Question

For the reaction below, describe the temperature and pressure conditions that would produce the highest yield of the forward reaction. Explain your answer in terms of Le Châtelier’s principle. N2O4 (g) Two arrows stacked on top of each other. The top arrow points to the right. The bottom arrow points to the left. 2 NO2 (g) deltaH = +57.2 kJ/mol

Answer 1

1) Chemical equation

N2O4 (g) ⇄ 2 NO2     ΔH = + 57.2 kj / mol

2) Temperature

ΔH > 0 means that products have higher enthalpy than the reactants, which means that the products have absorbed heat from the surroundings. So, the forward reaction is endothermic.

So you can rewrite the equation as ifd the heat were a reactant:

N2O4 (g) + calor ⇄ 2 NO2

Using Le Chatelier Principle, you can infere that adding heat to reaction displaces the equilibrum to the right. Then when the temperature is increased, which means that heat is added to the system, the backward reaction if favoured.

The conclusion is that the decrease of temperature will produce a higher yield of the foreard reaction.

2) Pressure.

N2O4 (g) ⇄ 2 NO2

1 mol of gaseous N2O4 produces 2 moles of gaseous NO2, so when the pressure is raised the equilibrium displaces to the side where the increase is compensated with the decrease of the number of moles.

So, if you want to increase the yield of the forward reaction, this is to  favor the increase of number of moles, you need to decrease the pressure.