• 2024-04-01
• sanjay
• Catalysis, Catalysis types
• 912

What are Catalysis and its types? Catalyst

Jöns Jakob Berzelius received a major observation in 1835 regarding the speed of the reactions. Jöns Jakob Berzelius observed that the velocities of many chemical reactions increase due to the presence of an external substance.

These external substances do not participate in the reaction and at the end of the reaction remain unchanged by the mass and chemical view. 

Example: If potassium chlorate is heated at 360°C, the oxygen is obtained in less amount, but when heated by adding a little manganese dioxide to potassium chlorate at 250°C then sufficient amount of oxygen is obtained.

At the end of the reaction manganese dioxide can be chemically separated from the mixture in an unchanged state, and the amount taken at the beginning is equal to the volume obtained at the end.

Berzelius named such external substances as catalyst and this phenomenon of increasing the speed of reaction gave the name of catalysis.

Subsequently, it was found that the speed of many chemical reactions also decrease due to the presence of an external substance.

Example: hydrogen peroxide (H₂O₂) is easily decomposed into water and oxygen, but in the presence of phosphorus acid, the speed of decomposition decreases. Hence Wilhelm Ostwald defined catalyst and catalysis as follows –

” A substance whose presence can increase or decrease the speed of a chemical reaction and which itself remains unchanged by weight and chemical point of view at the end of the reaction is called catalyst. This phenomenon of influencing the reaction speed is called catalysis. “

Classification of Catalysis

Depending on the physical state of the catalysts and reactants, catalysis is divided into the following two classes.

Homogeneous catalysis

When reactants and catalysts are in the same physical state in a chemical reaction, catalysis is called Homogeneous catalysis.

01) Oxygen gas is obtained by heating potassium chlorate with manganese dioxide.

2KClO₃ –MnO₂→ 2KCl + 3O₂

Potassium Chlorate is reactive in this reaction and manganese dioxide acts as a catalyst. Both reactants and catalysts are solid, meaning both are in the same physical state.

02) Glucose (C₆H₁₂O₆) and fructose (C₆H₁₂O₆) are obtained when sugarcane sugar(C₁₂H₂₂O₁₁) solution is heated with dilute sulfuric acid.

C₁₂H₂₂O₁₁ + H₂O –H₂SO₄→ C₆H₁₂O₆ + C₆H₁₂O₆

The sugarcane solution is reactant and dilute H₂SO₄ catalyst in this reaction. Both these substances are liquid, that is, in the same physical state.

In the Lead-chamber Process of making sulfuric acid, sulfur dioxide gas and oxygen gas are reactants and oxides of hydrogen(Nitric oxide gas) act as catalysts.

2SO₂ + O₂ –NO→ 2SO₃

The reactants and catalysts are in the same physical state in this reaction. This reaction is also an example of Homogeneous catalysis.

Heterogeneous catalysis

In a chemical reaction, when the physical states of the reactants and catalysts differ, the catalyst is called heterogeneous catalysis.

Example:

Iron acts as a catalyst in the hebar method of making ammonia by combining nitrogen and hydrogen.

N₂ + 3H₂ –Fe→ 2NH₃

N₂ and H₂ are reactants in this reaction and are in gaseous state. The catalyst is in solid state. Hence this reaction is an example of heterogeneous catalysis.

In the process of making sulfuric acid, platinum is used as a catalyst.

2SO₂ + O₂ –Pt→ 2SO₃

SO₃ + H₂O → H₂SO₄

SO₂ and O₂ are reactants in this reaction and are in gaseous state. The catalyst is in solid state. Hence this reaction is also an example of heterogeneous catalysis.

nickel is used as a catalyst in the preparation of vegetable ghee from vegetable oils.

vegetable oil + H₂ –Ni→ vegetable ghee

In this reaction, vegetable oil is in liquid state, H₂ is in gaseous state and Ni is in solid state. Hence this reaction is also an example of heterogeneous catalysis.

Classification of catalysts based on the process of catalysts

The catalyst is divided into four parts based on the action of the catalyst.

Positive catalysis

When a catalyst increases the speed of a reaction, it is called positive catalysis and the phenomenon of increase in reaction speed.

When potassium chlorate is heated to 360°C, its decomposition occurs slowly and the amount of oxygen obtained is low. When mixed with a little manganese dioxide(MnO₂) in potassium chlorate and heating it at 250°C, sufficient amount of oxygen is obtained at a rapid rate.

2KClO₃ –MnO₂→ 2KCl + 3O₂

In this reaction manganese dioxide acts as positive catalysis and this reaction is an example of positive catalysis.

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Iron (Fe) is used as a catalyst in the haber method of making ammonia by combining nitrogen and hydrogen. The reaction speed up in the presence of iron.

N₂ + 3H₂ –Fe→ 2NH₃

In this reaction iron acts as positive catalysis and this reaction is an example of positive catalysis.

Manufacture of sulfuric acid by sis chamber method –

2SO₂ + O₂ –NO→ 2SO₃

SO₃ + H₂O → H₂SO₄

In this reaction, nitric oxide gas acts as a positive catalysis.

Formation of sulfuric acid by contact method –

2SO₂ + O₂ – Pt or V₂O₅ → 2SO₃

SO₃ + H₂O → H₂SO₄

Pt or V₂O₅ act as positive catalysis in this reaction.

