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The equilibrium in which the involved substances are present in different phases is defined as heterogeneous equilibrium. In a heterogeneous system, when we involve a reversible reaction, the reactants and products are in different phases.
Also Check: Homogeneous Equilibrium
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Key Terms: Heterogeneous Equilibrium, Homogeneous Equilibrium, Reactants, Equilibrium State, Reversible Reaction, Decomposition, Equilibrium Constant
Heterogeneous Equilibrium
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Heterogeneous equilibrium is a chemical system of equilibrium in which the reactants and the products are present in more than one state of matter.
Let’s take as an example a situation where both ice and water can exist together simultaneously due to a certain permissible temperature. In this case, both the ice and water are in their equilibrium state. This state of equilibrium is known as the heterogeneous equilibrium.
Also Check: Process Of The Shift In Equilibrium Experiment
The above chemical equilibrium can be represented as follows:
H2O (Solid) ↔ H2O (Liquid)
We can observe that the states of the reactant and the product in the above situation are different.
Equilibrium In Heterogeneous System
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During the process of a chemical reaction, different reactants completely combine together and dissolve to form a chemical product.
However, if the chemical reaction is carried out in a closed container, many times we can see that the reaction is not completed and the reactants and products are present together simultaneously.
Also Check: Le Chatelier’s Principle, Henry’s Law, Chemical Equilibrium
After some time, the concentration of these reactants and the products become constant under the condition that the temperature and the pressure inside the container are also constant. In such a case, the reaction is said to be in equilibrium.
Similarly, in the heterogeneous system, if the reaction so conducted is reversible, the reactants and their products thereby are present together but in different phases.
Let us take an example of a reversible reaction.
Decomposition of CaCO3(s) which is the chemical representation of calcium carbonate. When decomposed, calcium carbonate splits into CaO and CO2.
The rate of decomposition is equal to the rate at which the reaction of a combination of the reactants takes place. Thus, this reaction is in equilibrium. The above chemical equilibrium can be represented as:
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| Equilibrium | Relation Between Kp and Kc | Energy Change due to Change in Equilibrium |
| Some Basic Concepts of Chemistry | Chemical Bonding and Molecular Structure | Structure of Atom |
Examples of Heterogeneous Equilibrium
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Some examples of heterogeneous equilibrium are given below:
- When solid ferrous oxide reacts with gaseous carbon monoxide, it produces solid iron and gaseous carbon dioxide.
- Bromine which is a natural liquid chemical element can easily convert into vapours. Here, the rate of evaporation of bromine and the rate of condensation of the bromine vapours are equal during the state of equilibrium. This state of equilibrium can be represented as
Br(l) ↔ Br(g)
- When red hot carbon reacts with steam, it produces hydrogen gas and carbon monoxide gas.
- When carbon dioxide gas reacts with solid carbon, it produces carbon monoxide gas.
- When phosphorus trichloride occurs in a natural liquid state and reacts with gaseous chlorine, it produces phosphorus pentachloride in a solid state. This reaction can be represented as PCl3(l) +Cl2(g) ↔ PCl5(s).
CaCO3 (s) ↔ CaO (s) + CO2 (g)
Equilibrium Constant of Heterogeneous Equilibrium Reaction
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The equilibrium constant in a reaction of heterogeneous equilibrium shows the relationship between the reactants and the products. The equilibrium constant is represented as K or KC, where C represents the concentrations of the reactants and the products respectively.
Also Check: Calculating Equilibrium Concentration
For example, in the case of the decomposition of the calcium carbonate chemical reaction, the product splits into two independent reactants, namely, calcium oxide and carbon dioxide.
Here, the equilibrium constant is only dependent on the gaseous component, as the other components, that is, solid and liquid remain constant.
CaCO3 (s) ↔ CaO (s) + CO2 (g)
Here CaCO3 and CaO are the solid components and therefore their molar concentrations remain constant throughout the entire reaction. Thus, the equilibrium constant in this reaction can be represented as
\(K_c = \frac{[CaO][CO_2]}{[CaCO_3]}\)
Kρ = ρCO2
Whereas, the Kp is written as
Where, p=the partial pressure. This means that under a given temperature the concentration or the partial pressure is constant in an equilibrium reaction.
Also Check: Rate of a Chemical Reaction
Equilibrium in Physical change
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The changes in phases of elements from one state of matter to another matter during a chemical reaction are also a part of the heterogeneous equilibrium system. The different types of equilibrium that are possible are as follows:
Also Check: Three States of Matter
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Solid to Liquid Equilibrium
The equilibrium between water and ice takes place at 0 and 1 atm pressure in a thermo flask.
H2O (s)⇄H2O (l)
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Liquid – Gas Equilibrium
Evaporation of water in a closed container at room temperature is the best example to explain this type of equilibrium.
