Catalysis

Catalysis Chemistry Notes

Catalysis :

→ Berzelius in 1835, found that the rate of various chemical reactions are altered in the presence of some foreign substances. He called these foreign substances ‘catalyst’.

→ Thus “a substance which can change the rate of a chemical reaction without itself undergoing any change in mass and chemical composition at the completion of reaction is called catalyst. This phenomenon is called catalysis.”

Types of Catalysts :

The various types of catalysts are as follows:

→ Positive Catalyst: The catalyst which can increase the rate of a chemical reaction is called positive catalyst. Examples :
In the thermal decomposition of potassium chlorate, MnO₂ behaves as positive catalyst.

→ In presence of positive catalyst, the value of activation energy decreases hence the rate of reaction increases.

→ Negative Catalyst: The catalyst which can decrease or retard the rate of a chemical reaction is called negative catalyst. Examples :
The rate of decomposition of hydrogen peroxide decreases in the presence of glycerol. That’s why, to store hydrogen peroxide, small amount of glycerol is added to it.

• Auto oxidation of benzaldehyde is stopped in presence of small amount of sulphur compound.
• To decrease the knocking of petrol, generally small amount of tetraethyl lead is added to it.
• Oxidation of sodium sulphite in presence of air is stopped in presence of glycerine. Here glycerine behaves as negative catalyst.

→ In presence of negative catalyst, the value of activation energy increases hence the rate of reaction decreases.

→ Auto Catalyst: During a chemical reaction, when one of the products formed behave as a catalyst and increases the rate of reaction then such catalyst is known as auto catalyst and the phenomenon is known as autocatalysis. Example :
→ The hydrolysis of ester is slow in the begining but becomes fast after some times because acetic acid formed during the hydrolysis of ester behave as auto catalyst in this reaction.

→ During the oxidation of oxalic acid by acidified potassium permanganate, manganese ions (Mn²⁺) are formed. This manganese ions behave as autocatalyst. As soon as Mn²⁺ ions formed, the rate of reaction increases.

→ Induced Catalyst : There are some reactions in which any substance in the form of catalyst is not added but the rate of reaction increases due to the induction of any other reaction occurring simultaneously with it. This phenomenon is known as induced catalysis and the reaction which is inducing the other reaction is known as induced catalyst.

→ For example : Sodium sulphite (Na₂SO₃) is oxidised in presence of air but the oxidation of sodium arsenite (Na₂ AsO₃) is not possible. As both the reactants (Na₂ SO₃ and Na₂ AsO₃) are mixed then both (Na₂ SO₃ and Na₂ AsO₃) are oxidised by air because the oxidation of sodium sulphite (Na₂ SO₃) induces the oxidation of sodium arsenite (Na₂ AsO₃) Hence, in this reaction sodium sulphite behaves as induced catalyst.

Types of Catalysis :

Generally catalysis are of two types :

Homogeneous Catalysis :

→ If the reactanta, products and catalyst are in same phase then this type of catalysis is called homogeneous catalysis.
Example:
Oxidation of SO₂ : In lead chamber process, the oxidation of SO₂ is an example of homogeneous catalysis.

→ Hydrolysis of methyl acetate : Hydrolysis of methyl acetate by H^– ion produced from hydrochloric acid (HCl) is an example of homogeneous catalysis.

Here all the substances are in the liquid phase.

→ Hydrolysis of can sugar : Hydrolysis of cane sugar, catalysed by H⁺ ions produced from H₂SO₄ is an example of homogeneous catalysis.

Here all the substances are in same phase.

→ Oxidation of CO : Oxidation of CO by O₂ takes place in presence of NO as catalyst.

→ Preparation of diethyl ether : Preparation of diethyl ether from ethyl alcohol using conc. H₂SO₄ is an example of homogeneous catalysis.

Heterogeneous Catalysis :

→ If the catalyst is not present in same phase as reactants and products in a chemical reaction then this type of catalysis is called heterogeneous catalysis.

Example :
Oxidation of SO₂ : Oxidation of so, into SO₃ in presence of Pt catalyst is an example of heterogeneous catalysis.

→ Haber’s Process : In this process, finely divided Fe is used as catalyst to produce ammonia from nitrogen and hydrogen gas.

→ Here all the reactants and products are in gas phase while catalyst iron is in solid phase.

→ Ostwald’s Process : In this process, oxidation of ammonia into nitric oxide takes place in presence of platinum gauze.

Mechanism of Catalysis :

Mechanism of catalysis are as follows:

→ Mechanism of Homogeneous Catalysis : Intermediate compounds formation theory According to this concept, catalyst forms an intermediate compound with one of the reactants. This intermediate compound is unstable which becomes free by reacting with other reactant to form product. A reaction A + B → AB takes place at very slow rate which takes place easily in the presence of catalyst X.
A + X → AX (Intermediate compound)
AX + B → AB + X

Low activation energy is required in the formation of intermediate AX. The reaction takes place at fast rate. Example:

Following facts can not be explained by intermediate compound theory:

• Mechanism of heterogeneous catalysis.
• Mechanism of catalyst promoter and catalyst poison.
• Importance of active centres.

