Question

The synthesis of ammonia gas from nitrogen gas and hydrogen gas represents a classic case in which a knowledge of kinetics and equilibrium was used to make a desired chemical reaction economically feasible. Explain how each of the following conditions helps to maximize the yield of ammonia.

a. running the reaction at an elevated temperature

b. removing the ammonia from the reaction mixture as it forms

c. using a catalyst

d. running the reaction at high pressure

Short answer

a) The velocity of molecules increases as temperature rises, causing molecules to collide more frequently.

b) The removal of ammonia will cause a higher production of ammonia and hence, the equilibrium position shifts to the right.

c) Catalystsspeed up a reaction.

d) The equilibrium position shifts to the right.

Step-by-step solution

Define exothermic or endothermic reaction

The temperature of the reaction mixture rises as energy is released in an exothermic process. The temperature drops when energy is absorbed in an endothermic reaction.

The chemical reaction is as follows:

_{${\text{N}}_2\left(g\right)+3{\text{H}}_2\left(g)\to 2{\text{NH}}_3(g)\right.$}

$\Delta \text{H}$value for this reaction is$-92\text{kJ}/\text{mol}$ which indicates an exothermic reaction

Effect on value of  K

(a)

This is an exothermic reaction. So, for an increase in temperature there will be decrease in the value of$K$.

This will lower the amount of${\text{NH}}_3\left(g\right)$ produced at equilibrium

The temperature increases. This must be due to the kinetics of the situation.

The process approaches equilibrium significantly faster as the temperature rises.

This reaction is extremely slow at low temperatures, far too slow to be useful.

As the temperature rises, the velocity of molecules increases as well.

There are more collisions per unit of time when molecules move faster.

Le Chatelier’s principle:

(b)

According to Le Chatelier's principle, “Every system at equilibrium has the ability to return to equilibrium after a change in conditions moves it away from that state”.

When a chemicalequilibrium system is disturbed, it re-establishes equilibrium by undergoing a net reaction that lessens or nullifythe influence of the disturbance.

The equilibrium position of a reaction is influenced by concentration, pressure, and temperature, while reaction rates are influenced by a catalyst.

Effect of change of concentration

In general, when a component's concentration changes, the equilibrium system reacts by consuming or producing some of the added or removed substance.

As a result, removing ammonia from the reaction mixture shifts the equilibrium position to the right.

In other words, the removal of ammonia will cause a higher production of ammonia and hence, the equilibrium position shifts to the right.

Effect of catalyst

(c)

Catalyst is a substance that enables a chemical reaction to proceed at a usually faster rate or under different conditions (as at a lower temperature) than otherwise possible.

Catalystsspeedup a reaction by providing an alternative mechanism with a lower activation energy , thereby increasing the forward and reverse rates to the same extent.

Thus, a catalyst shortens the time it takes to reach equilibrium but has no effect on the equilibrium position.

Le Chatelier’s principle

(d)

According to Le Chatelier's principle, “Every system at equilibrium has the ability to return to equilibrium after a change in conditions moves it away from that state”.

When a chemicalequilibrium system is disturbed, it re-establishes equilibrium by undergoing a net reaction that lessens or nullifythe influence of the disturbance.

The equilibrium position of a reaction is influenced by concentration, pressure, and temperature, while reaction rates are influenced by a catalyst.

 Step 7: Effect of high pressure

Change in pressure have significant effects only on equilibrium systems with gaseous components.

The system responds by lowering the quantity of gas molecules in order to limit the increase in gas pressure. And it does so by shifting the reaction to the side with fewer moles of gas. As a result, if the pressure is higher, the reaction shifts to the side with fewer gas molecules, reducing the total number of gas molecules.

In the reaction given below, a higher pressure will cause the equilibrium position to shift to the right.

${\text{N}}_2\left(g\right)+3{\text{H}}_2\left(g\right)\to 2{\text{NH}}_3$