9. (a) What is meant by chemical equilibrium? Why is a chemical equilibrium described as dynamic? (8) State Le Châtelier's principle: (6) (b) When a yellow solution of iron(III) chloride (FeCl₃) and a colourless solution of potassium thiocyanate (KCN) were mixed in a test tube, a red colour appeared and the following equilibrium was established: FeCl₃ (aq) + KCN (aq) ⇌ Fe(CNS)₂⁺ (aq) Explain: (i) the effect on the Fe²⁺ ion concentration of adding KCN to the equilibrium mixture, (ii) why changing the pressure has no effect on this equilibrium - Leaving Cert Chemistry - Question 9 - 2013

Le Châtelier's principle states that if a system at equilibrium is subjected to a stress (such as a change in concentration, temperature, or pressure), the system will adjust to counteract that stress and re-establish equilibrium. This principle highlights how dynamic systems respond to changes in their environment.

Adding KCN to the equilibrium mixture increases the concentration of CN⁻ ions. According to Le Châtelier's principle, the equilibrium will shift to the right to consume the added CN⁻ ions, thereby decreasing the concentration of Fe²⁺ ions as they form complex ions with CN⁻. Thus, the Fe²⁺ concentration will decrease.

The equilibrium established involves aqueous ions, which are not significantly affected by changes in pressure because the volume of the solution remains almost constant. Since both sides of the reaction include ions in solution and the number of moles remains balanced, pressure changes do not shift the equilibrium position.

The equilibrium constant expression, Kc, for the reaction is given by:
K_c = rac{[Fe(CNS)^{2+}]}{[Fe^{3+}][CN^-]}

To calculate Kc, we first need to determine the equilibrium concentrations:
Starting moles:

• FeCl₃ = 1.0 x 10⁻⁴ moles
• KCN = 1.0 x 10⁻⁴ moles
  At equilibrium:
• Let x be the amount that reacts.
  [Fe(CNS)²⁺] = x,
  [Fe³⁺] = (1.0 x 10⁻⁴ - x) moles,
  [CN⁻] = (1.0 x 10⁻⁴ - x) moles.
  In this case, the Kc can be calculated as follows, considering the equilibrium concentrations:
  K_c = rac{x}{(1.0 x 10^{-4}-x)(1.0 x 10^{-4}-x)}
  Given values:
• Starting concentration in 1.2 L:
  $K_c ext{ is calculated to be approximately } 138.87 (138.9, 139)$

The temperature at which the test tube was placed was lower than room temperature due to the influence of the ice-water bath.

The value of Kc for this reaction is smaller at the lower temperature, as the equilibrium shifts to favor the formation of reactants under such conditions.

The forward reaction is exothermic. By observing that the establishment of the red color (product) occurs upon mixing the reactants, a cooling effect is noted if the system is disturbed (e.g., when placed in an ice-water bath), indicating that the equilibrium shifts backward (to the left) to counteract the temperature drop, which corresponds to an exothermic process.