Problem 156 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will favour the formation of \(\mathrm{NH}_{3}\) by Haber's Process? (a) Increase of temperature (b) Increase of pressure (c) Addition of catalyst (d) Addition of promoter

Step-by-Step Solution

Answer

Options (b), (c), and (d) favor NH _nH_3 _n formation in the Haber Process.

Step 1: Understanding the Haber Process

The Haber Process synthesizes ammonia ( NH _nH_3 _n) from nitrogen ( N _2 _n) and hydrogen ( H _2 _h) using the reaction: nH_2 + } N _2 _n _{2} + 3H_{2} ightleftharpoons 2NH_{3}. This is an exothermic reaction, meaning it releases heat, and is favored at lower temperatures but high pressures.

Step 2: Analyze Each Option

(a) Increasing the temperature shifts the equilibrium away from the formation of NH _nH_3 _n, as the reaction is exothermic; thus, increased temperature is unfavorable. (b) Increasing the pressure shifts the equilibrium towards the side with fewer moles of gas, favoring NH _nH_3 _n production. The reaction moves from 4 to 2 moles of gas. (c) A catalyst speeds up the reaction without being consumed. (d) A promoter enhances the efficiency of the catalyst.

Step 3: Identify Which Conditions Favor NH3 Formation

High pressure (option b) compresses gases facilitating more NH _nH_3 _n production. A catalyst (option c) helps reach equilibrium faster, and a promoter (option d) increases catalytic efficiency. Low temperatures also favor formation, but option (a) increases temperature against energy favorability. Thus, pressures, catalysts, and promoters favor NH _nH_3 _n formation.

Key Concepts

Ammonia Synthesis

The Haber Process is a method used to synthesize ammonia from nitrogen and hydrogen. The overall chemical reaction can be represented as:\[ N_2 + 3H_2 \rightleftharpoons 2NH_3 \]This process combines nitrogen (from the air) with hydrogen (typically derived from natural gas) under specific conditions. Understanding ammonia synthesis in this context involves recognizing the balance of reactants—three moles of hydrogen gas react with one mole of nitrogen gas to produce two moles of ammonia.

• Ammonia is crucial for fertilizers, which support global food production.
• The synthesis relies on certain conditions to maximize efficiency and yield.

The Haber Process transformed agricultural productivity by making large-scale ammonia production feasible. Ideally, this process should be energy-efficient and safe to ensure a consistent supply of ammonia for various industrial applications.

Exothermic Reaction

In chemical terms, a reaction is considered exothermic if it releases heat into the surroundings. The synthesis of ammonia through the Haber Process is a prime example:\[ N_2 + 3H_2 \rightleftharpoons 2NH_3 + ext{heat} \]This means that as the reaction progresses, heat is released, making the process energetically favorable by itself. However, temperature plays a critical role in this dynamic. While expelling heat might seem beneficial, the goal of increasing ammonia production also involves leveraging temperature effectively. Lower temperatures typically favor the formation of products in exothermic reactions.

• At high temperatures, equilibrium shifts to favor reactants.
• Maintaining a balance between sufficient rate of reaction and temperature is vital for efficiency.

Thus, while low temperatures favor equilibrium for ammonia formation, reactions' speed is slower, requiring a strategic approach to optimize both speed and yield.

Reaction Equilibrium

The Haber Process achieves ammonia production by reaching and maintaining reaction equilibrium. The concept of equilibrium refers to the state in which the rates of the forward and reverse reactions are equal, leading to a constant concentration of both reactants and products over time.For ammonia synthesis:\[ N_2 + 3H_2 \rightleftharpoons 2NH_3 \]Here, reaction equilibrium is crucial because it dictates the overall yield of ammonia. A shift in conditions such as temperature or pressure can alter this balance:

• Increasing pressure favors the formation of ammonia, as it involves fewer gas moles on the product side.
• Temperature adjustments can cause shifts favoring either ammonia or the reactants depending on whether the other factors (catalyst presence, pressure) overrule these shifts.

The Haber Process manipulates these conditions to maximize ammonia output by preventing the backward shift towards reactants, thus maintaining higher yields.

Catalyst and Promoter Effects

In the context of the Haber Process, catalysts and promoters play an essential role in facilitating the reaction without being consumed. A catalyst speeds up the reaction rate, allowing equilibrium to be achieved faster and under milder conditions:

• Catalysts lower the activation energy required for the reaction to proceed.
• They provide a surface that facilitates the coming together of reactant molecules.

Iron is commonly used as a catalyst in the Haber Process, often with additional promoter elements, such as potassium or aluminum oxide, to enhance its effectiveness. These promoters maintain the efficiency of the catalyst over time, boosting reaction rates and improving yields:

• Promoters prevent catalyst deactivation due to impurities.
• They help in maintaining optimal particle size for better surface reactions.

Using catalysts and promoters ensures that the Haber Process not only proceeds efficiently but also remains cost-effective, making large-scale ammonia production viable.