Catalysts are the substances that are added to the reaction to speed up the rate of a chemical reaction. They are classified into homogeneous and heterogeneous catalysts.

“A catalyst is a substance that alters the rate of a chemical reaction but remains chemically unchanged at the end of the reaction.”

Characteristics of Catalysts

Catalysts can be positive or negative. The positive catalysts increase the rate of a chemical reaction by lowering the activation energy. Whereas the negative catalysts or poisons decrease the rate of the chemical reaction.

  • Catalysts are required in very minute amounts.
  • Their mass and chemical composition remain unchanged at the end of the reaction.
  • The catalysts change the rate of forward as well as reverse reactions. It does not alter the equilibrium constant.
  • They are sometimes very specific in their actions e.g. biological catalysts, and enzymes.
  • There are many factors that affect the activity of catalysts like temperature, pH, etc.

Types of catalysts

1. Homogeneous catalysts

“In homogeneous catalysis, catalysts and the reactants are in the same phase”.

The catalyst is evenly distributed throughout the system. In this process, the substrate molecule combines with the catalysts. Two important examples of homogeneous catalysts are:

  • Acid-base catalysts
  • Enzyme catalysts

Acid-base catalysts

Acid catalysts

Acid catalysts are proton donors.

Mechanism

  • In the first step, the proton is transferred from the acid catalyst to the substrate.
  • In the second step, the protonated substrate reacts with water to form the product.

S + HA ⇌ SH+ + A–

SH+ + H2O → P + H3O+

Acidic hydrolysis of lactose

When lactose is treated with an acid or lactase(enzyme), it is broken down into galactose and glucose molecule.

First lactose reacts with acid catalysts and is converted into protonated lactose which reacts with water to produce galactose and glucose.

Acidic hydrolysis of an ester

Esters are hydrolyzed into the carboxylic acid and alcohol in the presence of an acid catalyst. Both the reactants and the catalysts are present in the same phase.

Base catalysts

Base catalysts are proton acceptors.

Mechanism

  • The proton is transferred from the substrate to the base to form an intermediate.

Base catalysts equation

Aldol condensation reaction

Aldol condensation is a reaction in which the carbonyl compound reacts with an enolate ion to form 𝛃-hydroxy aldehyde or 𝞫-hydroxy ketone. It is a base-catalyzed reaction.

Enzyme catalysts

Enzyme catalysts are the most important homogenous catalyst. Enzymes are complex protein molecules.

Michaelis-Menten mechanism

Michaelis and Menter proposed the mechanism for the enzyme-catalyzed reaction in 1913.

Step 1

In the first step, the enzyme combines with the substrate to form an activated complex

Step 2

In the second step, this complex breaks into the products and enzymes.

E + S → ES → P + E

  • E = enzyme
  • S = substrate
  • ES = activated complex
  • P = product

Characteristics of enzyme catalysts

  • Enzymes are highly specific in nature. They catalyze only one type of reaction.
  • The presence of coenzymes enhances enzyme activity.
  • Temperature and pH play an important role in enzyme activity.

Examples:

  • Hydrolysis of urea

Urea reacts with water in the presence of the urease enzyme and is converted into ammonia and carbon dioxide.

  • Conversion of glucose into ethanol

Glucose is converted into ethanol in the presence of the zymase enzyme.

2. Heterogeneous catalysts

“In heterogeneous catalysis, the catalysts and reactants are in different phases”.

This type of catalyst is mostly solid, while the reactants are gases or liquids. In this process, the reactant is adsorbed on the surface of catalysts.

Metal catalysts

Transition metals are good catalysts because they can change the oxidation state. They can easily accept and lose 4s and 3d electrons so that they are capable to form a weak bond.

Example:

  • Haber’s process

Haber’s process is the manufacturing of ammonia from nitrogen and hydrogen in the presence of an iron catalyst. During reaction iron changes from one oxidation state into another, at the end of the reaction, it comes back to its original state.

  • Hydrogenation

The addition of molecular hydrogen into the unsaturated organic compound requires Ni, Pd, and Pt catalysts.

