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.
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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.