→ Mechanism of Heterogeneous catalysis : Adsorption Theory : Many gaseous reactions take place in the presence of solid catalyst. Active centres are present at the surface of solid catalyst due to free valencies. The molecules of reactants are adsorbed at the surface of solid catalyst by bonding with active molecules. Adsorbed molecules form activated complexes with catalyst; which are decomposed to form products and desorption starts at the surface of products.

Following points can be explained by this theory :

• Adsorption and desorption take place continuously at the surface of catalyst. So, small amount of catalyst is required to catalyse high amount of reactants.
• The surface of catalysts remains unchanged after desorption so, mass and composition of catalyst do not change at the end of reactions.
• Reactants form chemical bonds at the surface of catalyst, in which free adsorption energy compensates activation energy. So, reaction takes place rapidly.
• The molecules of catalyst poison are adsorbed rigidly on free valencies present at the surface of catalyst by which the molecules of reactants are not adsorbed.
• Promoters are adsorbed at the surface of catalyst that number of active centres increased due to this reason adsorption capacity and reactivity increases.

Points to be remember :

→ Catalytic Poison or Inhibitor : The substances which destroy the activity of a catalyst by their presence is called catalytic poison or inhibitor. Example: In Haber’s process, if small amount of CO is present with hydrogen gas then it decreases the activity of iron catalyst i.e, CO behaves as catalytic poison for iron catalyst.

• The platinum catalyst used in the oxidation of hydrogen is poisoned by CO.
• The poisoning of the catalyst is probably due to adsorption of poison on active centres present at the surface of catalyst. Thus reducing the free surface for the adsorption of reacting molecules.
• Due to this reason, the activity of catalyst either decreases or destroys and hence the rate of production of products also decreases.

→ Catalysis Promoters : The substances which can increase the efficiency of catalyst but themselves are not catalyst are known as catalytic promoters or activators.

Example :

→ In Haber’s process, for the synthesis of ammonia, iron powder behaves as catalyst but traces of molybdenum increases the activity of this catalyst. Hence Mo is catalytic promoter.

In the hydrogenation of vegetable oil, the activity of nickel catalyst can be increased by adding small amount of copper (Catalytic promoter).

Vegetable oil + H → Vegetable ghee

During the production of methyl alcohol from water gas (CO + H₂),chromic oxide (Cr₂O₃) is used as catalytic promoter to increase the activity of zinc oxide (ZnO) catalyst.

Reason : Catalytic promoter increases the number of activity centres on the surface of catalyst due to which the rate of reaction increases.

Enzyme Catalysis :

→ Enzymes are complex nitrogenous organic compounds. They are obtained from living plants and animals. They are actually protein molecules having high molecular mass ranging from 15,000 to 1,000,000 g mol^-1 and form colloidal solutions in water. Enzymes are very effective catalysts. Enzymes are effective generally for those chemical reactions which are related to natural processes.

→ They are also known as biochemical catalyst and the phenomenon of enzyme catalysis is known as biochemical catalysis Various enzymes are obtained from living cells. However, the first enzyme was synthesised in the laboratory in 1969. Enzymes are vital for the biological processes. Without enzyme, the life processes would be to slow to sustain life. The following are some of the examples of enzyme catalysis.

Inversion of can sugar : In the presence of enzyme invertase’ cane sugar is converted into glucose and fructose.

Conversion of glucose into ethyl alcohol : In presence of enzyme ‘zymase’, glucose is converted into ethyl alcohol and carbon dioxide.

Conversion of starch into maltose: In presence of enzyme ‘diastase’, starch is converted into maltose.

Conversion of maltose into glucose : In presence of enzyme ‘maltase’, maltose is converted into glucose.

Decomposition of urea into ammonia and carbon di oxide : In presence of enzyme ‘urease’, urea is converted into NH₃ and CO₂.

Conversion of proteins into peptides: In stomach, the enzyme ‘pepsin’ converts proteins into peptides.

Conversion of proteins into amino acids : In intestine, the pancreatic enzyme “trypsin converts proteins into amino acids by hydrolysis.

Conversion of milk into curd : The lacto bacilli’ which is present in curd is responsible to convert milk into curd by enzymatic action.

Conversion of ethyl alcohol into acetic acid : In presence of Mycoderma aceti’. dilute solution of ethyl alcohol is converted into acetic acid and water.

