Ch. 16 – Reaction Energy and Reaction Kinetics I. Rates of Reaction Ch. 16 – Reaction Energy and Reaction Kinetics

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

Unsuccessful Collisions A. Collision Theory Particle Orientation Required Orientation Unsuccessful Collisions Successful Collision

B. 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 Activation Energy

B. Activation Energy Activation Energy: - depends on reactants - is always positive - low Ea = fast reaction rate - takes less energy for the reaction to start Ea

C. Factors Affecting Rxn Rate 1. Nature of Reactants - 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 – increase locations of collisions dissolving in water – particles can mix & collide freely so reactions happen rapidly

C. Factors Affecting Rxn Rate 3. Concentration - high conc = fast rxn rate - more opportunities for collisions because there are more particles in the same volume that can react There are fewer red particles in the same volume so there is a lower chance of a collision There are more red particles in the same volume so there is more chance of a collision so the reaction goes faster

C. Factors Affecting Rxn Rate 4. Temperature - high temp = fast rxn rate - high KE - ↑ temperature, ↑ energy - particles move faster - more opportunities for collision - easier to reach activation energy

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

Ch. 16 – Reaction Energy and Reaction Kinetics II. Energy Diagrams Ch. 16 – Reaction Energy and Reaction Kinetics

A. Terms and Symbols Reactants: chemicals to start in a reaction Products: chemicals formed during reaction Heat of reaction (DHrxn): difference in energy between reactants and products (units are Joules) Activation Energy (Ea): amount of energy required for the reaction to take place. The higher the Ea, the slower the reaction (units are Joules)

B. Energy Diagrams Show relationship between time and energy during the course of a chemical reaction - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Reactants DH Products Forward Rxn (exothermic) Reverse Rxn (endothermic) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Ea Course of Reaction (time) Energy (E) in kJ/mol

C. Endothermic Reaction Products have higher enthalpy than reactants The pink curve is the uncatalyzed reaction The blue curve shows what happens when a catalyst is present Energies and amounts of the products and reactants stays the same, and the DHrxn stays the same – catalyst just allows reaction to reach equilibrium faster Course of Reaction Energy A reaction in which heat is absorbed

D. Exothermic Reactions Reactions in which heat is released Products have lower enthalpy than reactants The blue curve shows un-catalyzed reaction, red curve shows what happens when a catalyst is present Only change is the amount of time for the reaction to reach equilibrium Referred to as “spontaneous” because they can proceed to products without outside intervention Course of Reaction Energy