catalysisBy prof. Abhinav ShuklaIit.email@example.com 9096905695
Catalyst• Catalyst is a substance which speeds up and speeds down a chemical reaction without itself being used up.” orOstwald (1895) redefined a catalyst as, “A substance which changes the reaction rate without affecting the overall energetics of the reaction is termed as a catalyst and the phenomenon is known as catalysis
Types of catalysis• (1) Homogeneous catalysis : When the reactants and the catalyst are in the same phase (i.e. solid, liquid or gas). The catalysis is said to be homogeneous. The following are some of the examples of homogeneous catalysis.examples NO( g ) 2SO 2 (g) O 2 (g) 2SO 3 (g) HCl(l)CH 3 COOCH 3 (l) H 2 O(l) CH 3 COOH (l) CH 3 OH (l)
• (2) Heterogeneous catalysis : The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis. Some of the examples of heterogeneous catalysis are given below. Pt (s) 2SO 2 (g) O2 (g) 2SO 3 (g) BrV2 O5 Fe (s) N 2 (g) 3 H 2 (g) 2 NH 3 (g)• (3) Positive catalysis : When the rate of the reaction is accelerated by the foreign substance, it is said to be a positive catalyst and phenomenon as positive catalysis. Some examples of positive catalysis are given below MnO2 (s) 2 KClO3 (s) 2 KCl(s) 3O2 (g) o 270 C
• (4) Negative catalysis : There are certain, substance which, when added to the reaction mixture, retard the reaction rate instead of increasing it. These are called negative catalyst or inhibitors and the phenomenon is known as negative catalysis. Some examples are as follows. Diphenyl 2C6 H5 CHO (l) O2 (g) 2C6 H5 COOH (l) amine(l)• (5) Auto-catalysis : In certain reactions, one of the product acts as a catalyst. In the initial stages the reaction is slow but as soon as the products come into existences the reaction rate increases. This type of phenomenon is known as auto- catalysis. Some examples are as follows
• The rate of oxidation of oxalic acid by acidified potassium permanganate increases as the reaction progresses. This acceleration is due to the presence of ions which are formed during reaction. Thus ions act as auto-catalyst.5 H2C2O4 2KMnO 4 3 H2SO 4 2MnSO 4 K2SO 4 10 CO2 8 H 2O
Characteristics of catalysis• (1) A catalyst remains unchanged in mass and chemical composition at the end of the reaction.• (2) A small quantity of the catalyst is generally sufficient to catalyses almost unlimited reactions• (3) The catalyst can not initiate the reaction: The function of the catalyst is to alter the speed of the reaction rather than to start it. (4) The catalyst can not change the position of equilibrium : The catalyst catalyse both forward and backward reactions to the same extent in a reversible reaction and thus have no effect on the equilibrium constant.
• (5) The catalyst is generally specific in nature: A substance, which acts as a catalyst for a particular reaction , fails to catalyse the other reaction , different catalysts for the same reactant may for different products. Al 2O3 C2 H 4 (g) H 2 O(g) (Dehydration) C 2 H 5 OH (l) Cu CH 3 CHO (g) H 2 (g) (Dehydroge nation )(6) Catalytic promoters : Substances which themselves are not catalysts, but when mixed in small quantities with the catalysts increase their efficiency are called as promoters or activators. For example, in Haber’s process for the synthesis of ammonia, traces of molybdenum increases the activity of finely divided iron which acts as a catalyst.
• (7) Catalytic poisons : Substances which destroy the activity of the catalyst by their presence are known as catalytic poisons. For example, the presence of traces of arsenious oxide in the reacting gases reduces the activity of platinized asbestos which is used as catalyst in contact process for the manufacture of sulphuric acid.• (8) A positive catalyst lowers the activation energy
Uncatalysed complexChemica Energy l barrierpote Catalysedntial complexener Ea gy Ea ER Reactants G of Initial (A+B) reaction state EP Final state Products (C + D) Reaction sequence
Enzyme catalysis• (1) Enzymes are complex nitrogenous substances these are actually protein molecules of higher molecular mass.Numerous reactions occur in the bodies of animals and plants to maintain the life process. These reactions are catalysed by enzymes. The enzymes are thus, termed as bio-chemical catalysts and the phenomenon is known as bio-chemical catalysis.examples Invertase C12 H 22O11(l) H 2O(l) C6 H12O6 (l) C6 H12O6 (l) Glucose Fructose Zymase C6 H12O6 (l) 2C2 H 5 OH (l) 2CO2 (l) Diastasestarch Maltose (l)
Activity and Selectivity• Activity : Activity is the ability of catalysts to accelerate chemical reaction, the degree of acceleration can be as high as times in certain reactions• Selectivity : Is the ability of catalysts to direct reaction to yield particular products (excluding other).• Zeolite (Shape selective catalysis) Zeolite are alumino–silicates of the general formula, M x / n [ AlO 2 ]x .(SiO 2 )y .mH 2 Owhere, M may be simple cation , n is the charge on the simple cation, m is the number of molecules of water of crystallization.The zeolite catalyst ZSM-5 converts alcohols to gasoline (petrol) by dehydrating the alcohol