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.
N2 (g) + 3 H2 (g) Two arrows stacked on top of each other. The top arrow points to the right. The bottom arrow points to the left. 2 NH3 (g) delta H = -92 kJ/mol

Answer 1

@zepdrix @.Sam.

Answer 2

The two arrows on top of each other just mean it's an equilibrium reaction (the reaction can proceed in either direction).

According to Le Châtelier's principle, a system will try to counteract any challenge it is presented with. So if temperature is increased, the system will try to decrease temperature. Similarly if pressure is decreased, the system will try to increase the pressure.

In our situation, we have an exothermic reaction (the enthalpy is negative). That means the forward reaction PRODUCES heat. For the forward reaction to be favoured, we would want to lower the temperature, so that the system will try to counteract the challenge by producing heat via the forward reaction.

In terms of pressure, we need to look at the moles of gas on either side of the balanced equation. In general, if the volume is staying constant, the more moles of gas you have, the greater the pressure exerted (think PV=nRT, where P is proportional to n). On the left side, there are 4 moles of gas, while on the right side, there are only 2. If we want to maximize yield, it would better to increase the pressure - that way, the system will try to counteract this challenge by decreasing the pressure, and shifting to the right so that there are fewer moles of gas present.

To summarize: to maximize yield for this reaction, decrease the temperature and increase the pressure.

Did that make sense? If you have any questions let me know.