I I I o o o o Definition of Catalyst A substance that alters the reaction rate of a particular chemical reaction chemically unchanged at the end of the reaction 2 classes : I) positive catalyst increase the rate Il) negative catalyst (inhibitor) ==> decrease the rate How to change the rate of reaction???
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Catalysis By providing an alternative pathway (or mechanism) with lower/ higher activation energy, I
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Characteristics of catalysts I I o o o I The catalyst remains unchanged (in mass and chemical composition ) in the reaction (Activity of catalyst.) A small quantity of the catalyst is required. e.g. One mole of colloid Pt catalyses The catalyst does not change the equilibrium constant. But the equilibrium approaches earlier.
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I I I Specificity of catalyst The catalyst is specific in nature. It means o by the change of catalyst, nature of the products changes or specific catalyst for a specific reaction. co + co + 1120 CO + I-ICHO CO + 2112
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Characteristics of catalysts I I o o I The catalyst can not make impossible reaction to occur and does not intiate a reaction. Catalyst Poison: There are certain substances which decrease or destroy the activity of the catalyst. Such substances are known as catalytic poisons. E.g. arsenic destroys the catalytic activity of the platinum catalyst in the manufacture of sulphuric acid.
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Characteristics of catalysts I I I o o Catalyst Promoter: There are certain substances which increase the activity of the catalyst. Such substances are known as catalyst promoters e.g. MO acts as a promoter in the manufacture of ammonia by Haber's process. 2NH3 Ex. 2. In Bosch process of preparation of acts as a promoter for catalyst . Catalyst Poison or Promoter does not act like a catalyst.
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Characteristics of catalysts The catalyst exhibits maximum activity at a o particular temperature which is known as optimum temperature. I I I
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Catalysis o For example, Ea for the pathway with catalyst < Ea for the pathway without catalyst I I I potential energy E for act u nca talysed pathway activated complex or transitiion state activated complex or transition state A - catalyst E for act catalysed pathway products catalyst reactants + catalyst reaction pathway
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I I I Catalysis The reaction can then be speeded up by increasing the fraction of molecules that have energies in excess of the Ea for a reaction. Kinetic energy Ea2 Eal
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Theory of catalysis (1) Intermediate compound theory of o catalysis adsorption theory of catalysis o I I I
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I I Intermediate compound theory of catalysis According to this theory, the catalyst reacts o with one of the reactants to give an intermediate, which reacts with another reactant to yield products and the catalyst as follows: A + [ Catalyst] [Intermediate] [Intermediate] + B Product + [Catalyst] Examples: NO I 2 S02 + 02 Proceeds as: 2NO + 02 N02 + S02 2 S03 2N02 S03 + No
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I adsorption theory of catalysis The heterogeneous catalysis e.g. gaseous o reaction on a solid surface, is explained by this theory as follows: Catalyst
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Adsorption theory I I o o o o o I Following four steps are involved in the heterogeneous catalysis: (i)Diffusion of reactants at the surface of the catalyst. (ii)Adsorption of reactants at the surface. (iii)Reaction of reactants at the surface. (iv)Desorption of products from the surface.
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Type of catalysis Homogenous catalysis o Heterogeneous catalysis Positive catalysis o Negative catalysis o Acid Catalysis o Base catalysis o Autocatalysis o Enzyme catalysis o
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I I I Types of Catalyst 1, Heterogeneous Catalyst catalyst with different phase as the reactant usually solid state e.g. decomposition of H202 with Mn02 as catalyst e.g. hydrogenation of ethene (Ni as catalyst) c=c
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Heterogeneous Catalyst provides an active reaction surface for reactant reaction occurs with a lower Ea are usually transition metal such as m, Pd, V205 and Ni I I I
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2, Homogeneous Catalyst catalyst with the same phase as the reactant usually in aqueous state e.g. Oxidation of I- ion by S2032- with Fe3+ ion as catalyst I + S2082- 12 + 2S042- 21- + 2Fe3+ 2Fe2+ + 12 2Fe2+ 2 8 - 2Fe3+ + 2S0 2- I I
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I 3, Autocatalysis the product in the reaction be the catalyst of the reaction this product is called autocatalyst 2Mn04- + 16H+ + 5C2042- 2Mn2+ + 8H20 + IOC02 I [Mn04 - (aq)] I o 20 40 —d (Mn04 -l rate = dt time(s) 60 80 100 120 140 o 20 40 time 60 80 100 120 140
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Positive Catalysis I I o o I The catalyst which increases the rate of a chemical reaction is called positive catalyst and the phenomenon is known as positive catalysis Examples are Mn02 (j) 2 KC103 + 30 2 (ii) H 202 1—120 + [O]
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Negative catalysis I I I o o The catalyst which decreases the rate of reaction is called negative catalyst and phenomenon is called negative catalysis Examples are Acetanilide (j) H202 H20 + O (ii) Knocking of petrol by tetraethyl lead
