6.1 Write down the meaning of the term reversible reaction - NSC Physical Sciences - Question 6 - 2023 - Paper 2

Le Chatelier's principle states that when a system at equilibrium is subjected to a change in conditions (such as concentration, temperature, or pressure), the system will adjust itself to counteract that change and re-establish equilibrium.

At t = 80 s, the concentration of A2(g) was increased, which means that more of A2(g) was added to the container.

According to Le Chatelier's principle, increasing the concentration of A2(g) will shift the equilibrium position to the right to favor the formation of products (B2(g) and AB(g)). This means that the system will attempt to consume some of the added A2(g) by producing more B2(g) and AB(g).

To calculate the equilibrium constant (Kc) at t = 120 s, we use the concentrations at equilibrium. If at this time, [A2] = 2 mol/dm³, [B2] = 2 mol/dm³, and [AB] = 8 mol/dm³, then:

$K_c = \frac{[A_2][B_2]}{[AB]^2} = \frac{(2)(2)}{(8)^2} = \frac{4}{64} = 0.0625$

The potential energy diagram for this reaction would typically show the energy of the reactants at a certain level, then an activation energy peak for the transition state, followed by a lower energy level for the products, indicating that the reaction is exothermic. The diagram should clearly label the energy levels of the reactants, products, and the activation energy.

The equilibrium constant, Kc, at 100 °C will generally be less than Kc at 150 °C if the reaction is exothermic. This is due to the fact that decreasing the temperature shifts the equilibrium position to favor the reactants. Therefore, as the temperature decreases, the value of Kc at the lower temperature will be less than that at a higher temperature.

Between t = 0 s and t = 60 s, the graph shows increasing amounts of reactants and a gradual rise in products. The curve for reactants decreases as they are converted into products, while the product curves rise until they flatten out when equilibrium is reached. This indicates that the reaction is proceeding towards completion during this period.