Homogeneous Reaction

Dhristi JEE 2022-24

Introduction to Homogeneous Reaction

Homogeneous chemistry is a broader term that is specifically used in the context of Chemistry involved with homogeneous mixtures or homogeneous solutions. Homogeneous chemistry basically refers to the homogeneous reactions that take place in various conditions. So in order to understand the chemistry of homogeneous reactions, it is important to have a clear knowledge of homogeneous mixtures and solutions. 

A homogeneous mixture can be defined as a mixture that has a uniform composition throughout its mass. For example sugar in water or salt in water. That is, when sugar or salt is mixed in water they dissolve completely in the solvent which is water in this case. Many a time, a homogeneous mixture is confused with pure substance as they are both uniform in nature. 

The major difference between a homogeneous mixture and a pure substance is that the composition of the pure substance is uniform in all the samples of that particular substance. But, although the composition of a homogeneous mixture is uniform throughout a particular sample, the percentage of the composition will vary from sample to sample. 

Though all uniform compositions of various samples are broadly termed as homogeneous mixtures, they can be solid to liquid, solid to solid, liquid to liquid or gas to gas. But homogeneous solution is particularly used to denote solid to liquid mixture or liquid to liquid mixture.

What are Homogeneous and Heterogeneous Mixtures?

Homogeneous mixtures are the mixtures that have a uniform composition of substances throughout the mass. In other words, the particles in the homogeneous mixtures are uniformly distributed throughout the composition. Due to the uniformity of the mixture, it is often confused with pure solutions. But, unlike pure solutions, homogeneous mixtures can easily be separated by the precipitation method as the particles do not react with each other. 

On the other hand, a heterogeneous mixture can be defined as a mixture that has an uneven composition throughout the mass. Alternatively, it is stated as the mixture in which the particles are not uniformly distributed throughout the composition and can easily be identified with the naked eye or under a microscope. Heterogeneous has two phases. For example, when oil and water are mixed together, due to the low density of oil, it starts floating above the water surface. Thus, this mixture comprises two phases, one of water and the other of oil. 

The Major Differences Between Homogeneous and Heterogeneous Mixture are:-

Differences Between Homogeneous and Heterogeneous Mixture

Homogeneous Mixtures

Heterogeneous Mixtures

Particles are uniformly distributed throughout the mass

Particles are non-uniform throughout the mass

Homogeneous mixtures have the same composition throughout the mixture

Heterogeneous mixtures have different compositions throughout the mixture.

The particles cannot be separated physically

The particles can be separated physically

It has a single-phase throughout the mixture

It has two or more phases throughout the mixture

Example: sugar water mixture, salt water mixture

Example: smog particles in the air, oil-water mixture

Homogeneous Reaction and Heterogeneous Reaction

Homogeneous Reaction

Any class of reaction that occurs in a single-phase be that solid, liquid or gaseous phase is said to be a homogeneous reaction. Now the homogeneous reaction may occur in a single reaction or multiple reactions can take place. Therefore, sometimes a single stoichiometric equation or rate of reaction is considered to explain a certain reaction taking place in a single step. But most of the time, due to multiple intermediate reactions occurring in order to achieve the final product, two or more stoichiometric equations and rate of reactions are taken into consideration. A homogeneous reaction example is as follows: 

2H2 + O2 → 2H2O

Now this reaction follows multiple intermediate steps which are:

H2 + O2 → HO2 + H

H2 + HO2 → OH + H2O

OH + H2 → H2O + H

O2 + H → OH +O

H2 + O → OH + O

Though the formation of intermediate products is theoretically proven, in most of the reactions these elementary steps remain unidentified. The reason behind this is since the intermediate products formed are very small they escape the experimental values. The rate at which they form and break is very fast and therefore, difficult to detect in a reaction. Thus, for elementary reactions, the law of mass action states that the rate of elementary reaction is always proportional to the products of the concentration of the reactants at a constant temperature. Therefore for a single-step reaction: aA + bB →cC, 

the rate of reaction given by the law of mass action will be:-

⨏([A], [B], [C], T, t) = k[A][B] 

where K is called the rate of constant and is dependent on temperature. Thus further evaluation of the reaction species can be numerically expressed as:-

d[A] / dt = -ka [A] [B]

d[B] / dt = -kb [A] [B]

d[C] / dt = -kc [A] [B]

This is a reaction of order 2 and the mechanism of the reaction is more complicated to determine. Therefore, another easy way of determining the rate of reaction for any homogeneous reaction is

⨏([A], [B], [C], T, t) = k[A]α [B]β 

Where α is the rate of reaction of A, β is the rate of reaction of B and η = α + β and is known as the rate of complete reaction.

