Unit 12 Reaction Rates 12 1 Reaction Rates

Unit 12: Reaction Rates

12. 1: Reaction Rates Learning Target: I can describe reaction rate in terms of changing concentration.

12. 1: Reaction Rates Reaction Time is defined as: Amount of time needed for the reaction to go from reactants to products. Measured in seconds or minutes.

12. 1: Reaction Rates Reaction Rate is defined as: Change in amount of reactants/products per amount of time Examples: ▪ Grams/second ▪ Moles/second ▪ Molarity/second

12. 1: Reaction Rates Example 1: If 10 grams of Mg react with HCl in 150 seconds… What is the reaction time? What is the reaction rate?

12. 1: Reaction Rates Example 2: If 5 moles of O 2 reacts with H 2 in 2 seconds… What is the reaction time? What is the reaction rate?

12. 1: Reaction Rates Evidence of Chemical Reactions Color change Change in temperature Production of a gas (BUBBLES!) Change in odor Formation of a solid (precipitate) Light given off

12. 2 Collision Theory Learning Targets: I can use collision theory to describe how chemical reactions take place. I can define collision energy, activation energy, activated complex and transition state.

12. 2 Collision Theory Review: Reactants are… ▪ Initial substances used up during a chemical reaction Products are… ▪ Substances formed after a chemical reaction In your notes, label the reactants and products in the chemical equation.

12. 2 Collision Theory states: A chemical reaction only occurs when the molecules collide with one another Collisions ▪ Molecules must collide with sufficient energy ▪ Molecules must have correct orientation

12. 2 Collision Theory Definitions you must know: Collision energy: molecules colliding with sufficient energy Activation energy: minimum energy required for a chemical reaction to proceed Transition energy: highest amount of energy during the reaction

12. 2 Collision Theory Molecules must have correct orientation! Must collide at the right spots.

12. 2 Collision Theory Activated Complex Unstable molecule that forms when reactant molecules collide ▪ Old bonds start to break ▪ New bonds start to form

12. 3 Factors Affecting Rate Learning Target: I can list and describe the factors that affect reaction rates.

12. 3 Factors Affecting Rate Nature of Reactants Different chemicals react at different rates ▪ Fluorine is more reactive than oxygen State of Reactants ▪ ▪ Gases (fastest) Aqueous Liquids Solids (slowest)

12. 3 Factors Affecting Rate Surface Area Larger surface area, faster reaction rate Collisions only occur on the surface

12. 3 Factors Affecting Rate Concentration of Reactants Increasing the concentration (M) of reactants increases reaction rate More molecules = more collisions Elephant Toothpaste!

12. 3 Factors Affecting Rate Temperature Increasing temperature increases rate of reaction More molecules have sufficient energy to react Higher temp molecules moving more collisions!

12. 3 Factors Affecting Rate Catalyst Substance that lowers the activation energy. Lowers the hurdle of energy, more collisions have the right amount of energy. Not used up in reaction!

12. 3 Factors Affecting Rate Inhibitor Substance that interferes with a reaction Interferes with collisions, slower reaction

12. 4 Reaction Coordinate Diagram Learning Targets: Interpret reaction coordinate diagrams in terms of energy. Determine whether a reaction is exothermic or endothermic.

12. 4 Reaction Coordinate Diagram Graph that shows changes in energy during a chemical reaction.

12. 4 Reaction Coordinate Diagram Enthalpy (H) Potential Energy (bonds) of molecules Both reactants and products have enthalpy Δ H = Hproducts – Hreactants Ea = Hactivated complex – Hreactants

12. 4 Reaction Coordinate Diagram Exothermic Products lower in energy than reactants ΔH is negative (-) Energy is released

12. 4 Reaction Coordinate Diagram Endothermic Products higher in energy than reactants ΔH is positive (+) Energy is absorbed

12. 4 Reaction Coordinate Diagram Catalyst: Lowers potential energy of activated complex Only a catalyst changes Ea

12. 4 Reaction Coordinate Diagram H of reactants? 40 k. J H of activated complex? 100 k. J Ea of activated complex? 100 -40 = 60 k. J H of products 20 k. J

12. 5 Rate Laws & Mechanisms Learning Targets: Describe the mechanism for a chemical reaction. Use a balanced chemical equation to write the rate law for a reaction.

12. 5 Rate Laws & Mechanisms Reaction Mechanism: Steps involved in going from reactants to final products. Intermediate: Product formed at the end of a step and is a reactant in the next step

12. 5 Rate Laws & Mechanisms Reaction 1: Step 1: OCl-1 + H 2 O HOCl + OH-1 Step 2: HOCl + I-1 HOI + Cl-1 Step 3: HOI + OH-1 H 2 O + OI-1 What are the 3 intermediates? OH, HOCl and HOI

12. 5 Rate Laws & Mechanisms Rate-determining step Slowest step in the mechanism that controls speed of reaction

12. 5 Rate Laws & Mechanisms Reaction 2: Step 1: NO + F 2 NOF 2 Step 2: NOF 2 + NO 2 NOF slow fast What is the intermediate? NOF 2 What is the rate-determining step? Step 1 (slow)