H2O (l)⇄H2O (g)
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Solid – Gas Equilibrium (solid ⇆ Gas)
In this type of equilibrium, the elements directly change from the solid state to the vapour or gaseous state without passing through the liquid state. Such elements include ammonium chloride, iodine, camphor, etc. which undergo sublimation upon heating or at room temperature.
Things To Remember
- Heterogeneous equilibrium is a chemical system of equilibrium in which the reactants and the products are present in more than one state of matter.
- Decomposition of CaCO3(s) is a reversible reaction.
- Heterogeneous equilibrium is possible under the conditions of constant temperature and pressure.
- The equilibrium between water and ice takes place at 0 atm and 1 atm pressure in a thermo flask.
- The equilibrium constant is represented as K or KC.
- Different types of equilibrium are Solid Liquid Equilibrium, Liquid – Gas Equilibrium, and Solid – Gas Equilibrium.
Important PYQS Based On Heterogenous Equilibrium
- The Relationship Between K_p And K_c Is K_pK_c RTD is…. [KCET 2018]
- Ag3Bg4Cg Initially Concentration Of A Is Equal B find…. [KEAM]
- Find the value of kc…. [KEAM]
Sample Questions
Ques. Define heterogeneous equilibrium. (2 Marks)
Ans. Heterogeneous equilibrium is a chemical system of equilibrium in which the reactants and the products are present in more than one state of matter. For example, when both ice and water can exist together simultaneously due to a certain permissible temperature. In this case, both the ice and water are in their equilibrium state. This state of equilibrium is known as the heterogeneous equilibrium.
Ques. Define homogeneous equilibrium. (2 Marks)
Ans. In a homogeneous equilibrium system, the reversible reaction involving reactants and products is in the same phase. Examples of Homogeneous reactions include all the chemical reactions which are carried out in the gaseous phase such as the production of ammonia gas by the reaction of nitrogen gas and hydrogen gas.
Ques. Explain the equilibrium state of calcium carbonate. (2 Marks)
Ans. CaCO3(s), which is the chemical representation of calcium carbonate, when passed through disassociation, splits into CaO and CO2. The rate of decomposition is equal to the rate at which the reaction of a combination of the reactants takes place. Thus, this reaction is in equilibrium.
Ques. Give two examples of heterogeneous equilibrium. (2 Marks)
Ans. Two examples of a heterogeneous equilibrium are:
- When red hot carbon reacts with steam, it produces hydrogen gas and carbon monoxide gas.
- When carbon dioxide gas reacts with solid carbon, it produces carbon monoxide gas.
Ques. Calculate the equilibrium constant in the reaction of decomposition of calcium carbonate. (4 Marks)
Ans. The chemical formula for the reaction of calcium carbonate is
CaCO3 (s) ↔ CaO (s) + CO2 (g)
Here, CaCO3 and CaO are the solid components in the chemical reaction of calcium carbonate and therefore their molar concentrations remain constant throughout the entire reaction. Thus, the equilibrium constant in this reaction can be represented as
\(K_c = \frac{[CaO][CO_2]}{[CaCO_3]}\)
Kc = [CO2]
According to Le Chatelier’s principle, an increase in the concentration of Cl2 (one of the products) at equilibrium will favor the backward reaction, and thus the dissociation of PCl5 into PCl3 and Cl2 decreases.
Ques. What qualitative information can be obtained from the magnitude of the equilibrium constant? (4 Marks)
Ans. Qualitative information are:
- Large values of equilibrium constant (> 103) show that the forward direction is favored i.e. concentration of products is much larger than that of the reactants of equilibrium.
- Intermediate values of K (10-3 to 103) show that the concentrations of the reactants and products are comparable.
- The low value of K (< 10-3) shows that the backward reaction is favored, that is, the concentration of reactants is much larger than that of the products.
Ques. What is dynamic equilibrium? When does it occur? (3 Marks)
Ans. Dynamic equilibrium is a type of reversible reaction in which there exists an equal rate of transition between the reactants and the products, that is, the concentration of the reactants and the concentration of the products are equal.
In this system of equilibrium, there is no change in the concentration of both the reactants and the products with change in time and also there is no change in the properties of the system.
Ques. Define Henry's Law? (2 Marks)
Ans. Henry's law is one of the gas laws formulated by William Henry in 1803. According to Henry's Law, the mass of gas when dissolved at a given temperature, in a given mass of any solvent, is directly proportional to the gas present in the solvent.
Ques. What is Le Chatelier’s principle? (2 Marks)
Ans. The principle states that if a system at equilibrium is subjected to a change of pressure or temperature or number of moles of the component, there will be a tendency for a net reaction in the direction that reduces the effect of this change.
Ques. What is an Equilibrium Constant in an equation? (3 Marks)
Ans. Equilibrium Constant of an equation can be referred to as an expression signifying the concentration of reactants and products of a chemical reaction when it reaches at the stage of equilibrium. Equilibrium Constant is represented as K or KC.
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