  • Preparation of polyethylene

Polyethylene is prepared by the polymerization of terminal alkene in the presence of Ziegler-Natta catalyst(TiCl4+Al(C2H5)3.

Catalysts poisoning

Some metal catalysts become poison if another species adsorb on the surface of catalysts and blocks the active site.

Examples:

  • Iron catalysts used in Haber’s process can be poisoned in the presence of sulfur.
  • Platinum catalysts can be poisoned in the presence of lead or sulfur.

So that reactants must be purified before adsorption on the surface of catalysts.

Autocatalysts

“In some reactions, one of the products of the reaction act as a catalyst it is known as an autocatalyst.“

Example

  • Oxidation of oxalic acid by acidified KMnO4

In the beginning, the reaction between oxalic acid and acidified KMnO4 is slow. After some time MnSO4 is produced and Mn2+ acts as catalysts and increases the rate of reaction.

2KMnO4 + 3H2SO4 + 5(COOH)2 → K2SO4 + 2MnSO4 + 10CO2 + 8H2O

  • The reaction of copper with nitric acid

In the beginning, the reaction between nitric acid and copper is slow but when nitrous acid is formed it increases the rate of reaction.

3Cu + 8HNO3 → 3Cu(NO3)3 + 4H2O + 2NO

Photocatalysts

The catalysts which increase the rate of photochemical reactions are called photocatalysts. There are many photocatalysts, for example, ZnO, SrO2, CdS, ZnS, etc.

Types of photocatalysts

Homogenous photocatalysts

In this type, reactants and photocatalysts are present in the same phase.

Ozone system

The mechanism of hydroxyl free radical formation by ozone consists of two paths:

O3 + hv → O2 + [O]

[O] + H2O → ॱOH + ॱOH

[O] + H2O → H2O2

H2O2 + hv →ॱOH + ॱOH

Photo-Fenton system

The mechanism of hydroxyl free radical formation by the Fenton system is the following:

Fe2+ + H2O2 → ॱOH + Fe3+ + OH–

Fe3+ H2O2 → Fe2+ + HOॱ2 + H+

Fe2+ + HOॱ → Fe3+ + HO–

  • Advantage

It is a more efficient process, it can process in the presence of sunlight of wavelength 450nm.

  • Disadvantage

It can only work at low pH because at high pH iron will be precipitate.

Heterogenous photocatalysts

In this type, photocatalysts and reactants are in different phases. It involves many reactions like oxidation, dehydrogenation, water detoxification, etc.

Oxidative reaction

The mechanism of oxidative reaction due to the photocatalytic effect is the following:

H+ + H2O → H+ + HOॱ

2h+ + 2H2O → 2H+ + H2O2

H2O2 → 2ॱOH

Reductive reaction

The mechanism of reductive reaction due to the photocatalytic effect is the following:

E+ + O2 → ॱO2–

ॱO2– + H2O + H+ → H2O2 + O2

H2O2 → 2 ॱOH

Application of photocatalysts

  • the disinfection of water by TiO2.
  • self-cleaning glass in the presence of titanium dioxide.
  • Conversion of carbon dioxide to gaseous hydrocarbon in the presence of TiO2 and water.
  • Sterilization of instrument
  • Decomposition of crude oil.

Negative catalyst (poisons)

“A substance that retard the rate of a chemical reaction is known as a negative catalyst.”

Negative catalysts retard the rate of reaction by increasing the activation energy.

Example:

  • The addition of tetraethyl lead to petrol is negative catalysis. It saves the petrol from pre-ignition.

Transition state theory in terms of catalysts

Transition state theory is used to understand the effect of catalysts on reactions. It provides another path for the reaction that has lower energy of activation.

According to the transition state theory, the rate of reaction in the presence of catalysts changes from:

kf = (RT/NAh) exp(-ΔGf/RT)

to

kf’ = (RT/NAh) exp(-ΔGf’/RT)

  • ΔG = free energy for activation

ΔG’f is less than ΔGf so the rate of the forward reaction is increased.

