Question

Ammonia is synthesized industrially by reaction of nitrogen and hydrogen according to the equation \(3 \mathrm{H}_{2}(g)+\mathrm{N}_{2}(g) \longrightarrow 2 \mathrm{NH}_{3}(g) .\) The free-energy change for this reaction is \(\Delta G=-16 \mathrm{~kJ} / \mathrm{mol}\), yet this reaction does not readily occur at room temperature. List three ways to increase the rate of this reaction.

Short answer

Increase temperature, use a catalyst, and increase reactant concentration.

Step-by-step solution

Understand the Reaction Equation

The chemical equation for the synthesis of ammonia is given as \(3 \mathrm{H}_{2}(g) + \mathrm{N}_{2}(g) \rightarrow 2 \mathrm{NH}_{3}(g)\). This equation tells us that three moles of hydrogen gas react with one mole of nitrogen gas to produce two moles of ammonia gas. The reaction's free energy change (\(\Delta G\)) is \(-16 \mathrm{~kJ/mol}\), indicating that the reaction is spontaneous under standard conditions.

Identify the Temperature Influence

One way to increase the reaction rate is by increasing the temperature. Raising the temperature generally increases the kinetic energy of the molecules, leading to more frequent and energetic collisions, which can help overcome the activation energy barrier of the reaction. This is especially relevant given that the reaction is spontaneous (negative \(\Delta G\)), but might be limited by a high activation energy.

Use of a Catalyst

A catalyst can be used to increase the rate of the reaction without being consumed by it. In industrial ammonia synthesis, an iron catalyst is often used. The catalyst provides an alternative reaction pathway with a lower activation energy, thereby increasing the reaction rate at a given temperature.

Increase Reactant Concentration

Increasing the concentration of the reactants, \(\mathrm{H}_{2}\) and \(\mathrm{N}_{2}\), will also speed up the reaction. This is because a higher concentration of reactants leads to more frequent collisions between molecules, increasing the likelihood of successful reactions.

Key concepts

Reaction Kinetics

Reaction kinetics is a fascinating aspect of chemistry that helps us understand the speed of chemical reactions. In the case of ammonia synthesis, this is crucial to producing ammonia quickly and efficiently. Reaction kinetics focuses on the factors which influence the rate at which a reaction occurs. These factors include temperature, concentration, and the presence of a catalyst. By studying reaction kinetics, chemists can devise methods to enhance production rates in industrial settings.

• Temperature: Higher temperatures increase the energy of molecules, making them collide more often and with greater force.
• Concentration: More molecules present means more collisions likely occur.
• Catalysts: These substances offer alternative pathways for the reaction to proceed.

Learning about reaction kinetics allows us to fine-tune these parameters to achieve desired outcomes in chemical manufacturing.

Catalyst

Catalysts are remarkable substances that accelerate chemical reactions without being consumed themselves. In the context of ammonia synthesis, an iron catalyst is typically employed. This catalyst lowers the activation energy required for the reaction, which means that it allows the reaction to proceed more quickly than it would with no catalyst. - **How does it work?** Catalysts provide an alternative pathway for the reaction with lower activation energy. This means molecules can react more easily. - **Why use them?** Using a catalyst can significantly speed up industrial processes, such as ammonia production, making them more efficient and cost-effective. Catalysts are crucial for increasing the efficiency of reactions, as they bring about significant changes in the reaction rate without being changed themselves by the reaction. Their ability to be recycled within a process is both economical and beneficial.

Activation Energy

The concept of activation energy is central to understanding why some reactions happen slowly or not at all at lower temperatures. Activation energy is the minimum energy needed to initiate a chemical reaction. Even if a reaction is spontaneous based on thermodynamics (negative ΔG), it may proceed slowly if the activation energy is still high. In ammonia synthesis, the activation energy must be overcome for nitrogen and hydrogen molecules to rearrange into ammonia. By increasing temperature or using a catalyst, we can lower the effect of activation energy:

• **Increasing Temperature:** Provides the energy needed to overcome this barrier and initiate the reaction.
• **Catalysts:** Offer a pathway with lower activation energy, enabling the reaction to proceed at a faster rate even at lower temperatures.

Concentration Effect

The concentration effect plays a significant role in the rate of chemical reactions. By increasing the concentration of reactants, such as nitrogen and hydrogen in ammonia synthesis, we can enhance the likelihood of molecular collisions. More collisions increase the chances of successful interactions that lead to product formation. - **How does concentration affect reaction rate?** - Higher concentration means more molecules in a given volume, increasing the possibility of collision between reactants. - More collisions translate directly to a higher reaction rate. Understanding the concentration effect is vital for scaling up industrial processes. By strategically manipulating reactant concentrations, chemists can optimize the output of ammonia in large-scale production, ensuring the process is both efficient and economical.