At Equilibrium 0.100 Mol of O2 Is Present Calculate Kc

What is Kc?

The equilibrium constant (Kc) is a quantitative measure of the position of a chemical equilibrium. It is defined as the ratio of the product of the concentrations of the products to the product of the concentrations of the reactants, each raised to the power of their respective stoichiometric coefficients.

Kc is unitless because the concentration units cancel out. The value of Kc tells us whether the reaction favors products or reactants at equilibrium.

How to Calculate Kc

To calculate Kc when you know the moles of products and reactants at equilibrium, follow these steps:

1. Determine the stoichiometry of the balanced chemical equation.
2. Convert moles of each species to concentrations using the volume of the solution.
3. Substitute the concentrations into the Kc expression.
4. Calculate the numerical value of Kc.

Example Calculation

Consider the reaction: 2NO(g) + O2(g) ⇌ 2NO2(g)

At equilibrium, 0.100 mol of O2 is present in a 1.00 L solution. Let's calculate Kc.

1. First, convert moles to concentrations:
   • [O2] = 0.100 mol / 1.00 L = 0.100 M
2. Assume the reaction produces 0.200 mol of NO2 (from the stoichiometry):
   • [NO2] = 0.200 mol / 1.00 L = 0.200 M
3. For the Kc expression: Kc = [NO2]² / ([NO]² × [O2])
4. Assume [NO] = 0.050 M (from stoichiometry):
   • Kc = (0.200)² / ((0.050)² × 0.100) = 0.0400 / 0.0025 = 16.0

The equilibrium constant Kc is 16.0, indicating the reaction strongly favors the formation of NO2.

Interpretation

The value of Kc tells us about the equilibrium position:

• If Kc > 1: The reaction favors products at equilibrium.
• If Kc = 1: The reaction is at equilibrium with equal concentrations of products and reactants.
• If Kc < 1: The reaction favors reactants at equilibrium.

In our example, Kc = 16.0 shows the reaction strongly favors the formation of NO2.