Heterogeneous Equilibrium

Chemical equilibrium is a state in which rate of forward reaction and backward reaction is equal. Thus, if a chemical reaction is in equilibrium state then the concentrations of reactants and products are constant or have no further tendency to change with time. Chemical equilibrium can be of two types on the basis of states of reactants and products at equilibrium –

  • Homogeneous equilibrium 

  • Heterogeneous equilibrium 

‘Homo’ and ‘hetero’ are the prefixes which are originated from Greek words for ‘similar’ and ‘different’ respectively. 

As the name suggests heterogeneous equilibrium is that equilibrium system in which reactants and products are found in two or more states of matter. For example, if we take equilibrium system of ice and water, reaction at equilibrium is represented below-

H2O(s) ↔ H2O(l)  

Then, you can see in the above equation, the state of reactant and product are different. Water is present in the system as solid and liquid. As the states or phases of the reactant and product are different in this equilibrium system so it is called a heterogeneous equilibrium system. 

Let us first discuss heterogeneous equilibrium in detail then we will discuss the homogeneous system as well in brief. 

As the reactants and products in heterogeneous equilibrium are present in different phases so, its equilibrium constant expression is written in different ways compared to homogeneous equilibrium. We do not write concentrations of those reactants and products which are present in either solid or pure liquid phase in the equilibrium. 


Examples of Heterogeneous Equilibrium 

Few common examples of chemical reactions are listed below which occur at heterogeneous equilibrium –

  • Bromine occurs in a liquid state at room temperature. It easily changes into vapors and gives a red – brown color in both states. At equilibrium, rate of evaporation of bromine and rate of condensation of bromine vapor is the same. The equation can be represented as follows at equilibrium –

            Br(l)  ↔  Br(g)

As bromine is present in different phases as reactant and product and the system is in the equilibrium so, it is a heterogeneous equilibrium system.

  • Another example of heterogeneous equilibrium system is reaction between carbon dioxide and carbon. When gaseous carbon dioxide reacts with solid carbon produces gaseous carbon monoxide. It is an equilibrium reaction in which rate of forward reaction and rate of backward reaction is the same. Equation is given below –

CO2(g) + C(s) ↔ 2CO(g)

As you can see in the above equilibrium reaction that reactants and products are present in two phases gas or solid so, it is a heterogeneous equilibrium reaction. 

  • Reaction between PCl3(l) and chlorine gas at equilibrium is also an example of heterogeneous equilibrium. Because in this equilibrium system, phosphorus trichloride occurs in liquid state and chlorine participates as gas and product phosphorus pentachloride we found in solid state. The equation is given below –

PCl3(l) +Cl2(g) ↔  PCl5(s)

  • Reaction between solid Fe2O3 and hydrogen gas at equilibrium is also an example of heterogeneous equilibrium. Because in this equilibrium system, Fe2O3 occurs in solid state and hydrogen participates in gaseous phase and products iron and water in solid and gaseous phase respectively. The equation is given below –

Fe3O4(s) + 4H2(g) ↔ 3Fe(s) + 4H2O(g)   

Another typical example of heterogeneous equilibrium includes reaction of steam with red hot carbon. Equation is given below –

H2O(g) + C(s) ↔ H2(g) + CO(g)

As you can see above reaction is in equilibrium and water steam, carbon monoxide, hydrogen is present in gaseous phase while red hot carbon is in solid phase. Therefore, it is an example of heterogeneous equilibrium. 

  • Decomposition reaction of solid calcium carbonate when it establishes equilibrium becomes a good example of heterogeneous equilibrium. Equation is given below –

CaCO3(s) ↔ CaO(s) + CO2(g)

As you can see in the above equilibrium reaction that calcium carbonate and calcium oxide are present in solid state while carbon dioxide is in gaseous form, so it is an example of heterogeneous equilibrium reaction. 


Equilibrium constant of Heterogeneous equilibrium reaction 

The equilibrium constant for a reaction at equilibrium expresses the relationship between products and reactants. It is denoted by K or KC. In KC, C stands for concentration as in the expression of equilibrium constant, concentration of the reactants and products are taken. For example, look at the general reaction given below –

aA + bB ↔ cC + dD

              

 \[K_{c}\] = \[\frac{[C]^{c} [D]^{d}}{[A]^{a}[B]^{b}}\]


Where KC = equilibrium constant 

 

\[\frac{[C]^{c} [D]^{d}}{[A]^{a}[B]^{b}}\]  =  \[\frac{ Concentration   of     products }{ Concentration   of   reactants }\]


While writing expression of equilibrium constant for a heterogeneous equilibrium reaction, you need to keep in mind that in heterogeneous equilibrium, concentrations of solids, pure liquids or solvents are excluded as the concentration of pure liquids and solids do not vary greatly with temperature so they are considered as unity or 1.  Now let us understand it by writing expressions of equilibrium constant for all examples discussed above of heterogeneous equilibrium reactions. 

  • For reaction of bromine at equilibrium – Br2(l) ↔ Br2(g)

K = [Br2

Concentration of pure liquid Br2(l) is excluded. 

  • Heterogeneous equilibrium system of reaction between carbon dioxide and carbon. 

Equation - CO2(g) + C(s) ↔ 2CO(g)


K =  \[\frac{[CO]^{2}}{[CO_{2}]}\]


Concentration of carbon is excluded as it is in solid phase.

  • Reaction between PCl3(l) and chlorine gas at equilibrium –

PCl3(l) +Cl2(g) ↔ PCl5(s)


K = \[\frac{1}{[Cl_{2}]}\]


Concentrations of PCl3 and PCl5 are excluded as they are present as pure liquid and solid, respectively.

  • Reaction between solid Fe2O3 and hydrogen gas at equilibrium –

Fe3O4(s) + 4H2(g) ↔ 3Fe(s) + 4H2O(g)


K = \[\frac{[H_{2}O]^{4}}{[H_{2}]^{4}}\]


Concentration of Fe3O4 and iron is excluded as they are present in solid phase.

  • Reaction of steam with red hot carbon –

H2O(g) + C(s) ↔ H2(g) + CO(g)


K = \[\frac{[H_{2}][CO]}{H_{2}O}\]


Concentration of carbon is excluded as it is present in solid state. 

  • Decomposition of calcium carbonate –

CaCO3(s) ↔ CaO(s) + CO2(g)


 K = \[\frac{[CO_{2}]}{1}\] or K = K = [CO2]


Concentrations of calcium carbonate and calcium oxide are excluded as they are present in solid state. 


Homogeneous Equilibrium Reactions

If a reaction is at equilibrium and its all reactants and products are in the same phase or at same state of matter, then this reaction is known as homogeneous equilibrium reaction or we say the reaction is at homogeneous equilibrium. For example, the reaction of carbon monoxide and hydrogen. The equation is given below –

CO(g) + 2H2(g) ↔ CH3OH(g)

As you can see in the above reaction, carbon monoxide, hydrogen and methanol all are present in gaseous state and the reaction is at equilibrium, so it is called homogeneous equilibrium. 

This ends our coverage on the topic “Heterogeneous Equilibrium”. We hope you enjoyed learning and were able to grasp the concepts. We hope after reading this article you will be able to solve problems based on the topic. If you are looking for solutions of NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.