A chemical equilibrium is established when eleven moles of hydrogen and eleven moles of iodine are mixed at a temperature of 764 K - Leaving Cert Chemistry - Question 7 - 2008

No, when the colour of the mixture transitions to pale pink, the reaction has not ceased. Instead, it indicates that the formation of hydrogen iodide has reached a point where the rates of the forward and reverse reactions are equal. Thus, the concentrations of reactants and products remain constant, maintaining the equilibrium state.

The expression for the equilibrium constant (Kc) for the reaction is given by:

$K_c = \frac{[\text{HI}]^2}{[\text{H}_2][\text{I}_2]}$

where ([\text{HI}]), ([\text{H}_2]), and ([\text{I}_2]) represent the molar concentrations of hydrogen iodide, hydrogen, and iodine, respectively.

To calculate Kc, we can use the provided quantities:

• Total moles of hydrogen: 11 mol
• Total moles of iodine: 11 mol
• Moles of HI at equilibrium: 2x = 17 mol (given)

So, we can substitute the moles into the equilibrium constant expression:

1. Moles of H2 left = 11 - x = 11 - 8.5 = 2.5 mol
2. Moles of I2 left = 11 - x = 11 - 8.5 = 2.5 mol
3. Now using the expression:

$K_c = \frac{[17]^2}{[2.5][2.5]}$ 4. Given that the total volume is not specified, we can assume unit volume for simplification, yielding:

$K_c = \frac{289}{6.25} = 46.24$

Thus, the value of Kc at 764 K is approximately 46.24.

Le Chatelier's principle states that if an external change, such as a change in concentration, temperature, or pressure, is applied to a system at equilibrium, the system will adjust itself to counteract that change and restore a new equilibrium. This means it will shift in a direction that reduces the effect of the disturbance.