a) WRITE: $$ K_c = \frac{[H_2]^2[I_2]}{[HI]^2} $$ CALC: Let x = number of moles of hydrogen at equilibrium 2HI ⇌ H2 + I2 Let x = molar conc - Leaving Cert Chemistry - Question 9 - 2021

Let $x$ = number of moles of hydrogen at equilibrium.

For the reaction:

$2HI \rightleftharpoons H_2 + I_2$

Let $x$ = molar concentration of hydrogen at equilibrium, where the volume of the container is 12 liters.

1. Start with 5 moles.
2. At equilibrium: (5 - 2x) moles of HI.

Thus,

$[HI] = \frac{(5 - 2x)}{12}$

At equilibrium, we have:

For $H_2$: $\text{Equil} = 0.42 - 2z \text{ mol}$. The concentration can then be approximated by,

$[H_2] = 0.44 - 0.45 \text{ mol L}^{-1}$

Evidence of an equilibrium is observed when there are approximately equal amounts of both species present. According to Le Chatelier’s principle, if the concentration of one species is decreased, the equilibrium will shift forward (to the right) to compensate for that decrease in concentration.

The system shifts backward (to the left) to decrease the concentration of H2O, resulting in a pink color.

The equilibrium shifts forward (to the right) to decrease the concentration of Cl-.

The reaction is endothermic, which implies that heat is absorbed. Therefore, cooling will always shift in the exothermic direction (heat producing). Thus, a backward (left) shift indicates that this is exothermic for this reaction.