Consider the equilibrium N2O4(g) <---> 2NO2 (g) ΔH = 58.0kJ
In which direction will the equilibrium shift when
a. N2O4 is added
b. NO2 is removed
c. the pressure is increased
d. the temperature is decreased
Solutions:
a. When a substance is added, equilibrium shifts to the opposite side. N2O4 is on the left side and adding it, will result in a shift to the right side.
b. When a substance is removed, equilibrium shifts to the same side. NO2 is on the right side and removing it, will result in a shift to the left side.
c. When the pressure is increased, equilibrium shifts to the side with least moles of gas. (g) means that the substance is gaseous. We can use the coefficients in front of gaseous substances as moles. There is 1 mole of gas on the left side and 2 moles of gas on the right side. Increasing pressure, we need to shift to the side with least moles of gas, which is left.
d. In order to see the effect of changing temperature, we must first figure out on what side temperature is on. If the reaction is endothermic (ΔH is positive), heat is on the left side. If the reaction is exothermic (ΔH is negative), heat is on the right side.
Once this has been determined, we can treat temperature as one of the reactants or products. In this case, the reaction is endothermic because ΔH is positive and we can add heat to the left side. Decreasing temperature/heat will shift the equilibrium to the left side.