mathrm{HCl}\) (d) \(2 \mathrm{H}_{2}+\mathrm{C} \stackrel{1100^{\circ} \mathrm{C}}{\rightarrow} \mathrm{CH}_{4}\)

Short answer

Reaction (b) represents Haber's process.

Step-by-step solution

Identify Haber's Process

Haber's process is an industrial method for the synthesis of ammonia from nitrogen and hydrogen. This process typically requires a catalyst (usually iron, Fe), high pressure, and elevated temperature.

Match Process Descriptions

Evaluate each reaction to determine which one describes the synthesis of ammonia under conditions similar to those used in the Haber's process.

Select the Correct Reaction

Identify the reaction that involves the synthesis of ammonia (NH3) from nitrogen (N2) and hydrogen (H2) under the specific conditions (high pressure, elevated temperature, and presence of iron catalyst) indicative of the Haber's process.

Key concepts

Synthesis of Ammonia

The synthesis of ammonia is vitally important for creating fertilizers essential to modern agriculture. By combining nitrogen (\textbf{N}\(_2\)) from the air with hydrogen (\textbf{H}\(_2\)), we obtain ammonia (\textbf{NH}\(_3\)), through a direct chemical reaction known as the Haber's process. In simple terms, ammonia synthesis can be represented by the following balanced chemical equation:
\[\textbf{N}_2(g) + 3\textbf{H}_2(g) \rightarrow 2\textbf{NH}_3(g)\]
This reaction is exothermic, releasing energy, making it favorable when considering energy requirements. The fundamental challenge, however, is procuring the right conditions to facilitate an adequate reaction rate. These include high pressure and temperature, along with the use of a catalyst, which in this context, is often iron. The intricacies of the reaction require an intricate balance of conditions that are derived from an understanding of chemical thermodynamics and the principles of catalysis.

Industrial Production of NH3

Industrial production of ammonia, or NH3, is chiefly accomplished through Haber's process. This procedure is highly efficient at a commercial scale, mainly due to the advancements in chemical engineering. To sustain industry demands, the reaction is carried out in large reactors, where nitrogen and hydrogen gases are combined at approximately 673 K (400°C) and 200 atmospheres of pressure in the presence of an iron catalyst.

Optimizing Conditions

The production facilities are designed to recirculate unreacted gases to maximize yield and resource efficiency. Despite the reaction being exothermic, it requires continuous energy input to maintain the necessary conditions and to overcome the activation energy barriers. The infrastructure also includes methods for the continuous removal of ammonia from the reaction mixture, as drawing the ammonia out of equilibrium by condensation helps drive the reaction further towards product formation, thus increasing yield.

Catalysis in Chemical Reactions

Catalysis is a term that frequently comes up in discussions of chemical reactions, such as the synthesis of ammonia. Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. They function by providing an alternative reaction pathway with lower activation energy.

Role of Iron in Haber's Process

Within Haber's process, iron serves as the catalyst. It accelerates the combination of nitrogen and hydrogen to form ammonia. However, just employing iron is not enough; the catalyst surface is often 'promoted' with small amounts of other chemicals, like potassium and aluminum oxides, which enhance its efficiency. Although the catalyst is not consumed during the reaction, it can degrade over time due to high temperatures and the presence of impurities, which may necessitate periodic replacement or regeneration.

Chemical Thermodynamics

Chemical thermodynamics is the branch of physical chemistry that deals with the relationship between heat and other forms of energy in chemical reactions. It lays the groundwork for understanding why certain reactions occur spontaneously while others require external conditions.

Thermodynamics of Ammonia Synthesis

In the context of ammonia synthesis, thermodynamic principles dictate that high pressures are favorable for the production of ammonia due to Le Chatelier's Principle---as the reaction results in a decrease in the number of gas molecules. Furthermore, although the reaction is exothermic, suggesting lower temperatures would be favorable for maximum yield, elevated temperatures are needed to increase the reaction rate. The Haber's process is an excellent example of how thermodynamics must be balanced with reaction kinetics to optimize industrial chemical processes—the thermodynamic conditions must be manipulated to achieve the desired reaction rate and yield in the most resource- and energy-efficient manner possible.