Consider the following reaction where Kc = 1.80×10-2 at 698 K: 2HI(g) → H2(g) + I2(g)

A reaction mixture was found to contain 0.280 moles of HI (g), 2.09×10^-2 moles of H2 (g), and 4.14×10^-2 moles of I2 (g), in a 1.00 liter container.

Required:
a. Is the reaction at equilibrium?
b. What direction must it run in order to reach equilibrium?
c. The reaction

1. must run in the forward direction to reach equilibrium.
2. must run in the reverse direction to reach equilibrium.
3. is at equilibrium.

The system is not in equilibrium and the reaction must run in the forward direction to reach equilibrium.

Explanation:

The reaction quotient Qc is a measure of the relative amount of products and reagents present in a reaction at any given time, which is calculated in a reaction that may not yet have reached equilibrium.

For the reversible reaction aA + bB⇔ cC + dD, where a, b, c and d are the stoichiometric coefficients of the balanced equation, Qc is calculated by:

In this case:

Since molarity is the concentration of a solution expressed in the number of moles dissolved per liter of solution, you have:

So,

Qc= 0.011

Comparing Qc with Kc allows to find out the status and evolution of the system:

If the reaction quotient is equal to the equilibrium constant, Qc = Kc, the system has reached chemical equilibrium.

If the reaction quotient is greater than the equilibrium constant, Qc> Kc, the system is not in equilibrium. In this case the direct reaction predominates and there will be more product present than what is obtained at equilibrium. Therefore, this product is used to promote the reverse reaction and reach equilibrium. The system will then evolve to the left to increase the reagent concentration.

If the reaction quotient is less than the equilibrium constant, Qc <Kc, the system is not in equilibrium. The concentration of the reagents is higher than it would be at equilibrium, so the direct reaction predominates. Thus, the system will evolve to the right to increase the concentration of products.

Being Qc=0.011 and Kc=1.80⁻²=0.018, then Qc<Kc. The system is not in equilibrium and the reaction must run in the forward direction to reach equilibrium.