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Acid catalvsis mechanism for the acid-catalyzed hydrolysis of ап ester он снэ — —он с он снз—с—он slow + H2Q: sIow снзон он + ОН он он :бн он
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I I Base catalysis A base catalyst increases the rate of the reaction by removing a proton from the reaction specific-base catalyzed dehydration G.öH CICH2CCH2C1 OH a hydrate CICH2CCH2C1 + H 20 slovv CICH2CCH2C1 + -OH I
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Enzyme catalysis I I o o Enzymes are Biological catalysts Enzymes control chemical reactions that take place in the cytoplasm, Catalase in an example of an enzyme made by living cells I
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I I I I Catalase The enzyme catalase breaks down the waste substance hydrogen peroxide into water and oxygen. H202 Reactant Catalase 02 Enzyme + H20 product
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Properties of enzymes I I I o o o o o o Speed up reactions, Made of protein, Are specific Not used up during the reaction Require optimum conditions at which they work best At high temperature they become denatured
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Mechanism and kinetics of enzyme catalysed reactions
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The Michaelis-Menten Equation [ESI k -1 Assumption: k 1 k2 i.e. the equilibrium of [E], [S] and [ES] is not I I I affected by 1
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The Michaelis-Menten approach k LES] (1) total Solving equation (2) for [E] and substituting [E] in equation (1): I k -1 total We also know that the velocity of the reaction equals: v = k2 LES] Solving equation (3) and (4) for [ES] and then substituting [ES] in I equation (3) with [ES] = v / k2 then yields: (3) (4) I
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total k -1 total k -1 I I I We define k 1/ kl as KM, the Michaelis-Menten constant and the maximal velocity as v total This simplifies the above equation to: max if [S] KM then v = if [S] = KM then v = max max 2 Therefore KM can be viewed as the substrate concentration with half- maximal velocity (dimension M, typically mM to nM)
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Michaelis-Menten plot Linear plot of substrate concentration versus velocity yields a hyperbolic relationship: 1st order zero order I I max I max 2 I
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I ndustrial Application of Catalysts I A) Usage of Catalysts in Chemical Industries O Cost is always the greatest concerns of manufacturers O How can we get the highest yield of product? High temperature High pressure High Concentration
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Industrial Application of Catalysts Haber Process 3H2 + 2NH3 (Fe) o Contact Process 2S02 + 02 2S03 (Pt/V205) o Hydrogenation of C=C (hardening of oil - vegetable oil to margarine) CH2CH2 + 1-12 CH3CH3 (Ni/Pd/Pt) I I
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The Haber Process An essential industrial process
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The Haber Process This reaction makes ammonia out of hydrogen and nitrogen. The nitrogen comes from the air (78% N). You don't need to worry about where the hydrogen comes from!
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The Haber Process The Haber process is a REVERSIBLE reaction o I N + 3H 2NH (+ heat) 3(g) I nitrogen + hydrogen —v ammonia A reversible reaction is one where the products of the reaction can themselves I react to produce the original reactants.
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The Haber Process I I I Catalyst Reactor (400 oc, 200 atm) N2(g) + 3 H2(g) Cornpressor Unreacted N2 and H2 Condenser NH3(1) S torage
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The Haber Process I I o o You need to LEARN the industrial conditions this reaction occurs in off by heart — this is a favourite exam I question!! ! Industrial conditions: PRESSURE: TEMPERATURE: CATALYST: 200 atmospheres 4500c Iron
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The Haber Key facts I I 1. 2. 3. 4. H and N are mixed in a 3:1 ratio Because the reaction is reversable not all the nitrogen and hydrogen will convert to ammonia. The ammonia forms as a gas but cools and liquefies in the condenser The H and N which do not react are passed through the system again so they are not wasted. I
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Manufacture of H2S04 two basic methods: lead chamber process contact process
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acid. I-Iso + so 4(l) 3(g) I 1. 2. I 1. I Steps of The Contact* process The combustion of sulfur makes sulfur dioxide S + 802 8so 2(g) The sulfur dioxide is converted into sulfur trioxide (the reversible reaction at the heart of the process); 2SO + 0 2so The sulfur trioxide is converted into concentrated sulfuric 2 2 7(1) H S 0 + HO -.........> 2H2SO 2 2 7(1) * Called "contact" since the molecules of the gases 02 and S02 are in contact with the surface of the solid catalyst, V 205
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Contact Process superheater I I drying all' furnace waler I absorber saa heat exchanger cata st converter diluter con: I-I,sa
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I I I Ostwald process The Ostwald process is a chemical process for making nitric acid (HN03). Wilhelm Ostwald developed the process. The Ostwald process is a mainstay of the modern chemical industry It provides the main raw material for the most common type of fertilizer production. Historically and practically, the Ostwald process is closely associated with the Haber which provides the requisite raw process material, ammonia (NH3).