The rate of forwarding or reverse reaction in the presence of catalysts are

kf’ = (RT/NAh) exp(-ΔGf’/RT)

k’r = (RT/NAh) exp(-ΔGr’/RT)

Kc = kf’/kr.’ = (RT/NAh) exp(-ΔGf’/RT)/(RT/NAh) exp(-ΔGr’/RT)

= exp(-( -ΔG’f – ΔG’r)/RT)

= exp ( – ΔG*/RT)

The catalysts increase the rate of forward as well as the reverse reaction so that the free energy of activation is lowered for both reactions. The equilibrium constant does not change in the presence of catalysts because the overall free energy change of the reaction( ΔG*)remains unchanged.

Theories of catalysis

Intermediate compound formation theory

This theory explains homogenous catalysis.

According to this theory:

  • First, the catalyst forms an intermediate with one of the reactants.
  • Later, this intermediate reacts with another reactant and forms a product.

Mechanism

Examples

Oxidation of SO2 to SO3

In the oxidation of SO2 to SO3, NO acts as a catalyst. NO2 is formed as an intermediate.

Mechanism:

2NO + O2 → 2NO2

NO2 + SO2 → SO3 + NO

Adsorption theory

This theory explains heterogeneous catalysis. In this process, reactants are adsorbed on the surface of solid catalysts.

There are the following steps involved in adsorption theory:

Step 1

In the first step, reactant molecules are adsorbed on the surface of catalysts.

Step 2: the reactant molecules form an intermediate complex that exists for short time.

Step 3: In this step, the intermediate complex is broken down and forms products, which are also adsorbed on the surface of catalysts.

Step 4: the products are desorbed from the surface of catalysts.

Example

Hydrogenation of ethene

Concepts berg

What are the types of catalysts?

There are the following types of catalysts:

  • Homogeneous catalysts
  • Heterogeneous catalysts
  • Biocatalysts
  • Autocatalysts

Are different catalysts used for different reactions

Catalysts are very specific in their action. Each type of catalyst is used for a different type of reaction. For example,

  • Ni is used for the hydrogenation of unsaturated organic compounds.
  • Sucrase enzymes are used for the hydrolysis of sucrose.

Do different catalysts have different effects on the activation energy of the same reaction

The different catalysts have different effects on the activation energy of the same reaction. For example,

  • Positive catalysts lower the activation energy.
  • Negative catalysts increase the activation energy.

What are some common catalysts in daily life?

Almost everything in our daily life depends on catalysts. For example,

  • papain, pancreatin, and subtilisin enzymes are used to clean our contact lenses at night.
  • The lactase enzyme is used to convert milk into yogurt.
  • Platinum is used in catalytic converters, it converts carbon monoxide into carbon dioxide.

How can a catalyst initiate a reaction?

Catalysts do not initiate a reaction. It only changes the rate of the reaction by providing another path with lower activation energy.

What is the use of the ZnCl2 catalyst?

ZnCL2 is used as a catalyst for the halogenation of alcohol. It attaches to the hydroxyl group of alcohol and increases the rate of conversion of alcohol into alkyl halide.

Which factors affect the activity of the catalyst?

There are the following factors that affect the activity of the catalyst:

  • Temperature
  • pH
  • The surface area of catalysts
  • Reactant concentration

Does the surface area of a catalyst affect the rate of a reaction

By increasing the surface area of catalysts increase the chance of collision between a molecule in surface catalysis. So that increasing the surface area of catalysts increases the rate of a reaction.

Can an enzyme catalyze both forward and backward reactions?

Enzymes increase the rate of forward as well as the reverse reaction so that it does not affect the equilibrium.

What are some examples of metabolic enzymes?

There are the following examples of some metabolic enzymes:

  • Amylase → digest starch into smaller molecules.
  • Lipases→ break down fats
  • Proteases→ break down proteins

References Books

  • A Textbook of Physical Chemistry for B.Sc by Chaudary Sana Ullah (Ph.D. Chemistry)
  • A Textbook of Physical Chemistry Volume 5 By K.L. Kapoor (University of Delhi)

Reference links

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