Question

For the reaction below, describe the temperature and pressure conditions that would produce the highest yield for the forward reaction. Explain your answer in terms of Le Châtelier's principle. (2 points)
N2(g) + 3H2(g) Two arrows stacked on top of each other. The top arrow points to the right. The bottom arrow points to the left. 2NH3(g) delta H = -92 kJ/mol

Answer 1

Answer:

According to Le Chatelier's Principle, low temperature and high pressure are the condition that will produce the highest yield of ammonia.

Explanation:

The Haber process is a reaction that combines nitrogen with hydrogen to form ammonia according to the following balanced equation:

• N₂ ₍g₎ + 3 H₂ ₍g₎ ⇆  2NH₃ ₍g₎             ΔH = -92 kJ/mol

Equilibrium considerations for Temperature changes:

You need to shift the equilibrium position  as far as possible favoring the forward reaction (reaction to the right) in order to obtain the highest yield of ammonia in the equilibrium mixture.  the reaction is exothermic and can be expressed as following:

• N₂ ₍g₎ + 3 H₂ ₍g₎ ⇆  2NH₃ ₍g₎ + heat

If the temperature is increased, the heat content of the system will increase, so the system would consume some of that heat by shifting the equilibrium to the left, thereby producing less ammonia.

According to Le Chatelier's Principle, the forward reaction will be favored if you lower the temperature you will get more ammonia (i.e. highest yield). The system will respond by moving the position of equilibrium to counteract this - in other words by producing more heat.

Equilibrium considerations for Pressure changes:

for the Haber process reaction

• N₂ ₍g₎ + 3 H₂ ₍g₎ ⇆  2NH₃ ₍g₎

One can note that there are 4 moles on the left-hand side of the equation, but only 2 on the right.

According to Le Chatelier's Principle, if you increase the pressure the system will respond by favoring the reaction which produces fewer no. of  moles. That will cause the pressure to fall again.

In order to get as much ammonia as possible in the equilibrium mixture, you need as high a pressure as possible producing more ammonia (i.e. highest yield)..