The key difference between equilibrium constant and formation constant is that equilibrium constant is the ratio between the concentrations of products and the concentrations of reactants at equilibrium whereas formation constant is the equilibrium constant for the formation of a coordination compound from its components.
The equilibrium constant is useful in explaining the behaviour of different equilibrium states. Formation constant is a type of equilibrium constant which is specific for the formation of a coordination compound; for example, complex ion.
CONTENTS
1. Overview and Key Difference
2. What is Equilibrium Constant
3. What is Formation Constant
4. Side by Side Comparison – Equilibrium Constant vs Formation Constant in Tabular Form
5. Summary
What is Equilibrium Constant?
Equilibrium constant is the ratio between the concentrations of products and the concentrations of reactants at equilibrium. This term is used only with reactions that are in equilibrium. The reaction quotient and the equilibrium constant are the same for reactions that are in equilibrium.
Equilibrium constant is also given as the concentrations raised to the power of stoichiometric coefficients. The equilibrium constant is dependent on the temperature of the system considered since the temperature affects the solubility of components and the volume expansion. However, the equation of the equilibrium constant does not include any details about solids that are among the reactants or the products. Only the substances in the liquid phase and gaseous phase are considered.
For example, let us consider the equilibrium between carbonic acid and bicarbonate ion.
H2CO3 (aq) ↔ HCO3–(aq) + H+ (aq)
The equilibrium constant for the above reaction is given as below.
Equilibrium Constant (K) = [HCO3–(aq)] [H+ (aq)] / [H2CO3 (aq)]
Figure 01: Equilibrium Constants for Different Compounds when they are in an Aqueous Solutions
What is Formation Constant?
Formation constant is the equilibrium constant for the formation of a coordinate complex from its components in a solution. We can denote it as Kf. This equilibrium is mainly applied for the formation of complex ions. The components we need for the formation of a complex ion are metal ions and ligands.
A complex ion forms as a result of Lewis acid-base interactions of metal ions and ligands. The metal ion always carries a positive charge and it acts as the Lewis acid while the ligand should carry one or more lone electron pairs to act as a Lewis base. Small metal ions have a great tendency to form complex ions because they have a high charge density.
Generally, the complex ion formation is a stepwise reaction which includes all the steps involved in the addition of ligands one by one; thus, these steps have individual equilibrium constants as well. For example, the formation of copper-ammonium complex ion has four steps. Therefore it has four different equilibrium constant values: K1, K2, K3 and K4. Then, the formation constant for the overall reaction is as follows:
Kf = K1K2K3K4
What is the Difference Between Equilibrium Constant and Formation Constant?
Equilibrium constant is useful in explaining the behaviour of different equilibrium states, while the formation constant is a type of equilibrium constant. The key difference between equilibrium constant and formation constant is that the equilibrium constant is the ratio between the concentrations of products and the concentrations of reactants at equilibrium, whereas the formation constant is the equilibrium constant for the formation of a coordination compound from its components.
Below infographic summarizes the difference between equilibrium constant and formation constant.
Summary – Equilibrium Constant vs Formation Constant
Equilibrium constant is useful in explaining the behaviour of different equilibrium states while formation constant is a type of equilibrium constant. The key difference between equilibrium constant and formation constant is that the equilibrium constant is the ratio between the concentrations of products and the concentrations of reactants at equilibrium, whereas the formation constant is the equilibrium constant for the formation of a coordination compound from its components.
Reference:
1. “17.3: The Formation of Complex Ions.” Chemistry LibreTexts, Libretexts, 17 June 2019, Available here.
Image Courtesy:
1. “HydrationKs” By Nikolaivica – Own work (CC BY-SA 3.0) via Commons Wikimedia