Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

PART 1 RATES OF REACTION

Collision Theory Reactions occur at different rates Speed = Distance in a given amount of time –Shelia runs 10 meters in 4 seconds 10 m / 4s 5 m / 2s 2.5 m/s

Collision Theory Rate = a measure of the speed of any change that occurs within an interval of time. –Rate of a Chemical Change or reaction = amount of reactant changing per unit time

Collision Theory Collision Theory: atoms, ions and molecules can react to form products when they collide with one another, provided that the colliding particles have enough kinetic energy –Kinetic energy = the energy possessed by a body because of its motion

Collision Theory Particles lacking the necessary kinetic energy to react bounce apart unchanged with they collide

Collision Theory If enough energy is applied to a molecule, the bonds holding the molecule together can break! Substances supplied with enough energy decompose to simpler substance or reorganize themselves into new substances.

Collision Theory Activation Energy = the minimum energy that colliding particles must have in order to react

Collision Theory When 2 reactant particles with enough activation energy collide, an activated complex (new entity) is formed

Collision Theory Activated Complex = an unstable arrangement of atoms that forms momentarily at the peak of the activation- energy barrier (Brief existence!) –Forms only if: Colliding particles have sufficient energy Atoms are oriented properly –Also called… Transition state Reformation of the reactants or formation of products. Both outcomes equally likely!

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Collision Theory Particle size –Surface area of reactant affects the reaction rate Smaller particle size  larger the surface area High surface area  –High amount of reactant exposed –High collision frequency –High reaction rate

Collision Theory Increasing surface area –Solid: dissolve into solution, grind into powder

Collision Theory Catalysts = a substance that increases the rate of reaction without being used up during the reaction –Permit reactions to proceed along a lower energy path –More reactants have the energy to form products within a given time

Reaction with catalyst vs. no catalyst

Collision Theory Catalysts –Important to the human body –Enzymes = catalysts –Without enzymes, digestion would take years!!

Collision Theory Inhibitor = a substance that interferes with the action of a catalyst –Reduces the amount of functional catalyst available –Reactions slow or stop

Collision Theory Heterogeneous catalyst = the catalyst exists in a different physical state that the reaction it catalyzes –Ex. Catalytic converter Homogeneous catalyst = the catalyst exists in the same physical state as the reaction it catalyzes –Ex. Both the enzyme and reaction are in aqueous solution

PART 2 REVERSIBLE REACTIONS & EQUILIBRIUM

Reversible reactions Reversible reaction = a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously

Reversible reactions Chemical Equilibrium = a state of balance; when rates of the forward and reverse reactions are equal

Reversible reactions Chemical Equilibrium –No net change occurs in the amounts of the components of the system –Dynamic state: both forward and reverse reactions continue

Reversible reactions Equilibrium position = the relative concentrations of the reactants and products at equilibrium –Indicates whether the reactants or products are favored If A reacts to give B, eq. mixture contains more B, then…

Reversible reactions Catalysts –Speeds up both forward and reverse reactions equally

Le Chatelier’s Principle When the equilibrium of a system is disturbed, the system makes ajustments to restore equilibrium –Shift in Equilibrium position

Le Chatelier’s Principle Le Chatlier’s Principle If a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress.  Stresses that upset the equilibrium  changes in the concentration of reactants or products  changes in temperature  changes in pressure

Le Chatelier’s Principle Concentration –System adjusts to minimize the effects of the change –Adding product to a reaction pushes in the direction of reactants and vice a versa

Le Chatelier’s Principle Temperature –Increasing the temperature causes the equilibrium position of a reaction to shift in the direction that absorbs heat Heat is considered a product!!

Le Chatelier’s Principle Pressure –Change in pressure only affects gaseous equilibria that have an unequal number of moles of reactants and products

Le Chatelier’s Principle

Equilibrium Constants Equilibrium constant = (K eq ) the ratio of product to reactant concentrations at equilibrium –Each concentration raised to a power equal to the # of moles of that substance in the balanced chemical equation

Equilibrium Constants

Brackets indicate concentration (mol/L) Value of K eq depends on the temperature The size of Keq shows whether products or reactants are favored at equilibrium

Equilibrium Constants

K eq =12

Equilibrium Constants