Shaillee Kaushal is teaching live on Unacademy Plus
XII 05 SURFACE CHEMISTRY- IV
Characteristics of catalysis: (i) A catalyst remain unchanged in mass and chemical composition at the end of the reaction The amount of catalyst is the same as at the end of the feaction. There is no change in its composition. (ii) A small amount of catalyst is sufficient to catalyst a reaction. >For example in the decomposition of hydrogen peroxide one gram of colloidal platinum can catalyse 108 litre of hydrogen peroxide However in some reactions the rate of the reaction is proportional to the concentration of the catalyst. RCOOR+H20RCOOH+R'OH H+ In the above reaction the rate of the reaction is proportional to the concentration of OH- ions, or H+ ions In certain reactions the rate of the reaction increases with increase of area of catalytic surface.
(iii) The catalyst cannot initiate the reaction: The catalyst only alters the speed of the reaction but does not start the reaction However there are certain instances where it is observed thatthe reaction cannot be started in the absence of the catalyst. For example there is no reaction with H2 and 02 at room temperature but the reaction occurs very readily in presence of platinum blac Room temperaturs No reaction 2H2(g)+02(g)c 2H2 + 02--> 2H20 Pt black
(iv) The catalyst is generally specific in nature: A substance which acts as a catalyst for one reaction may not act as a the other reaction. clys Different catalyst for the same reactant may form different products Sometimes catalyst may alter the reaction product Al203 C2H4+H20 (Dehydration) C2Hs0H H3 CH2Cl Cu 383K CH3CHO H2 (Dehydration) + Cl2 +HCI Sunlight Chlorophenyl methane Cu (Benzyl chloride) CO2 + H2 (Dehydration) HCOOH CH3 CH3 CH3 Al20 CP +H20 (Dehydration) Fecl + CI2 +HCI o-Chlorotoluene CI p-Chlorotoluene
(v) The catalyst does not change the position of equilibrium: In the case of reversible reactions, the concentration of the products and the reactants cannot be affected by the catalyst if the equilibrium position has been established. However the use of catalyst can help to achieve the equilibriu state in lesser time (vi) Catalytic promoter: Substances which themselves are not catalysts but when mixe quantities with the catalysts increase their efficiency are called promoters or activators. For example: (a) In Haber's process for the manufacture of NH3 traces of molybdenum increases the activity of finely divided iron which acts as a catalyst (b) In the manufacture of methyl alcohol from water gas (CO+ H2), Cr203 is used as a promoter with the catalyst Zno al
(vi) Catalytic Poisons: Substance which destroy the activity of catalyst by their prese e called poisons. For example (a) Presence of As203 in the reacting gases reduces the activity of platinized asbestos which is used as a catalyst in contact process. (b) The activity of iron catalyst is destroyed by the presence of H2S or Co in the synthesis of NH3
(vi) Catalytic Poisons: Substance which destroy the activity of catalyst by their presence are called poisons. For example (a) Presence of As2 03 in the reacting gases reduces the activity of platinized asbestos which is used as a catalyst in contact process. (b) The activity of iron catalyst is destroyed by the presence of H2S or Co in the synthesis of NH3
(vii) Change of temperature alters the rate of catalytic reaction: By increasing the temperature there is an increase in catalytic power of catalyst but after a certain temperature its power begins to decrease. This an optimum temperature is required for the activity of the catalyst. However incase of colloidal solution acting as catalysts activity decrease by the rise of temperature due to coagulation of the colloidal solution.
(ix) A positive catalyst lowers the activation energy The catalyst provides a new pathway involving lower amount of activation energy. Thus, larger number of effective collisions occur in presence of a catalyst in comparison to effective collisions at the same temperature in absence of a catalyst. Therefore catalyst increases the rate of a reaction
Activated complex Ea (uncat) Uncatalyzed reaction Ea (cat) Reactant Catalyzed reaction Product Reaction coordinate