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At Equilibrium 0.170 Mol of O2 Is Present Calculate Kc

Reviewed by Calculator Editorial Team

When a chemical reaction reaches equilibrium, the equilibrium constant (Kc) describes the ratio of product concentrations to reactant concentrations. This calculator helps determine Kc when you know the moles of a product at equilibrium.

Introduction

The equilibrium constant (Kc) is a fundamental concept in chemical equilibrium. It quantifies the position of a reaction at equilibrium and helps predict the direction in which a reaction will proceed to reach equilibrium.

When dealing with gas-phase reactions, Kc is expressed in terms of molar concentrations. The formula for Kc is derived from the stoichiometry of the balanced chemical equation and the concentrations of reactants and products at equilibrium.

Formula for Kc

The general formula for the equilibrium constant Kc is:

Kc = [Products] / [Reactants]

Where:

  • [Products] = concentration of products at equilibrium
  • [Reactants] = concentration of reactants at equilibrium

For a specific reaction, the formula becomes more detailed. For example, consider the reaction:

aA + bB ⇌ cC + dD

Kc = ([C]c × [D]d) / ([A]a × [B]b)

Where a, b, c, and d are the stoichiometric coefficients from the balanced chemical equation.

Worked Example

Let's calculate Kc for a reaction where 0.170 mol of O2 is present at equilibrium. Suppose we have the following reaction in a 1.00 L container:

2NO(g) + O2(g) ⇌ 2NO2(g)

At equilibrium, we find:

  • 0.170 mol of O2
  • 0.340 mol of NO2 (since 2 mol of NO2 are produced for each 1 mol of O2)
  • 0.170 mol of NO (since 2 mol of NO react for each 1 mol of O2)

First, convert moles to concentrations (M = mol/L):

  • [O2] = 0.170 mol / 1.00 L = 0.170 M
  • [NO2] = 0.340 mol / 1.00 L = 0.340 M
  • [NO] = 0.170 mol / 1.00 L = 0.170 M

Now apply the Kc formula:

Kc = ([NO2]2) / ([NO]2 × [O2])

Kc = (0.3402) / (0.1702 × 0.170)

Kc = (0.1156) / (0.00489)

Kc ≈ 23.64

The equilibrium constant Kc for this reaction is approximately 23.64.

Interpreting Results

The value of Kc tells you about the position of equilibrium:

  • If Kc > 1, the reaction favors products at equilibrium.
  • If Kc = 1, the reaction is at equilibrium.
  • If Kc < 1, the reaction favors reactants at equilibrium.

In our example, Kc ≈ 23.64 indicates the reaction strongly favors the formation of NO2 at equilibrium.

Note: The actual value of Kc depends on the specific reaction and conditions. Always ensure your reaction is balanced and your concentrations are correctly calculated.

FAQ

What is the difference between Kc and Kp?

Kc is the equilibrium constant expressed in terms of molar concentrations, while Kp is expressed in terms of partial pressures. Kp is used for gas-phase reactions where concentrations are expressed in terms of pressure.

How do I know if a reaction is at equilibrium?

A reaction is at equilibrium when the rates of the forward and reverse reactions are equal, and the concentrations of reactants and products no longer change over time.

Can Kc be negative?

No, Kc cannot be negative. The equilibrium constant is always a positive value, as it represents the ratio of concentrations or pressures of products to reactants.