Difference Between Physical and Chemical Equilibrium

Key Difference – Physical vs Chemical Equilibrium
 

An equilibrium state describes the concentrations of reactants and products in a reaction mixture in a closed system. An equilibrium can take place only in a closed system. In equilibrium, the concentrations of reactants and products remain at constant values. If the amount of reactants or products is changed, the concentration of other constituents is also changed spontaneously in order to hold the equilibrium. Based on the characteristics of equilibrium, there are two types; physical equilibrium and chemical equilibrium. The key difference between physical and chemical equilibrium is that a physical equilibrium is an equilibrium in which the physical state of the system does not change whereas chemical equilibrium is the equilibrium state in which the concentrations of reactants and products is not changed with time.

CONTENTS

1. Overview and Key Difference
2. What is Physical Equilibrium
3. What is Chemical Equilibrium
4. Similarities Between Physical and Chemical Equilibrium
5. Side by Side Comparison – Physical vs Chemical Equilibrium in Tabular Form
6. Summary

What is Physical Equilibrium?

A physical equilibrium is an equilibrium state in which the physical state of the system does not change. Change of phase of matter from one phase to another is a physical process. Hence the equilibrium state in which the physical state does not change with time is known as the physical equilibrium. There are three main types of physical equilibrium;

1. Solid-Liquid Equilibrium

For example, the equilibrium between ice and water is a physical equilibrium because there are no chemical reactions taking place. Any pure substance can coexist in both solid and liquid phases at the melting point of that substance. The melting point is the temperature at which the solid substance starts to melt (converted into its liquid form).

2. Liquid-Vapour Equilibrium

The equilibrium between water and steam is a physical equilibrium in which no chemical reactions are taking place. However, this type of equilibrium occurs only inside closed systems, unless the vapour escapes from the equilibrium.

3. Solid-Vapour Equilibrium

This type of physical equilibrium can be observed in substances that undergo sublimation. There, the equilibrium occurs at the sublimation temperature (sublimation is the conversion of a solid directly into the vapour phase, without passing a liquid phase). For example, conversion of solid ammonium chloride (NH₄Cl) into gaseous ammonium chloride.

What is Chemical Equilibrium?

Chemical equilibrium is the equilibrium state in which the concentrations of reactants and products is not changed with time. This type of equilibrium can be observed in reversible chemical reactions. The chemical equilibrium results when the forward and backward reactions of the reversible chemical reaction occur at the same rate. There are no net changes in concentrations of each reactant and product that are involved in equilibrium reactions.

Figure 1: Graph of the Reaction Between A and B; the Reactant concentrations become constant after the Reaction mixture obtains an Equilibrium

For example, the reaction between hydrogen gas and iodine vapour in a closed system gives a deep violet colour at the beginning which then fades with time. The deep violet colour is given by iodine vapour. The colour fades because of the reaction between iodine vapour and hydrogen gas. After some time, the colour remains constant. It is the point at which the reaction has obtained an equilibrium state. This is a chemical equilibrium.

H_2(g)    +     I_2(g)        ↔        HI_(g)

What are the  Similarities Between Physical and Chemical Equilibrium?

• Both Physical and Chemical Equilibrium are forms of equilibrium states.
• Both Physical and Chemical Equilibrium forms have parameters that are constant with time.

What is the Difference Between Physical and Chemical Equilibrium?

Summary – Physical vs Chemical Equilibrium

The equilibrium state of a system is the state of having constant parameters inside that system. Based on the characteristics of the equilibrium state of a system, there are two forms of equilibrium; physical equilibrium and chemical equilibrium. The difference between physical and chemical equilibrium is that a physical equilibrium is an equilibrium in which the physical state of the system does not change whereas chemical equilibrium is the equilibrium state in which the concentrations of reactants and products is not changed with time.

Reference:

1.“Types of Equilibrium : physical equilibrium and chemical equilibrium.” Redefining Knowledge, 2 Mar. 2018. Available here
2.“Chemical equilibrium.” Wikipedia, Wikimedia Foundation, 2 Mar. 2018. Available here 
3.“Characteristics Of The Equilibrium State.” Chemistry LibreTexts, Libretexts, 21 July 2016. Available here 

Image Courtesy:

1.’Dynamic equilibrium’By Petergans at English Wikipedia – Own work (Public Domain) via Commons Wikimedia

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