The key difference between equilibrium constant and rate constant is that the equilibrium constant is expressed using both the concentrations of reactants and products, whereas the rate constant is expressed using either concentration of reactants or products.
Both, the equilibrium constant and rate constant, are constant values for a particular reaction. That means, at a constant reaction, the conditions such as temperature, value of the equilibrium constant, and rate constant do not change with time. Furthermore, in expressing the equilibrium constant, we have to consider the stoichiometric coefficient as well. But, for the rate constant, we have to determine the value using only an experimental method.
CONTENTS
1. Overview and Key Difference
2. What is Equilibrium Constant
3. What is Rate Constant
4. Side by Side Comparison – Equilibrium Constant vs. Rate Constant in Tabular Form
5. Summary
What is Equilibrium Constant?
The equilibrium constant is the ratio between the concentrations of products and the concentrations of reactants at equilibrium. We can use this term only with reactions that are in equilibrium. The reaction quotient and the equilibrium constant are the same for reactions that are in equilibrium.
Moreover, we have to give this constant using the concentrations raised to the power of stoichiometric coefficients. The equilibrium constant depends on the temperature of the system since the temperature affects the solubility of components and the volume expansion. However, the equation for the equilibrium constant does not include any details about solids that are among the reactants or products. Only the substances in the liquid phase and gaseous phase are considered.
For example, the equilibrium between acetic acid and acetate ion are as follows:
CH3COOH ⇌ CH3COO– + H+
The equilibrium constant, Kc for this reaction is as follows:
Kc = [CH3COO–][ H+]/[CH3COOH]
Figure 01: Equilibrium Constants for Different Compounds
What is Rate Constant?
Rate constant is a coefficient of proportionality relating the rate of a chemical reaction at a given temperature to the concentration of the reactants or the products of the reaction. If we write the rate equation in relation to the reactant A for the reaction given below, it is as follows.
aA + bB ⟶ cC + dD
R= -K [A]a [B]b
In this reaction, k is the rate constant. It is a proportionality constant that depends on the temperature. We can determine the rate and the rate constant of a reaction by experiments.
What is the Difference Between Equilibrium Constant and Rate Constant?
The key difference between equilibrium constant and rate constant is that the equilibrium constant is expressed using both the concentrations of reactants and products, whereas the rate constant is expressed using either the concentration of the reactants or products. Furthermore, the equilibrium constant is given for an equilibrium reaction, while the rate constant can be given for any reaction.
Moreover, in expressing the equilibrium constant, we can use the concentrations of reactants and products along with the stoichiometric coefficients while, in expressing the rate constant, we cannot use the stoichiometric coefficient because we have to determine the value of the constant experimentally. Besides, the equilibrium constant describes an unchanging reaction mixture, while the rate constant describes a reaction mixture that changes with time.
Summary – Equilibrium Constant vs. Rate Constant
In summary, both, the equilibrium constant and rate constant, do not change with time if reaction conditions such as temperature are not changed. However, the key difference between equilibrium constant and rate constant is that the equilibrium constant is expressed using both the concentrations of reactants and products, whereas the rate constant is expressed using either concentration of reactants or products.
Reference:
1. Helmenstine, Anne Marie. “What Is the Rate Constant in Chemistry?” ThoughtCo, Sep. 27, 2018, Available here.
2. “The Equilibrium Constant, K.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.
Image Courtesy:
1. “HydrationKs.” By Nikolaivica – Own work (CC BY-SA 3.0) via Commons Wikimedia