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I I I Ostwald process Ammonia is converted to nitric acid in 2 stages. It is oxidized (in a sense "burnt") by heating with in the presence of a catalyst such oxygen platinum with 10% rhodium, to form nitric as oxide and water This step is strongly exothermic, making it a useful heat source once initiated: 4 NH3 (g) + 5 02 (g) 4 NO (g) + 6 H20 (g) (AH = -905.2 kJ)
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I I I Ostwald process Stage two encompasses two reactions and is carried out in an absorption apparatus containing water. Initially nitric oxide is oxidized again to yield nitrogen dioxide This gas is then readily absorbed by the water, yielding the desired product (nitric acid, albeit in a dilute form), while reducing a portion of it back to nitric oxide 2 NO (g) + 02 (g) 2 N02 (g) (AH = -114 kj/mol) 3 N02 (g) + H20 (l) 2 HN03 (aq) + NO (g) (AH = -117 kJ/m01)
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Ostwald process I I o o o The NO is recycled, and the acid is concentrated to the required strength by distillation Alternatively, if the last step is carried out in air: 4 N02 (g) + 02 (g) + 2 H20 (l) 4 HN03 (aq) I
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Ostwald process I I I o o o Typical conditions for the first stage, which contribute to an overall of about 98%, yield are: between 4 and pressure atmospheres (approx. 400-1010 kPa or 10 60-145 psig) and temperature is about 500 K (approx. 217 Cor 422.6
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I I I Hydrogenation of oils Hydrogenation means adding hydrogen to a substance. Liquid vegetable oils that are unsaturated will react with hydrogen at about 60 oc in the presence of a nickel catalyst This is an example of an addition reaction where hydrogen adds across the double bond leaving only single bonds. The picture below shows hydrogenation of a double bond
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I I I Hydrogenation of oils Hydrogenation raises the melting point above room temperature and makes the liquid oil become solid in a process called hardening. I HYDROGEN GAS -c—c- HEAT, NICKEL CATALYST Unsaturatod Saturatod Solid Fat Liquid Oil
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I I I Saturated Fats 'Contain no C=C double bonds 'Generally are solids or semisolids at room temp 'Animal fat is a major source 'Should not make up more than 30% of your total fat intake per day Unsaturated Fats 'Contain one or more C=C double bonds 'Generally are liquids at room temp 'Vegetable oils are major source
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Palmitic acid Stearic Acid Oleic Acid Linoleic Acid Linolenic Acid Arachidonic Acid I Saturated Fatty Acids COOH COOH Unsaturated Fatty Acids COOH COOH COOH COOH I NOTE: Linoleic, linolenic, and aracidonic acids are examples of polyunsaturated fatty acids. Linoleic and linolenic acids are also the two essential fatty acids your body needs.
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This is a chemical reaction that decreases the amount of unsaturation in a fat or oil by the addition of H2 in the presence of a metal catalyst. CH3 C=C + H2 CH3CH3 H—C—C—H
I I I Fermentation If no oxygen is available, cells can obtain energy through the process of anaerobic respiration. A common anaerobic process is fermentation. Fermentation is not an efficient process and results in the formation of far fewer ATP molecules than aerobic respiration. There are two primary fermentation processes: 1. Lactic Acid Fermentation 2. Alcohol Fermentation
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I I I Alcoholic fermentation Alcoholic fermentation, also referred to as o ethanol fermentation, is a biological process in which molecules such as glucose, fructose, and sucrose are converted into cellular energy and thereby produce ethanol and carbon dioxide as metabolic waste products
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