For example, when bromine and hydrogen undergo following reaction:-

H2 + Br2→ 2HBr

Then the rate of reaction is expressed as 

⨏([H2], [Br2], [HBr]) = k1[H2] [Br2]½ / K2+( [HBr] / [Br]2 )

Heterogeneous Reaction

A heterogeneous reaction is a kind of reaction in which the reactants are actually the components of two different phases, that can be, solid and liquid, liquid and gas, solid and gas or two immiscible liquids. In other words, when in a reaction one of the reactants undergoes chemical changes at the interface, be that on the surface of the catalyst or any other similar surfaces, that reaction is referred to as heterogeneous reaction. For example, the process of corrosion or metal acid reaction or the electrochemical reaction happens inside a battery. Some of the examples of heterogeneous reactions are:

Solid to Solid Reaction: CoO(s) + Al2O3→CoAl2O4

Liquid to Gas (Dissolution With Chemical Reaction): 3NO2(g) + H2O(l)→2HNO3(l) + NO(g)

Solid to Liquid (Dissolution of Solids): MgCO3(s) + HNO3(l)

Solid to Fluid (Sublimation Reaction): U(s) + 3F2(g)→ UF6(g)

Solid to Fluid (Metal’s Oxidation): Zn(s) + O2(g))→ ZnO(s)

Solid to Fluid (Reduction Reaction of Solid): NiO(s) + H2(g))→ Ni(s) + H2O(l)

Now, there are many factors that affect the rate of reaction of a heterogeneous reaction. These are primarily the concentration of reactants, temperature, catalyst effect, solvent effect and surface area effect.

  1. Concentration of Reactants: As the concentration of the reactants increases, the molecules of reactants will tend to collide more with one another and thus the rate of reaction also rises.

  2. Temperature: When the temperature of a reaction increases, the molecules gain more kinetic energy and start moving faster. Due to this, they collide with a much higher frequency with one another and hence increase the rate of reaction. 

  3. Surface Area Effect: In a heterogeneous reaction both the reactants are in different phases altogether. Therefore they can interact mutually with each other only at the interface of the phases. Thus the collision of the molecules of reactants per unit time decreases due to limited interface area. Hence the rate of reaction reduces.

  4. Solvent Effect: Viscosity of the solvent plays an important role in determining the rate of reaction. If the solvent is more viscous in nature then the movement of the molecules in the solvent is restricted as compared to a less viscous liquid. Thus more collision of molecules takes place easily in less viscous liquid and thus improves the rate of reaction.

  5. Catalyst Effect: A catalyst is a substance that accelerates the rate of reaction without actually going through any major chemical change in itself. Since most of the catalysts used in a  reaction are selective in nature, thus, it accelerates only one of the several possible elementary reactions and therefore plays an important role in determining the product of the reaction.

What is Homogeneous Equilibrium and Heterogeneous Equilibrium?

A homogeneous equilibrium is a state in which all the reactants and products are present in one single solution. A homogeneous equilibrium can be established only when the reactions taking place between various solutes in the liquid solutions are in the same homogeneous equilibrium. The chemical species involved in the reaction can be molecules, ions or mixtures. Example,

C2H2(aq) + 2Br2(aq) ↔  C2H2B4(aq)

Therefore, equilibrium constant will be, K =  [C2H2B4] / [C2H2] [Br2]2

A heterogeneous equilibrium is a state in which the reactants and the products of a reaction are in two or more phases. These phases can be in a combination of solid, liquid, gases or solutions.

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FAQs (Frequently Asked Questions)

Q1. What is a Homogeneous Solution? 

Ans. A homogeneous solution is the same as a homogeneous mixture and can be defined as a mixture that has a uniform composition throughout the mass. The term homogeneous solution is used when there is a solid-liquid or liquid-liquid mixture.

Q2. Give Some Homogeneous and Heterogeneous Examples.

Ans. Salt solution, milk, lotions, cool- aids are few examples of homogeneous mixtures.  Most of the alloys, rocks in the sand, oil in water are few examples of heterogeneous mixtures.