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Complete The Equations for The Following Equilibria and Calculate Kc

Reviewed by Calculator Editorial Team

Chemical equilibria describe the dynamic balance between reactants and products in a chemical reaction. Completing equilibrium equations and calculating the equilibrium constant (Kc) are fundamental skills in chemistry. This guide explains how to approach these tasks systematically.

What is Chemical Equilibrium?

Chemical equilibrium occurs when the rates of the forward and reverse reactions become equal, resulting in constant concentrations of reactants and products. The equilibrium constant (Kc) quantifies the position of equilibrium and is calculated using the concentrations of species at equilibrium.

Equilibrium is dynamic, not static. While concentrations appear constant, molecules are continuously reacting in both directions.

Completing Equilibrium Equations

To complete an equilibrium equation, follow these steps:

  1. Write the balanced chemical equation for the reaction.
  2. Add a double arrow (⇌) to indicate the equilibrium state.
  3. Include the state symbols (g, l, s, aq) for each species if needed.

Example

For the reaction between hydrogen gas and iodine vapor to form hydrogen iodide:

H2(g) + I2(g) ⇌ 2HI(g)

This equation is now complete and ready for equilibrium calculations.

Calculating the Equilibrium Constant (Kc)

The equilibrium constant (Kc) is calculated using the concentrations of species at equilibrium. The formula is:

Kc = [Products] / [Reactants]

Where:

  • [Products] = Concentration of each product raised to the power of its coefficient
  • [Reactants] = Concentration of each reactant raised to the power of its coefficient

For the example equation above:

Kc = [HI]2 / ([H2][I2])

Note that solid (s) and liquid (l) species are omitted from the expression for Kc because their concentrations are constant.

Example Calculation

Let's calculate Kc for the following equilibrium at 500°C:

N2(g) + 3H2(g) ⇌ 2NH3(g)

At equilibrium, the concentrations are:

  • [N2] = 0.10 M
  • [H2] = 0.20 M
  • [NH3] = 0.35 M

Plugging these into the Kc formula:

Kc = [NH3]2 / ([N2][H2]3)

Kc = (0.35)2 / (0.10 × (0.20)3)

Kc = 0.1225 / (0.10 × 0.008)

Kc = 0.1225 / 0.008 = 15.31

The equilibrium constant for this reaction is 15.31 at 500°C.

FAQ

What is the difference between Kp and Kc?

Kp uses partial pressures for gases, while Kc uses concentrations. The relationship between them is Kp = Kc(RT)Δn, where Δn is the difference in moles of gas between products and reactants.

How do I know if a reaction reaches equilibrium?

A reaction reaches equilibrium when the concentrations of reactants and products stop changing over time, and the rates of the forward and reverse reactions become equal.

Can Kc be greater than 1?

Yes, Kc can be greater than 1, equal to 1, or less than 1. A Kc greater than 1 indicates the reaction favors products, while a Kc less than 1 favors reactants.

What units are used for concentrations in Kc calculations?

Concentrations are typically expressed in molarity (M), which is moles per liter (mol/L).