10.01 Kinetics. Dr. Fred Omega Garces. What determines the speed of a reaction? Chemistry 100. Miramar College. 1 Kinetics and Equilibrium

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1 10.01 Kinetics What determines the speed of a reaction? Dr. Fred Omega Garces Chemistry 100 Miramar College 1 Kinetics and Equilibrium

2 Kinetics and Equilibrium Kinetics is a concept that address, how fast will the reaction occur Equilibrium address, to what extent will the reaction proceed. The rate of iron rusting depends on reaction conditions. 4 Fe (s) + 3 O 2(g) + xh 2 O (g) D 2Fe 2 O 3 XH 2 O (s) Whether it occurs overnight or over many years, the reaction conditions influence how fast it occurs. Conversion of graphite to diamonds is thermodynamically favorable (DG - ). C (graphite) D C (diamond) Kinetics makes this reaction nearly impossible. 2 Kinetics and Equilibrium

3 Reaction Favorable What influence the speed of a chem reaction? Chemical changes occurs when: Reactant bonds are broken Product bonds are formed For a reaction or a chemical change to take place, reactant bonds must be broken (require energy, endothermic) and product bonds must form (produce energy exothermic). Any factors which favors bond breakage and bond formation, favors the rate (kinetics) of the reaction. 3 Kinetics and Equilibrium

4 Collision Theory Theory which explains how molecules come together before product is formed. Deciding Factor on the speed of a reaction- Frequency Factor How often they collide Orientation (steric) alignment How correct they collide Impact of Collision How hard they collide 4 Kinetics and Equilibrium

5 Collision Frequency Rate of a reaction depends on the number of encounter the reactants have to form product. One particle system 1 type collision A A Two particle system 4 type collision A A B A B B The greater the amount of reactant, the greater the number of encounters, the more likely a reaction takes place. 5 Kinetics and Equilibrium

6 Collision Orientation Rates of reaction depends on proper alignment of chemical species upon collision for product formation. With proper alignment Product formation 6 Kinetics and Equilibrium

7 Collision Impact For reaction to take place, species must collide with enough energy so that reactant bonds breaks. The faster the reactant energy (kinetic energy) the harder the collision. Two car collision: Minor 5 mph, major damage if speed > 50 mph. correct orientation but not enough energy to break bonds correct orientation and enough energy to break bonds Speed of reactants must be high enough so that bonds can be broken 7 Kinetics and Equilibrium

8 Energy of Reaction The energy of a reaction depends on the energies of the reactant relative that of the product. If the reaction is downhill (a) then consider energy as a product (Exothermic Reaction). If the reaction is uphill (b), then consider energy as a reactant (Endothermic) (a) Heat product (-)Exothermic Reactant Product + E (b) Heat reactant (+)Endothermic Reactant + E Product 8 Kinetics and Equilibrium

9 Activation barrier E act - Minimum energy a reactant must possess in order to convert to products. The activation barrier (E act ) can determine how fast a reaction occurs. In general, the higher the activation barrier, the slower the reaction rate. The lower the Activation barrier, the faster the reaction. Consider the process of someone trying to roll a boulder over a hill. The E act (forward) higher the hill, the slower the task. The lower the hill the faster the process. The height of the hill correspond to the energy of activation (E act ). 9 Kinetics and Equilibrium

10 Reaction (Profile) Coordinate Diagram What are the events at the molecular level when a reaction takes place? And what are the energies involved? Conversion of methyl isonitrile, CH 3 NC to acetonitrile, CH 3 CN 10 Kinetics and Equilibrium

11 Energy (Reaction) Coordinate Diagram An Exothermic reaction between A & B to produce A-B A + B (Reactant) g A-B (Product) 50kJ A B In this reaction: E react > E product Exothermic reaction 20kJ A+B 10 kj A B E transition St. = E reactant = E product = E reaction = E prod - E react = E f act = E r act = 11 Kinetics and Equilibrium

12 Energy (Reaction) Coordinate Diagram An Exothermic reaction between A & B to produce A-B A + B (Reactant) g A-B (Product) 50kJ A B In this reaction: E reactant > E product Exothermic reaction 20kJ A+B 10 kj A B E transition St. = 50 kj E reactant = 20 kj E product = 10 kj E reaction = E prod - E react = -10 kj E f act = 30 kj E r act = 40 kj 12 Kinetics and Equilibrium

13 Features of Reaction Coordinate Diagram An Endothermic reaction between A & B to produce A-B A + B (Reactant) g A-B (Product) 45kJ A B In this reaction: E react < E product Endothermic reaction A B E transition St. = E reactant = E product = 25kJ A+B E reaction = E prod -E react = 20kJ E f act = E r act = 13 Kinetics and Equilibrium

14 Features of Reaction Coordinate Diagram An Endothermic reaction between A & B to produce A-B A + B (Reactant) g A-B (Product) 45kJ A B In this reaction: E reactant < E product Endothermic reaction 25kJ 20kJ A+B A B E transition St. = 45 kj E reactant = 20 kj E product = 25 kj E reaction = E prod -E react = +5 kj E f act = 25 kj E r act = 20 kj 14 Kinetics and Equilibrium

15 Catalyst Affect Catalyst lowers the activation energy so that minimum energy needed for the reaction to take place is lowered. Consider the task of moving coal over a barrier. A pathway with a lower barrier is analogous to a reaction affected by a catalyst. The task becomes easier for a pathway with a lower barrier. 15 Kinetics and Equilibrium

16 Catalytic Converter A catalytic converter works by taking exhaust gases from the engine, including CO and NO, passing them through the catalytic converter, where they are converted to harmless CO 2 and N 2 by catalyzed reactions. 16 Kinetics and Equilibrium

17 Summary: Reaction Conditions Affecting Rates Conditions to Increase Rate: Nature of Reactants - Different substances will have different reaction rates. Concentration of Reactant- More reactants result in more collision yielding more products. Surface area- More surface area, the greater the chance for reactants to encounter to form product. Temperature- Higher temperature result in more energetic collisions. Catalyst- Lowers the activation energy for the reaction. 17 Kinetics and Equilibrium