What is meant by a chemical equilibrium? Why is it described as a dynamic state? State Le Chatelier’s principle - Leaving Cert Chemistry - Question 7 - 2017

It is described as a dynamic state because, although the concentrations of reactants and products remain constant, the individual molecules remain in motion. The forward and reverse reactions continue to occur at equal rates, meaning that the system is in a state of continuous change.

Le Chatelier's principle states that if an external change is applied to a system at equilibrium, the system will adjust itself to counteract that change and restore a new equilibrium.

The equilibrium constant (Kc) expression for the reaction is given by: $K_c = \frac{[NO_2]^2}{[N_2O_4]}$

To find the equilibrium concentrations, we start with the initial concentration of dinitrogen tetroxide, where:

• Initial concentration of N2O4 = 0.1 mol/L
• Initial concentration of NO2 = 0 mol/L

Let x represent the change in concentration at equilibrium:

• N2O4 at equilibrium = 0.1 - x
• NO2 at equilibrium = 2x

Using the value of Kc = 0.2: $K_c = \frac{(2x)^2}{(0.1-x)} = 0.2$ After solving this equation, we find:

• x = 0.05 mol/L Therefore, at equilibrium:
• [N2O4] = 0.1 - 0.05 = 0.05 mol/L
• [NO2] = 2(0.05) = 0.1 mol/L.

The colour of the equilibrium mixture is paler at 0 °C than at T, where T > 0 °C. This implies that at lower temperatures, the concentration of denser dinitrogen tetroxide is higher, indicating that the reaction is exothermic. An increase in temperature would shift the equilibrium towards the endothermic direction, producing more nitrogen dioxide and leading to a darker colour.

No, there would be no change in the value of Kc at a given temperature if a different initial concentration of dinitrogen tetroxide were used. The equilibrium constant Kc is only affected by temperature, not by concentrations or the amounts of reactants or products present in the system.