The summary of some important enzymatic reactions are given in table 5.2

Some enzymatic reactions

Enzyme	Source	Enzymatic reaction
Invertase	Yeast	Sucrose → Glucose and Fructose
Zymase	Yeast	Glucose → Ethyl alcohol and Carbon dioxide
Diastase	Malt	Starch → Maltose
Maltase	Yeast	Maltose → Glucose
Urease	Soyabean	Urea → Ammonia and C02
Pepsin	Stomach	Proteins → Peptides
Trypsin	Intestine	Proteins → Amino acids
Amylase	Saliva	Starch → Glucose
Lactobacilli	Curd	Fermentation of milk
Mycoderma	Vinegar	Ethyl alcohol → Acetic acid
Lipase	Castor seed	Fat → Glycerol
Ptyalin	Saliva	Starch → Sugar

Characteristics of Enzyme Catalysis :

Enzyme catalysis is quite similar to inorganic heterogeneous catalysis. It is unique in its efficiency and high degree of specificity. The main characteristic of enzyme catalysis are given below:

→ Highly Specific Nature : Enzymes are highly specific in their nature i.e., one enzyme can not catalyse more than one reaction.

For example:

• Enzyme ‘urease’ can hydrolyse urea only. It can not hydrolyse the other amide.
• Zymase converts only glucose into alcohol and CO₂, It can not hydrolyse fructose.

→ High efficiency : Enzymes are highly effcient in their action. One molecule of an enzyme may catalyse one million molecules of the reactant per minute. It is due to the fact that the activation energy of chemical reaction is quite low in presence of enzyme.

→ Highly active at optimum temperature : The rate of an epzymatic reaction is maximum at a definite temperature, called the optimum temperature. Beyond the optimum temperature the activity of enzyme decreases and ultimately becomes zero. It is observed that the optimum temperature range for enzymatic activity is 298-310 K, which is human body temperature (i.e., 37°C). In fever (i.e., at higher temperature) the enzyme activity becomes slow.

→ Highly active at optimum pH : The rate of enzymatic reaction is maximum at a particular pH called optimum pH, which is in between 5-7. In human body, the optimum pH for enzymatic reaction is 7.4.

→ Effect on equilibrium state : Like catalyst, enzyme cannot disturb the final state of equilibrium of a reversible reaction. It only increases the rate of forward and backward reaction equal to extent so that equilibrium may achieve faster.

→ Colloidal nature : Enzymes form colloidal solutions in water. Their efficiency is decreased in presence of electrolytes.

→ Increasing activity in presence of activators and co-enzymes : Those substances, whose presence can increase the activity of enzyme is known as co-enzymes. For example : small amount of non-protein substance like vitamins, present along with enzyme may increase the catalytic activity of enzyme considerably.

→ Activators are generally metal ions such as Na⁺, Mn²⁺, CO²⁺, Cu²⁺ etc. These metal ions, when weakly bonded to enzyme molecules, increase their catalytic activity. For example, amylase in presence of sodium chloride i.e.. Na⁺ ions are catalytically very active.

→ Effect of Inhibitor or poisons : Like catalyst, enzymes are also poisoned in presence of certain substances. These substances are known as inhibitor or poisons. The inhibitors or poisons interact with the active functional groups on the suface of enzyme and often reduce or completely destroy the catalytic activity of the enzymes. For example, many drugs work as enzyme inhibitor and they destroy the activity of enzyme in the body.

Mechanism of Enzyme Catalysis :

The mechanism of enzyme catalysis involves the following steps:

→ Formation of Complex : There are various cavities on the surface of colloidal particles of enzymes. These cavities have characteristic shape and have active groups like-NH₂, -COOH, -SH, -OH etc. on its surface. These active groups actually are active centers at the surface of enzyme particles. The reactants or substrate which have complementary substances, fit into these cavities just like key fits into a lock and form a activated complex

Formation of products : The complex is activated hence hase higher energy. So it decomposes easily into products.

Zeolite Catalysis or shape selective catalysis :

→ The catalysis process, which depends upon the pore structure of the catalyst and molecular size of reactants and products, is known as shape selective catalysis. Zeolites are good examples of shape selective catalysis due to its honey comb like structure. Zeolites are naturally occurring or synthetic microporous alumino silicates having general formula

M_x/n [(AlO₂)x(SiO₂)y].mH₂O
where, M = Na⁺, K⁺, Ca²⁺ like metals
n = Valency of cation
m = Number of molecules of water of crystallisation Zeolites have three dimentional network structure of silicates in which some silicon atoms are replaced by aluminium atoms giving Al-O-Si frame work. Zeolites are formed in nature as well as synthesised artificially.

→ Zeolites, which are used as catalysts, first of all heated strongly in vacuum so that water of crystallization can be lost and it may become porous. The size of pores in zeolites varies between 260 pm to 740 pm. In this way only those molecules can be adsorbed whose size is small enough to enter in these pores and can also bo left catalyst easily.

→ Zeolites do not work as catalyst for those molecules which are too big to enter. Thus zeolites act as selective adsorbents and hence work as molecular sieves.

Example : (i) ZSM-5 (Zeolite sieve of molecular porosity 5) is used in petroleum industry. ZSM-5 converts alcohols directly into gasoline (petrol) by dehydrating the alcohol so that a mixture of hydrocarbons is obtained.

Zeolite is also used in softening of hard water in ion exchange method

Chemistry Notes