Thermodynamics Tells if a reaction will occur

Kinetics Tells how fast a reaction will occur

Reaction Rate speed of the reaction –found experimentally measure of change in concentration of reactant or product over time rate =  concentration  time

How do you measure rates? measure concentration of 1 or more reactants/products over time reactants disappear as products appearreactants disappear as products appear reaction rate = change in concentration of reactants & products over given amount time

Concentration of Reactants, Products Disappearance of reactants Appearance of products

How do reactions occur? Collision Theory “ Collision Theory ” –must have –must have effective collisions between reacting particles collision must be energetic collision must occur at an effective angle

Particle Diagram of Collision Activated complex or transition state. Reactants Products NO + O 3  NO 2 + O 2 Activated Complex is NOT in equation!

Reaction Rates depend on … frequency of collisions – how often occur And efficiency of collisions – percentage that are effective

Collision Theory molecules must collide in order to react –effective collisions lead to formation of products –ineffective collisions do not lead to products

Effective Collisions must: be energetic have favorable orientation

Effective vs. Ineffective Collision

most collisions are NOT effective!

Why Do Collisions Have to be Energetic?

Activation Energy & Reaction

Energy Diagram of a Reaction Reaction Pathway Enthalpy or Potential Energy Reactants Activated Complex Products

Activation Energy energy needed to initiate reaction energy needed to overcome reaction barrier (hill) difference between where reactants start & top of hill

Examples of Activation Energy using match to start a fire spark plug in car engine

Potential Energy Curve: Endothermic endothermic rxn: products have more PE than reactants start low, end high

Potential Energy Curve: Exothermic exothermic rxn: products have less PE than reactants start high, end low

label on both endo & exo PE curves PE diagram: must be able to label label on both endo & exo PE curves 1.PE reactants 2.PE products 3.PE activated complex 4.E a forward reaction 5.E a reverse reaction  H

Time What kind of reaction is represented? PE of reactants PE of activated complex PE of products E a for forward rxn E a for reverse rxn

 H of reaction

Time What kind of reaction is represented? PE of reactants PE of activated complex PE of products E a forward reverse

 H of reaction

Why does collision have to be energetic? KE of colliding reactant particles is used to overcome reaction barrier KE is transformed into PE

Factors that determine reaction rates 1) nature of reactants (ions vs molecules) 2) temperature 3) concentration 4) pressure (gases only) 5) surface area 6) presence of catalyst

Nature of the reactants: Ions vs Molecules? Type of particles: ions in solution react quickly covalently bonded molecules react slowly –takes time to break all those bonds! Phase of particles: 2 gas phase reactants react more quickly than 2 liquid reactants or 2 solid reactants

Temperature measure of average KE of molecules in system more oftenfaster molecules are moving, more often will collide more energeticfaster molecules are moving, more energetic the collisions

Increase in Temperature increases frequency of collisions increases percentage of collisions that lead to reaction

Concentration increase in concentration: – more particles per unit volume – more collisions in given amount time

Pressure only pertains to systems involving gases: – Δ pressure analogous to Δ concentration  pressure,  # particles per unit volume ↓ pressure, ↓ # particles per unit volume

Surface Area higher surface area –more particles exposed for reaction higher surface area means smaller particle size for heterogeneous reactions only

Vocabulary Interlude homogeneous reactionhomogeneous reaction: –all reactants in same phase heterogeneous reactionheterogeneous reaction: –reactants in different phases

Catalyst substance that increases rate of reaction without itself being consumed –does not participate in reaction –provides alternate rxn pathway with lower energy barrier

Reaction Mechanism series of steps that leads from reactants to products also referred to as transition state process of rxn during which: bonds break atoms rearrange & new bonds form

Elementary Steps each individual step in reaction mechanism lowest elementary step called rate-determining step rate-determining step –step that must get enough energy to occur or the whole reaction can’t occur