Catalysis

1. Catalysis • Reaction Rate can be increased by • increasing the concentration of reactants • increasing the surface area of reactants • increasing the temperature

2. Catalysis • Sometimes increasing the temperature to increase the reaction rate is not feasible • in biological systems, high temperatures can destroy living cells • in a factory, high temperatures require a lot of energy, often at high cost

3. Catalysis • Enzymes • increase the rates of reactions in living cells • almost every biologically important reaction is assisted by a specific enzyme

4. Catalysis • Catalyst • a substance that speeds up a reaction without being consumed • an enzyme is a biological catalyst • catalysts are used in industry to speed up reactions

5. Catalysis • How does a catalyst work? • Lowers the activation energy of a reaction by • taking the reaction through a different pathway (requiring lower energy) • acting as a surface to “hold” a reactant, to weaken the bonds in the reactant • Why does lowering the activation energy increase the reaction rate?

6. Catalysis • Energy diagram for catalyzed vs. uncatalyzed reaction:

7. Catalysis • Energy Distribution Curve: • So the reaction rate increases because with a lower activation energy, the fraction of molecules (collisions) that have the energy needed to react is greater.

8. Catalysis • Heterogeneous Catalyst • a catalyst that exists in a different phase (usually a solid) from the reactants • often involves gaseous reactants being adsorbed (collected) onto the surface of a solid catalyst • ex: the catalytic hydrogenation of unsaturated hydrocarbons

9. Catalysis • C2H4(g) + H2(g) ---> C2H6 • normally slow at room temperature • use platinum or palladium as a solid catalyst • H2 and C2H4 are adsorbed on the catalyst surface • the catalyst allows the metal and hydrogen to interact, weakening the H - H single bond

10. Catalysis • Other examples of heterogeneous catalysts: • SO2 ---> SO3 • ...catalyzed by dust and water droplets • environmentally bad, SO2 is created when sulfur containing fuels are burned, SO3 will react with water to form acid rain • catalytic converters in automobile exhaust engines • converts CO ---> CO2 • hydrocarbons ---> CO2 + H2O • nitric oxide ---> N2 • “poisoned” by lead...hence the need for lead free gasoline

11. Catalysis • Homogeneous Catalyst • exists in the same phase as the reactants • often reacts with a reactant, but will be reformed in the course of the reaction. • Ex: Catalytic behavior of NO (nitric oxide) and ozone • NO is a pollutant made by automobiles • NO catalyzes ozone production in the troposphere (the layer closest to earth), but catalyzes ozone decomposition in the stratosphere (a layer where ozone is needed for protection against uv).

12. Catalysis • In the troposphere: • 2NO + O2 --> 2NO2 • 2NO2 --light--> 2NO + 2O • 2O + 2O2 ----> 2O3 (ozone) • Net equation: 3O2 --> 2 O3 • NO does not appear in the overall reaction...it is the catalyst...a substance that reacts, but is reformed • Ozone - an oxidizing agent, toxic, and reacts with other air pollutants to form substances irritating to the eyes and lungs

13. Catalysis • In the stratosphere: • NO + O3 ---> NO2 + O2 • O + NO2 ---> NO + O2 • Net Equation: O + O3 ---> 2 O2 • So NO again takes part in the reaction, but gets reformed to do its dirty work over and over. • Ozone disappears where its needed, resulting in uv being passing through

14. Catalysis • More threats to the ozone layer • Freon breaks down in light to make Cl, which catalytically breaks down ozone • (CCl2F2 --light--> CClF2 + Cl) • Cl + O3 ----> ClO + O2 • O + ClO ---> Cl + O2 • Net equation: O + O3 ---> 2O2

15. Catalysis • Inhibitors • results in a decrease in the reaction rate by • preventing reactants from colliding and reacting • e.g. Paint • reacts preferentially with a reactant • e.g. Anti-oxidants and O2 in potato chips (to prevent the fats from becoming rancid) • knocks out a catalyst • e.g. Lead knocks out the catalyst in a catalytic converter