CHEM Chemical Kinetics

Transcription

1 Chemical Kinetics Catalysts

2 A catalyst is a substance that increases the rate of the reaction but is neither created nor destroyed in the process. Catalysts can be divided into two broad categories. Homogeneous catalysts are those that are in the same phase as the reacting substances. Heterogeneous catalysts are in a different phase from the reacting species. 2

3 CF 2 Cl 2 (aka R-12, CFC-12, Freon 12) Example: The catalytic ti destruction ti of ozone in the stratosphere t involves chlorine as a catalyst. (homogeneous catalysis) UV radiation breaks off a chlorine atom from a CFC molecule. l Step 1: CF 2 Cl 2 + UV CF 2 Cl + Cl The chlorine atom attacks an ozone molecule (O 3 ), breaking it apart and destroying the ozone. Step 2: Cl + O 3 ClO + O 2 ClO is attacked by an ozone molecule, l The result is an oxygen molecule (O 2 ) and a chlorine monoxide molecule (ClO). Step 3: ClO + O 3 Cl + 2 O 2 Chlorine atom is released and an oxygen molecule (O 2 ) is formed. 3

4 Example: The catalytic ti destruction ti of ozone in the stratosphere t involves chlorine as a catalyst. (homogeneous catalysis) Step 1: CF 2 Cl 2 + UV CF 2 Cl + Cl Step 2: Cl +O 3 ClO +O 2 Step 3: ClO + O 3 Cl + 2 O 2 Overall reaction: 2 O 3 (g) 3 O 2 (g) Note: The chlorine atoms serve as a catalyst for ozone decomposition because They are not part of the reaction stoichiometry t They are not consumed by the reaction They increase the rate of the reaction One chlorine atom may cycle through this mechanism to destroy 100,000 ozone molecules. 4

5 Example: The reaction between persulphate ions and iodide ions (homogeneous catalysis) S 2 O 8 2 (aq) + 2 I (aq) 2 SO 4 2 (aq) + I 2 (aq) Slow reaction Powerful oxidizing agent Easily oxidized The reaction needs a collision between two negative ions. Repulsion is going to get seriously in the way of that! The catalysed reaction avoids that problem completely! Catalysts Cu (II) Experiment 5 Fe (II) or Fe (III) ions which are added to the same solution. 5

6 Example: The reaction between persulphate ions and iodide ions (homogeneous catalysis) Fe 2+ catalyst: S 2 O 2 8 (aq) + 2 Fe 2+ (aq) 2 SO 2 4 (aq) + 2 Fe 3+ (aq) 2 Fe 3+ (aq) + 2 I (aq) 2 Fe 2+ (aq) + I 2 (aq) Cu 2+ catalyst: 2 Cu 2+ (aq) + 2 I (aq) 2 Cu + (aq) + I 2 (aq) S 2 O 2 8 (aq) + 2 Cu + (aq) 2 SO 2 4 (aq) + 2 Cu 2+ (aq) Both of these individual stages in the overall reaction involve collision between positive and negative ions. This will be much more likely to be successful than collision between two negative ions in the uncatalyzed reaction. 6

7 Hydrogenation of ethene (heterogeneous catalysis) CH 2 CH 2(g) H 2(g) C H Ni 26(g) The four pictures show the effect of a catalyst on hydrogenation of ethylene (or any C=C compounds). Metal atoms attract The free radicals move The ethane, CH 3 CH 3 is not as electrons away form the H 2 molecules. The bonds break. along the surface of the metal until they collide with an ethene molecule. "electron rich" as the ethylene, and breaks away from the catalyst. 7

8 Schematic diagram of how catalytic converters in automobiles Remove NO from exhaust gas. (heterogeneous catalysis) Pt 2molecule Pt2NO 2/ Pd / Rh NO(g) N 2(g) O (g) Nitrogen atom collides with the Pt catalyst. NO Oxygen atom NO molecule adsorbs on the Pt surface. NO molecule dissociates and bonds with the Pt atom. Platinum surface (Pt, Pd, Rh) N 2 O 2 Pairs of N atoms react. Pairs of O atoms react. N 2 and O 2 desorb from the Pt surface, leaving more surface to catalyze further reaction. 8

9 The uncatalyzed reaction must overcome a relatively high activation barrier. Catalysts increase the rate of reaction by providing an alterative reaction pathway with lower activation energy. Catalysis leads to a new reaction pathway that has a relatively lower activation barrier. 9

10 Catalyzed reaction E a = 25 kj/mole 2505Example: Reaction at 293K E a RT e e. x Uncatalyzed reaction E a = 50 kj/mole e. x Ea RT. e 5 _ 3.47x10 0 Catalyzed reaction speed increases. 1.21x x 10

11 Uncatalyzed Reaction: OCl - + I - OI - + Cl - High E a ( reaction involves two negative ions) Requires high energy input 11

12 OCl - + I - OI - + Cl - Catalyzed Reaction: Water acts as a catalyst for this reaction. It is not consumed in the reaction. It provides an alternate mechanism with a lowering of E a. ClOH, OH - and IOH are intermediate species Forms intermediates Forms intermediate Forms the product, Cl - Forms the product, IO - 12

13 OCl - + I - OI - + Cl - Catalyzed Reaction: E a3 E a2 E a1 13

14 OCl - + I - OI - + Cl - Compare the Catalyzed and Uncatalyzed Reaction: E uncatalyzed E a3 E a1 E a2 14

15 Important in billion dollar industries such as petroleum refining Research of materials and design process for better catalysts Selective so that it speeds up one reaction and not all the reactions Historically, selection of catalysts is by trial and errors Modern days, catalysts are better understood by: 1. Reaction mechanisms 2. Molecular structure 3. Material properties The importance of Chemistry!! 15