Heterogeneous Equilibrium in Chemical Systems

Chemical equilibrium is a state in which the rate of forward reaction and backward reaction is equal. Thus, if a chemical reaction is in the 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. 

Heterogeneous Equilibrium

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 the 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 states 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, 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, 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 vapours and gives a red-brown colour in both states. At equilibrium, the rate of evaporation of bromine and rate of condensation of bromine vapor are 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 a heterogeneous equilibrium system is the 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 the rate of forward reaction and rate of backward reaction are the same. The 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. 

• The 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 produces phosphorus pentachloride 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 produces 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 the reaction of steam with red hot carbon. Equation is given below

 H2O(g) + C(s) ↔ H2(g) + CO(g)

As you can see, the above reaction is in equilibrium and water steam, carbon monoxide, and hydrogen are present in the gaseous phase while red hot carbon is in the 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, 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, concentrations 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 

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

While writing expression of the 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 concentrations 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 the equilibrium constant for all examples discussed above of heterogeneous equilibrium reactions. 

• For the 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 = \[\frac{[H_{2}O]^{4}}{[H_{2}]^{4}}\]

Concentrations of Fe₃O₄ and iron are 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 –

CaCO_3(s) ↔ CaO_(s) + CO_2(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 the 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.