Principles of Chemical Catalysis PDF

Principles of Chemical Catalysis Dr. Asaad F. Hassan Assoc. Prof. Of Physical Chemistry 1  Contents: ❖Introduction ❖ Acid–Base Catalysis ❖The Properties Of 1-Specific Acid-Catalysis Catalysts 2-Specific Base –Cataly...

Principles of Chemical Catalysis Dr. Asaad F. Hassan Assoc. Prof. Of Physical Chemistry 1  Contents: ❖Introduction ❖ Acid–Base Catalysis ❖The Properties Of 1-Specific Acid-Catalysis Catalysts 2-Specific Base –Catalysis ❖Mechanism Of Catalysis 3-Mechanism Of Acid - Base Catalysis ❖Catalysts And Reversible 4-Examples On Acid - Base Reactions Catalysis ❖Catalyst Activity & ❖ Enzyme Catalysis Selectivity &Autocatalyst ❖Homogeneous Catalysis 2  Ways to Make Chemicals Faster Temperature Pressure Disadvantage--Too hot! Disadvantage--Cause Explosions Add other Chemicals Catalysts!!!! Disadvantage—Separation of chemicals Disadvantage--Costly 3 Who Uses Catalyst? Industries Your body (Enzymes) Chemical/Pharmaceutical Companies Catalytic Converters in your car 4 What is a Catalyst? Catalysts speed up a chemical reaction without being used up... Catalyst-Reactants Catalyst + Reactants Catalyst + Products 5 Examples of Reactions With Catalysts Propane + Catalyst = Benzene + Catalyst = + C-C-C + Food + Enzymes = Small Molecules + Enzymes Proteins Food + = Fats + Simple Sugars 6  Types of Catalysts & Catalytic Reactions ❑Classification based on the its physical state, a catalyst can be Gas Liquid Solid ❑Classification based on the substances from which a catalyst is made Inorganic (gases, metals, metal oxides, inorganic acids, bases etc.) Organic (organic acids, enzymes etc.) 7  Types of Catalysts & Catalytic Reactions, Cont’d ❑Classification based on the ways catalysts work Homogeneous - both catalyst and all reactants are in the same phase (gas or liq.) Heterogeneous - reaction system involves multi- phase (catalysts + reactants) ❑Classification based on the catalysts’ action Acid-base catalysts Enzymatic Photocatalysis Electrocatalysis, etc. 8  Definitions of catalyst ❑A catalyst is a substance that increases the rate of a reaction without affecting the position of equilibrium. It follows that the rate in the reverse direction must be increased by the same factor as that in the forward direction ❑A catalyst is a compound that increases the rate of a reaction by providing an alternative reaction mechanism, for a chemical process. This alternative mechanism has a lower 9 activation energy.  A Catalyst: a substance that increases the rate of a reaction and can be recovered chemically unchanged at the end of the reaction. A catalyst Lower the activation energy Avoiding the slow, rate-determining step of the uncatalyzed reaction Results in a higher reaction rate at the same temperature Activation energies of catalyzed reactions Reaction Catalyst Ea / kJmol-1 2 HI → H 2 + I 2 None 184 Au 105 Pt 59 2NH3 → N2 + 3H 2 None 350 W 162 10  Action of Catalysts Catalysis action - Reaction kinetics and mechanism Catalyst action leads to the rate of a reaction to change. This is realised by changing the course of reaction (compared to non-catalytic reaction) Forming complex with reactants/products, controlling the rate of elementary steps in the process. This is evidenced by the facts that uncatalytic The reaction activation energy is altered The intermediates formed are different from catalytic energy those formed in non-catalytic reaction reactant The rates of reactions are altered product (both desired and undesired ones) reaction process Reactions proceed under less demanding conditions Allow reactions occur under a milder conditions, e.g. at lower temperatures for those heat sensitive materials 11  The action of a catalyst Arrhenius expression for reaction rate:  − Ea  Rate = A exp   kT  12  The properties of catalysts The properties of catalysts a) Only a small amount of catalyst is needed to alter the rate of reaction indefinitely. b) Catalysts are usually specific. This means that most catalysts can only be used for a specific reaction. If they were tried on an another reaction, other than their specified one, they would have no effect. c) During a course of a reaction a catalyst frequently undergo a physical change. For example if Manganese (iv) Oxide is used as a catalyst its changes from Crystalline (like Crystal) to an Amorphous (without distinct shape) powder form. d) A catalyst does not effect the position of equilibrium in reversible reaction. It speeds up the backward reaction to the same extent of the forward reaction. 13