Collision Theory slide 1 Collision Theory is an explanation of the mechanism by which reactants change to products Chemical Reaction: the intramolecular forces of atoms of reactants break apart and rearrange into products 2 Requirements for a reaction to occur: Collide with sufficient force to overcome Activation Energy, or Ea Collide with proper orientation

Collision Theory slide 2 “Effective Collision” a collision which results in product formation Must possess BOTH sufficient force and proper orientation “Ineffective Collision” a collision which does not result in product formation Lacks EITHER or BOTH sufficient force or proper orientation

Reaction Rate slide 3 The Rate of a Reaction is the speed in which a reaction occurs (i.e. the speed in which a reactant becomes product) Rate can be altered by manipulation of the two requirements needed for an effective collision Force: change kinetic energy of moving particles Orientation: we cannot physically re-orient particles can manipulate the frequency of collisions and, therefore, increase the chances for more collisions with proper orientation

Factors Affecting Reaction Rate slide 4 4 Factors can be used to affect reaction rate You are responsible for WHAT is meant by the factor Describing HOW the factor affects particles and WHY this factor can alter the rate of a reaction Which of the 2 requirements for an effective collision are being manipulated and how so 4 factors affecting reaction rate: Temperature Concentration & Pressure Nature of Reactants (phase & size) Catalysts

Factors Affecting Reaction Rate slide 5 Temperature: WHAT: Measure of average KE of particles HOW/WHY: Increasing KE increases rate (direct) Particles are moving more quickly and will collide with greater force as well as more often Manipulated Requirement(s) and HOW: Force—changing speed Orientation—increasing frequency of collision

Factors Affecting Reaction Rate slide 6 Concentration & Pressure: WHAT: Concentration is a measure of number of particles in a given volume Pressure (by changing size of volume) affects density of particles HOW/WHY: Increasing concentration and/or pressure increases rate (direct) Increasing concentration results in more particles in a given volume, so more collisions Increasing pressure results in same number of particles in a smaller volume, so more collisions Manipulated Requirement(s) and HOW: Orientation—increasing frequency of collision

Factors Affecting Reaction Rate slide 7 Nature of Reactants: Generic term referring to phase of reactants and exposed surface area PHASE of reactants: WHAT: Reactants can be in the solid, liquid (or aqueous), or gas phase, each differing in amount of KE, order, and density HOW/WHY: Amount of KE is lowest in solids and highest in gases, therefore particles in the gas phase move with greater speed Space between particles is greatest in the gas phase, allowing for freedom of motion and increased mixing of different reactants Manipulated Requirement(s) and HOW: Force—more KE (gas phase) results in more force Orientation—increasing frequency of collision

Factors Affecting Reaction Rate slide 8 Nature of Reactants: Generic term referring to phase of reactants and exposed surface area Surface Area: WHAT: Surface area (or “particle size”) refers to the amount of reactant exposed to the other reactant(s) HOW/WHY: By increasing surface area, the rate increases (direct) Surface area can be increased by separating a “clump” of reactant so more particles of each reactant can mix Manipulated Requirement(s) and HOW: Orientation—increasing frequency of collision

Factors Affecting Reaction Rate slide 9 Catalysts WHAT: Substance that do NOT act as reactants or products—a catalyst is unchanged after the reaction HOW/WHY: Catalysts reduce the amount of energy needed for an effective collision (reduce the required activation energy) Manipulated Requirement(s) and HOW: Force—the amount of required force is reduced

Unit 8 Set 5 slide 10 1.Does every collision result in a chemical reaction? What is an effective collision? What are the 2 requirements? 2.If one were to increase the frequency of collisions, which of the two requirements is being manipulated and why? 3.Solid iron (Fe) can be used as a catalyst to create ammonia, NH 3, from N 2 (g) and H 2 (g). Will the iron used in this reaction be available for future reactions? Explain. 4.After being snapped, one glow stick is put in the freezer and another in a pre-heated oven. a.Which will produce light most quickly? b.Which will stop producing light most quickly? c.Which factor is being altered in this scenario? d.Describe what is happening on a molecular level to the reactants of each of the two glow sticks e.How does changing this factor affect the requirements for an effective collision?