Question

About \(80 \%\) of the ammonia produced by industry goes into the production of fertilizers. Ammonia is produced in industry by the reaction of \(\mathrm{N}_{2}\) and \(\mathrm{H}_{2}\) in the following reversible reaction: \(\mathrm{N}_{2}(g)+3 \mathrm{H}_{2}(g) \rightleftharpoons 2 \mathrm{NH}_{3}(g)\) exothermic (a) Which conditions of pressure and temperature favor the formation of the most amounts of ammonia? (b) Which conditions of temperature will increase the rate of formation of ammonia? (c) Use your answers to parts (a) and (b) to determine whether the reaction should be run at extremely high temperatures, extremely low temperatures, or an intermediate temperature.

Short answer

Optimal conditions for producing maximum amounts of ammonia are high pressure and low temperature. However, since a lower temperature slows the reaction rate, an intermediate temperature is often chosen for practical purposes.

Step-by-step solution

Understand Le Chatelier's Principle

According to Le Chatelier's principle, if you change the conditions of a reversible reaction at equilibrium, it will adjust to counteract the change. Here, the reaction will shift in the direction that will decrease the pressure increase. Thus, a higher pressure will favor the side with fewer gas molecules. Likewise, since the reaction is exothermic (releases heat), an increase in temperature would shift the reaction to the left to absorb the extra heat, while a decrease in temperature would shift it to the right to produce more heat.

Apply Le Chatelier's Principle to the reaction

In the given reaction, the formation of ammonia from nitrogen and hydrogen is accompanied by a decrease in the number of gas molecules (from 4 moles on the reactants’ side to 2 moles on the products’ side). So, according to Le Chatelier's Principle, increasing the pressure would shift the equilibrium to the right, favoring the production of ammonia. Also, because the reaction is exothermic, lowering the temperature would also shift the equilibrium to the right, producing more ammonia.

Understand the impact of temperature on reaction rate

A higher temperature generally increases the kinetic energy of the molecules involved in a reaction, causing them to collide more frequently and with greater energy. This typically results in an increase in the rate of the reaction.

Consider the combined impact of pressure and temperature on ammonia production

While a lower temperature is favorable for the production of the most amounts of ammonia, it will slow the rate of reaction. Therefore, a compromise must be reached - an intermediate temperature might be chosen to produce ammonia effectively. Likewise, a high pressure is favorable, so the reaction should be run at as high a pressure as is practical.

Key concepts

Chemical Equilibrium

Chemical equilibrium plays a central role in industrial processes, especially in the synthesis of chemicals such as ammonia. It is a balance between the forward and reverse reactions where the rate at which the reactants are transformed into products equals the rate at which products revert to reactants. The equilibrium can be disturbed by altering reaction conditions such as pressure and temperature.

The production of ammonia (NH_{3}) via the Haber process is a classic example where chemical equilibrium is essential. Employing Le Chatelier's Principle, industrial chemists understand that increasing the pressure will drive the reaction towards the formation of NH_{3}, as there are fewer gas molecules on the product side. Conversely, raising the temperature would favor the reactants because the reaction is exothermic. Therefore, equilibrium considerations are fundamental to optimizing ammonia yield efficiently.

Reaction Rates

The rate at which a chemical reaction proceeds is crucial for industrial and laboratory processes. Reaction rates are influenced by various factors, including temperature and the presence of a catalyst. A fundamental principle is that higher temperatures increase molecular collisions, leading to faster reactions. This means that in the production of ammonia, a higher temperature would increase the reaction rate.

However, there's a delicate balance between the thermodynamics and kinetics of a reaction. While higher temperatures can speed up the production of ammonia, they can simultaneously shift the chemical equilibrium unfavorably, as the process is exothermic. Therefore, reaction rates are a vital consideration when determining the optimal conditions for manufacturing ammonia.

Impact of Pressure and Temperature on Reactions

Pressure and temperature are two variables that deeply impact chemical reactions. When considering gas reactions, increasing the pressure generally favors the formation of the side with fewer gas molecules. For ammonia production, a higher pressure will push the equilibrium towards the production of ammonia due to the reduced volume and gas molecules on the product side.Balance is key, as extremely high temperatures, while increasing reaction rates, would not be ideal for ammonia yield due to the exothermic nature of the process. Alternatively, extremely low temperatures would favor ammonia production but slow the reaction greatly. Thus, an optimal intermediate temperature is often chosen in the industry. This optimal condition allows satisfactory reaction rates while still maintaining a favorable yield of ammonia, demonstrating the intricate interplay of pressure and temperature on reaction dynamics.