Manufacture of nitric acid by ostbord method –

4NH₃ + 5O₂ –Pt→ 4NO + 6H₂O

2NO + O₂ → 2NO₂

3NO₂ + H₂O → 2HNO₃ + NO

Pt works in this reaction for catalysis.

Hydronisation of vegetable oils –

Vegetable oil + H₂ –Ni→ vegetable ghee

Ni positive catalysis works in this reaction.

Decomposition of Hydrogen peroxide –

2H₂O₂ –Pt→ 2H₂O + O₂

Pt works in this reaction for catalysis.

Inversion of cane sugar –

C₁₂H₂₂O₁₁ + H₂O –H₂SO₄→ C₆H₁₂O₆ + C₆H₁₂O₆

In this reaction dilute H₂SO₄ acts as a positive catalysis.

The terms catalyst and catalysis are generally used only for positive catalyst and positive catalysis.

Negative Catalysis

When a catalyst reduces the speed of a reaction, it is called negative catalysis or inhibitor. The phenomenon of decrease in reaction speed is called negative catalysis.

example:

Hydrogen peroxide is easily decomposed into water and oxygen, but in the presence of phosphoric acid its speed of decomposition decreases.

2H₂O₂ –H₃PO₄→ 2H₂O + O₂

In this reaction, phosphoric acid performs negative catalysis.

In the presence of light, the chloroform reacts with oxygen to decompose. In this reaction toxic gas phosgene (COCl₂) and hydrogen chloride gas are formed.

2CHCl₃ + O₂ –hv→ 2COCl₂ + 2HCl

To prevent the decomposition of chloroform, store it in colored bottles so that the colored glass absorbs the light. In addition, a small amount of ethyl alcohol is added to it, which acts as a negative catalysis for the above reaction.

Auto-Catalysis

When a product of a chemical reaction itself acts as a catalyst and no catalyst has to be added from outside, this type of catalyst is called auto catalyst and this type of catalysis is called auto catalysis.

Example: Water decomposition of ethyl acetate (CH₃COOC₂H₅) provides acetic acid (CH₃COOH) which acts as an auto catalyst.

CH₃COOC₂H₅ + H₂O ⇋ CH₃COOH + C₂H₅OH

The speed of this reaction is initially low, but as the amount of acetic acid produced as a result of the reaction increases, so does the speed of the reaction.

The acidic solution of potassium permanganate (KMnO₄) in oxalic acid (H₂C₂O₄) solution causes the following reaction and the pink color of potassium permanganate (KMnO₄) disappears.

2 KMnO₄ + 3H₂SO₄ + 5 H₂C₂O₄ → K₂SO₄ + 2MnSO₄ + 10CO₂ + 8H₂O

This titration initially takes longer for the pink color of potassium permanganate (KMnO₄) to fade. After some time the color of potassium permanganate (KMnO₄) rapidly disappears. The reason for this is that the Manganese(II) sulfate (MnSO₄) formed in this reaction acts as a auto catalyst.

Induced catalysis

If any two reactions are occurring simultaneously, in some cases one reaction increases the speed of the other reaction. These types of events are called induced catalysis.

example:

The chemical reaction of mercuric chloride (HgCl₂) and oxalic acid is slow. The reaction of acidic potassium permanganate (KMnO₄) and oxylic acid is faster than the above reaction.

If the vessel containing the first chemical reaction is mixed with some amount of acidic potassium permanganate (KMnO₄), then the speed of the first chemical reaction also increases.

Here the chemical reaction of potassium permanganate (KMnO₄) and oxalic acid induces chemical reaction of mercuric chloride and oxalic acid, which increases its speed. This phenomenon is an example of induced catalysis.

The solution of sodium arsenite (Na₂AsO₃) is not oxidized by air, but the solution of sodium sulphite(Na₂SO₃) is oxidized by air. If both solutions of sodium sulfite and sodium arsenite are mixed and kept in the presence of air, both these substances get oxidized.

2Na₂SO₃ + O₂ → 2Na₂SO₄

2Na₂AsO₃ + O₂ → 2Na₂AsO₄

In this action, the oxidation reaction of sodium sulfite acts as a catalyst for the oxidation reaction of sodium arsenite and this phenomenon is an example of induced catalysis.

Catalyst Promoters and Poisons

Promoters Catalyst

Those substances, when used in small quantities with catalyst, stimulate the power of that catalyst, that is, they increase but do not automatically act as catalyst, called catalytic promoters.

example:

In the haber process of making ammonia, iron powder is used as catalyst. If a small amount of molybdenum is added to the iron powder, the catalysis capacity of iron powder increases. Here molybdenum (Mo) catalyst acts as promoters.

Nickel catalyst acts in the manufacture of vegetable ghee from vegetable oil. If a small amount of tellurium (Te) is added to the nickel, its catalysis capacity increases. Here tellurium catalyst acts as promoters.

Catalyst Poisons

Substances that destroy or reduce the strength of the catalyst when present in a small amount with a catalyst are called poisons catalyst or anti catalyst.

example:

In the hebar process of making ammonia, iron powder is used as catalyst. If a small amount of hydrogen sulfide gas(H₂S) is added to the reaction mixture or, if already present, it destroys the catalysis ability of iron. Here hydrogen sulfide gas catalyst acts as poisons.

Platinum catalyst acts in the manufacture of sulfuric acid by the contact method. If a small amount of arsenic trioxide is present with the catalyst, the functional capacity of the catalyst is greatly reduced. Here arsenic tri oxide catalyst acts as poisons.