Heterogeneous Reaction: Definition, Catalytic Reaction & Example

Heterogeneous reaction is a class of chemical reactions in which the reactants are components of two or more phases. In chemistry, chemical reaction is a process that leads to the transformation of one set of chemical substances to another. There are certain reactions that do not proceed directly, but take place on the surface of another substance known as catalyst that also falls under the heterogeneous reaction.

Key terms: Heterogeneous reaction, Homogeneous reaction, Heterogeneous Catalyst, Chemical reaction, Catalyst, Hydrogen

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What is a Heterogeneous Reaction?

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Heterogeneous reaction can be defined as a reaction which happens between two or more reactants which are present in two or more different phases. The phases of the reactants can be solid and liquid, solid and gas or two immiscible liquids (liquids which cannot be mixed).

In the heterogeneous system two reactants undergo a chemical change on the interface of a catalyst that can be in a solid or a liquid phase. According to studies, the catalyst is mostly in the solid form.

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Examples Of Heterogeneous Reaction

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Examples of heterogeneous reactions are the reaction of solid metals with acids, the corrosion of iron and the electrochemical reaction occurring in batteries and electrolytic cells. Most of the studies and research are done on the heterogeneous catalyst such as the reaction between liquids or gases that happens on the surface of solid catalyst. It helps in initiating the reaction or in increasing the reaction rate.

The most important reactions are the reaction between gases and reaction between liquids or substances that dissolve in these liquids. For example, reaction between oxygen and natural gas which leads to the production of flame and the reaction of solutions of acid and bases in aqueous solutions.

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Heterogeneous Chemical Reaction

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Various heterogeneous chemical reactions of reactants of different phases are illustrated below.

1. Solid-Fluid (liquid, gas)

• Dissolution of solids [MgCO₃(s) + 2HNO₃(L) → Mg(NO₃)₂(aq) + H₂O(L) + CO₂(g)]
• Sublimation [U(s) + 3F₂(g) → UF₆(g)]
• Chemical vapor deposition [SiH₄(g) → Si(s) + 2H₂]
• Reduction of solid oxides [NiO₂(s) + H₂(g) → Ni6(g) + H₂O(g)]
• Metal reduction [Zn(s) + O₂(g) → ZnO₂(s)]

2. Liquid-Gas

• Dissolution with chemical reaction
• [Cl₂(s) + 2NaOH(L) → NaOCl(L)) + NaCl(L) + H₂O(g)]
• [3NO₂(g) + HO(L) → 2HNO₃(L) + NO(g)]

3. Solid and Solid

• Heterogeneous precipitation and calcination reaction
• [CoO(s) + Al₂O₃(s) → CoAl₂O₄(s)]

Before any heterogeneous chemical reaction occurs, the reactant in a bulk of a particular phase is transferred to either the interface of the two bulk phases or is completely transported to the bulk of another phase. It is one common thing in all the heterogeneous reactions.

Heterogeneous reactions take place in a number of steps which are one or more chemical steps accompanied by the intermediate steps that are purely physical in nature and are only responsible for transporting the bulk phase of certain reactants to the bulk phase of other reactants.

The concentration of the reactant in one bulk phase is different from the reactants at the interface of the two different bulk phases. This difference in the concentration is the driving force for this phase transfer process. 

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Rate of Chemical Reaction 

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In chemistry, the reaction rate is the speed at which a chemical reaction proceeds. There are two factors that are important to study the rate of chemical reaction:

1. The mass transfer from one phase to the other.
2. The contact pattern of the reacting phase.

Therefore in a steady state, when the heterogeneous reaction takes place in multiple steps, the overall rate of reaction is equal to the rate of reaction of the individual reactions that are taking place in a series. Hence, the rate of reaction can be given by:

r_overall = r₁ = r₂ =…= r_n

When the mass transfer takes place, the rate of reaction is calculated on the basis of molar reflux. Molar reflux is the number of moles per unit time per surface. 

