Unit 12: Reaction Rates & Equilibrium Chemistry Unit 12: Reaction Rates & Equilibrium Lecture #1

Objectives Define Reaction Rates Describe the collision theory Interpret Reaction Profile (Coordinate) Graphs List the factors that affect reaction rates and explain HOW they do so.

Big Idea Not all reactions happen at the same rate! Some are really fast… Explosion of dynamite (completely finished in a fraction of a second!!) Some are really slow… Oxidation of copper (patina) (not completely reacted for many years!!)

Reaction Rate Two Ways to Define: The rate at which the concentration of the reactants decreases (disappear) The rate at which the concentration of the products increases (appears)

Controlling Reaction Rates Often chemists try to control the rate of a chemical reaction! Sometimes we want to speed it up… Like a paint formula that will dry quickly Sometimes we want to slow it down… Like a paint formula that will resist fading

Collision Theory Molecules must COLLIDE to react In effective collisions existing bonds are broken as new bonds are formed, thus transforming reactants into products.

Requirements for Effective Collisions Must have frequent collisions Collisions must have sufficient energy Collisions must have proper orientation (geometry)

Behind-the-Scenes of a Chem. Rxn. For a chemical reaction to occur, the reactants must collide with sufficient energy to react. This energy is required to achieve the transition state (activated complex) required to form the products (b) Without sufficient energy, the reaction does not occur (a)

Reaction Profiles Shows the energy of the reactants and the products during a chemical reaction. The highest point is the transition state (activated complex) Activation Energy Heat of Reaction (ΔH or q)

Endothermic Reactions An endothermic reaction absorbs heat as the reaction proceeds: 2H2O + heat  O2(g) + 2H2(g) For endothermic reactions, the energy of the reactants is lower than the energy of the products

Exothermic Reactions An exothermic reaction releases heat as the reaction proceeds: NO2 + CO  NO + CO2 + heat For exothermic reactions, the energy of the reactants is higher than the energy of the products

Catalysts A catalyst is a substance that allows a reaction to go faster by lowering the activation energy. It does this by fining an alternate pathway for the reaction to occur. Catalysts sometimes also help with orientation Do not change ΔH Speed up both fwd. and reverse rxns. An inhibitor is the opposite of a catalyst…it slows down a chemical reaction

Factors That Affect Reaction Rate Temperature: An increase in temp. means faster moving particles. This means more collisions, AND collisions with greater energy.  FASTER Rxn. Concentration: An increase in concentration means more particles. This means more collisions  FASTER Rxn Pressure: If reactants OR products are gases, then increasing the pressure crowds the particles. This means more collisions.  FASTER Rxn. Surface Area: Rxns only happen on the surface. More surface area  FASTER Rxn. Catalysts: Aid in orientation, or provide alternate pathways.  FASTER Rxn.

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