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

PART 3 Solubility Equilibrium

Solubility Product Constant Ionic compounds have different solubilities Most salts are somewhat soluble –Most salts of the alkali metals are soluble in water (slightly / sparingly soluble) –Compounds that contain phosphate, sulfide, sulfite or carbonate ions are generally insoluble

Solubility Product Constant When salts completely dissolve it is a one way reaction… When salts do no completely dissolve it becomes a reversible reaction and an equilibrium is established…

Solubility Product Constant Solubility product Constant K sp = an equilibrium constant for the dissolving of a sparingly soluble ionic compound in water  The product of the concentrations of the ions each raised to the power of the coefficient of the ion in the dissociation equation

Solubility Product Constant

The Common Ion Effect Common Ion = an ion that is found in both salts in a solution Common Ion effect = the lowering of the solubility of an ionic compound as a result of the addition of a common ion

The Common Ion Effect Pb 2+

The Common Ion Effect

The K sp can be used to predict whether a precipitate will form when solutions are mixed. If the product of the concentrations of two ions in the mixture is greater than K sp of the compound formed from the ions, a precipitate will form.

The Common Ion Effect