Question

The equilibrium constant, Kp, for the reaction H2(g) + I2(g) → 2HI(g) is 55.2 at 425°C. A rigid cylinder at that temperature contains 0.127 atm of hydrogen, 0.134 atm of iodine, and 1.055 atm of hydrogen iodide. Is the system at equilibrium? a)Yes. b)No, the forward reaction must proceed to establish equilibrium. c)No, the reverse reaction must proceed to establish equilibrium. d)Need to know the volume of the container before deciding. e)Need to know the starting concentrations of all substances before deciding

Answer 1

Answer: No, the reverse reaction must proceed to establish equilibrium.

Explanation:

$K_p$ is the constant of a certain reaction at equilibrium while $Q_p$ is the quotient of activities of products and reactants at any stage other than equilibrium of a reaction.

For the given chemical reaction:

$H_2(g)+I_2(g)\rightleftharpoons 2HI(g)$

The expression of $Q_p$ for above equation follows:

$Q_p=\frac{(p_{HI})^2}{p_{H_2}\times p_{I_2}}$

We are given:

$p_{HI}=1.055atm\\p_{H_2}=0.127atm\\p_{I_2}=0.134atm$

Putting values in above equation, we get:

$Q_p=\frac{(1.055)^2}{0.127\times 0.134}=65.41$

We are given:

$K_p=55.2$

There are 3 conditions:

• When $K_{p}>Q_p$; the reaction is product favored.
• When $K_{p}$; the reaction is reactant favored.
• When $K_{p}=Q_p$; the reaction is in equilibrium.

As, $Q_p>K_p$, the reaction will be favoring reactant side or the reaction must proceed in the reverse direction.

Hence, no, the reverse reaction must proceed to establish equilibrium.