I. Rates of Reaction

1. I. Rates of Reaction Ch. 17 – Reaction Energy and Reaction Kinetics

2. Collision Theory Reaction rate depends on the collisions between reacting particles. The particles collide and make new substances. Successful collisions occur if the particles... collide with each other have the correct orientation have enough kinetic energy to break bonds

3. Collision Theory Particle Orientation

4. Activation Energy Activation Energy (Ea) minimum energy required for a reaction to occur Activated Complex: the transitional structure in a collision that exists while old bonds are breaking and new bonds are being formed.

5. Activation Energy Cont… Activation Energy: - depends on reactants - is always positive - low Ea = fast reaction rate - takes less energy for the reaction to start. Reaction Rate: the change in concentration of reactants per unit time as a reaction proceeds.

6. Factors Affecting Rxn Rate 1. Nature of Reactants - substances vary greatly in their tendencies to react. - bonds are broken and other bonds are formed in reactions. - the rate of reaction depends on the particular reactants and the bonds involved. 2. Surface Area high SA = fast rxn rate more opportunities for collisions Increase surface area by… using smaller particles – if we make the pieces of the reactants smaller, we increase the number of particles on the surface which can react. dissolving in water – gases & dissolved particles can mix & collide freely. Reactions happen rapidly.

7. Factors Affecting Rxn Rate Cont. 3. Concentration - high conc = fast rxn rate - more opportunities for collisions because there are more particles in the same volume that can react.

8. Factors Affecting Rxn Rate Cont. 4. Temperature - high temp. = fast rxn. rate - high KE - when we increase the temperature, we give the particles energy - this makes the particles move faster - so there are more opportunities for collision - it is easier to reach activation energy

9. Factors Affecting Rxn Rate Cont. 5. Catalyst substance that increases rxn rate without being consumed in the rxn lowers the activation energy

10. II. Energy Diagrams Ch. 17 – Reaction Energy and Reaction Kinetics

11. Terms and Symbols Reactants: the chemicals you start with in a reaction. Products: the chemicals formed during the reaction. Heat of reaction (DE or DHrxn): the difference in energy between reagents and products (units are Joules). Transition State: the highest point on the energy diagram, representing the point at which the reaction is half-completed. Activation Energy (Ea): the amount of energy required for the reaction to take place. The higher the Ea, the slower the reaction (units are Joules).

12. Energy Diagrams

13. Endothermic Reaction A reaction in which heat is absorbed Products have higher potential energy than the reactants. The pink curve shows the uncatalyzed reaction. The blue curve shows what happens when a catalyst is present. The energies and amounts of the products and reactants stays the same, and the DE stays the same. The catalyst just allows the reaction to reach equilibrium faster.

14. Exothermic Reactions Reactions in which heat is released. Products have lower potential energy than the reactants. The blue curve shows the uncatalyzed reaction. The red curve shows what happens when a catalyst is present. Again, nothing changes but the amount of time it takes for the reaction to reach equilibrium. Exothermic rxns are referred to as “spontaneous” because they can proceed to products without outside intervention.

15. Formulas * DEforward = Eproducts – Ereactants * DEreverse = Ereactants – * Ea = energy of activated complex – energy of reactants * Ea’ = energy of activated complex – energy of products

16. Practice #1 For the energy diagram provided, label the reactants, products, DE, Ea, and Ea’. Also, determine the values of DE for the forward and reverse reactions, and the values of Ea and Ea’.

17. Practice #1 Continued * DEforward = Eproducts – Ereactants = 55 kJ/mol – (-20 kJ/mol) = 75 kJ/mol * DEreverse = Ereactants – Eproducts = -20 kJ/mol – 55 kJ/mol = -75 kJ/mol

18. Practice #1 Continued * Ea = energy of activated complex – energy of reactants = 80 kJ/mol – (-20 kJ/mol) = 100 kJ/mol * Ea’ = energy of activated complex – energy of products = 80 kJ/mol – 55 kJ/mol = 25 kJ/mol

19. Practice #2 Draw and label an energy diagram that depicts the following reaction. Place the reactants at energy level of 0 kJ/mol. DEforward = -10 kJ/mol and Ea’ = 40 kj/mol DEforward = Eproducts – Ereactants -10 kJ/mol = Eproducts – 0 kJ/mol = -10 kJ/mol Ea’ = activated complex – Eproducts 40 kJ/mol = a.c – (-10 kJ/mol) = 20 kJ/mol