Thus, reaction based on unit volume is,

Reaction based on the mass of a solid is,

Reaction based on unit interfacial surface/unit surface of solid is,

All these rates of chemical reaction are based on

Example: For a non-elementary heterogeneous reaction rate is as follows:

 C₆H₅CH₃(L) + H₂(g) → C₆H₆(L) + CH4(g)

The rate of reaction for this chemical reaction will be

-rY' =kPH2PT1 + KBPB + KTPT

Where, K_B and K_T are the absorption constants with unit kPa^-1 (atm^-1). The units of reaction rate is

kk = mol toluene (C₆H5CH₃) / kgcat • s • kPa²

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Heterogeneous Catalytic Reaction

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Heterogeneous catalytic reactions are the reactions that occur in the presence of a third element known as a catalyst. Catalysts are used in reaction to increase the rate of reaction without themselves getting consumed in the process. In heterogeneous catalytic reactions, the catalysts are mostly in the solid phase and reactants are mostly in the gaseous phase.

The solid surface has three functions when it works as a catalyst for a chemical reaction. 

1. It adsorbs the reactants and makes the required bond
2. It Holds the reactants in close proximity so that they can react
3. Surface lets the products desorb back into the surrounding phase.

Therefore, the heat transfer or mass transfer affects the rate of heterogeneous catalytic reaction. One of the examples of heterogeneous catalyst reaction is hyper thermal O-atom exchange reaction which involves adsorption, reaction and desorption.

Reaction Mechanism

Many chemical reactions are catalysed by solid surfaces which include most large scale chemical processes in industry and a number of environmental protection processes such as the exhaust gas cleaning in automobiles. 

The energy sector is highly dependent on heterogeneous catalysis. Heterogeneous catalytic hydrogenation reactions have been used (10-20%) mostly in industries like pharmaceuticals, flavour and fragrance industries, fine chemicals, agrochemicals and dietary supplements. Reason for the wide use of it is because generally catalytic reactions are highly selective in nature and are easy to work upon.

Adsorption is the most essential part of any kind of catalytic reaction. Adsorption is classified into two parts: physisorption and chemisorption. In the adsorption process, the adsorbate and adsorbent share a chemical bonding by sharing electrons. Heterogeneous catalytic reaction falls between these processes.

Physisorption: it is a weak adsorption bonding and molecules of the reactants are bound to the surface of the catalyst by Van Der Waal forces. This includes forces like dipole-dipole, induces dipole interactions and London dispersion forces.

Chemisorption: it is a strong adsorption bonding where the molecules of the reactant approach the atoms closely on the surface of the catalyst by electron cloud overlapping.

Mechanism of The Catalytic Reaction in Heterogeneous System

Mechanism of the catalytic reaction in a heterogeneous system involves following steps:

1. Diffusion of reactants to the surface. Where, the rate of diffusion of the reactants on the surface is directly proportional to the concentration of the reactants and the thickness of the boundary layer.
2. Adsorption of reactants on the surface of the catalyst. It happens when the adsorbate forms bonding with the adsorbent. Therefore, the ability of the molecules to form bonds with the atoms on the surface of the catalyst can be efficiently calculated as a sticking coefficient. Sticking coefficient is the percentage of the ratio of the number of molecules or atoms that have stuck to the surface of the catalyst.
3. The reaction is indicated by the bonds formed by the atoms of reactants and the catalyst.
4. Desorption of the products. Desorption takes place when the bonds between the products are formed and the catalyst breaks.
5. The product formed is diffused from the surface of the catalyst. It happens when the bond is cleaved and the product diffuses from the surface of the catalyst without changing the characteristics.

The contact processes used in the industries to produce sulphuric acid in high concentrations is the example of this. Platinum was used as a catalyst in this process earlier, but it may react with the arsenic impurities that are present in the sulphur feedstock. Therefore, vanadium oxide (V₂O₅) is used as the active catalyst. In this process sulphur oxide and oxygen are gases and vanadium oxide is solid.

• 2SO₂(g) + O₂(g) → 2SO₃(g) in presence of V₂O₅(S)
• 2V₂O₅(S) + 2SO₂(g) → 2SO₃(g) +2V₂O₄(S)
• 2V₂O₄(S) + O₂(g) → 2V₂O₅(S)
• 2SO₂(g) + O₂(g) → 2SO₃(g)

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Things to Remember

• Solvothermal is a heterogeneous reaction process of precursors dissolved in a solvent under high pressure and temperature.
• According to collision theory, the rate of reaction increases with the increase of the concentration of the reactants.
• The heterogeneous catalytic reactions are atomic efficient and the catalyst does not get consumed and can be recovered and recycled.
• Heterogeneous catalysis is very important because it enables faster and large-scale production.
• Metals like platinum, nickel and iron make good catalysts.

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