Reaction RatesEquilibrium Water, Solutions, and Colloids

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Reaction Rates/EquilibriumWater, Solutions, and
Colloids
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Reaction Mechanisms and Rates

• Reactions do not all take place at the same
  speed, and the reasons for the rate of each
  reaction can be deduced from the chemicals
  involved and the theory of how reactions take
  place
• Collision model of reactionsreactions take place
  as atoms and/or molecules collidein order for a
  reaction to occur there must be
• 1. Sufficient energy to break bonds
• 2. Correct orientation of molecules to each
  other

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Activation Energy, Eact

• For a reaction to begin
• Colliding particles must overcome repulsions
• Colliding particles must form activated
  complex
• Energy diagrams

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Energy Yield From Reactions

• Endothermicenergy is required to make the
  reactants change to products, the energy of the
  products is higher than the reactants
• Exothermicenergy is released in making the
  reactants change to products, the energy of the
  products is lower than the reactants
• Net heat of reaction, ? Hthe change in energy
  content between reactants and products, if
  endothermic, - if exothermic

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Factors Affecting Rate of Reaction

• 1. Nature of reactantswhat the reactants are and
  how stable their bonds are
• 2NO O2 ? 2NO2 very fast at 25?
• 2CO O2 ? 2CO2 very slow at 25?
• 2. Concentration of reactantsthe more particles
  there are, the more often they can collide
• 3. Surface area of solidsthe more molecules
  available at the surface, the more collisions
  which can occur

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Factors Affecting Rate of Reaction

• 4. Temperature of the reactionhigher temperature
  means higher speed of molecules thus more
  energetic collisions
• 5. Presence of catalystschemicals which help a
  reaction go faster, but which are not affected in
  the course of the reaction
• a. In living systems, enzymes do this job
• b. Enzymes are specific, catalysts much less so

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Homework-8a

• CYU p. 191 All
• p. 204ff 4, 5, 7, 11

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Chemical Equilibrium

• Reactions not totally one way, balance between
  reactants and productsdynamic
• The state where the concentrations of all
  reactants and products remain constant with time.
• On the molecular level, there is frantic
  activity. Equilibrium is not static, but is a
  highly dynamic situation.

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The Law of Mass Action

• For
• jA kB ? lC mD
• The law of mass action is represented by the
  equilibrium expression

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Equilibrium Expression

• 4NH3(g) 7O2(g) ? 4NO2(g) 6H2O(g)

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Le Châteliers Principle

• . . . if a change is imposed on a system at
  equilibrium, the position of the equilibrium will
  shift in a direction that tends to reduce that
  change.

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Changing Equilibrium

• Concentration
• a. increase pushes equilibrium away from that
  increased
• b. decrease pushes reaction toward that
  decreased
• Temperature
• a. increase favors endothermic direction of
  equilibrium
• b. decrease favors exothermic direction
• Catalysts
• Dont affect equilibrium, just the rate at which
  it is established

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Effects of Changes on the System

• 1. Concentration The system will shift away
  from the added component.
• 2. Temperature K will change depending upon the
  temperature (treat the energy change as a
  reactant).

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Effects of Changes on the System (continued)

• 3. Pressure
• a. Addition of inert gas does not affect the
  equilibrium position.
• b. Decreasing the volume shifts the
  equilibrium toward the side with fewer moles.

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Example

• Consider the reaction
• 2 NO2 (g) ?? N2O4 (g) 13.6 kcal
• What effect would each of these changes have on
  the equilibrium (final) concentration of NO2 ?
• Removing N2O4 from the container
• Putting the cylinder in an ice-water bath
• Putting the cylinder in a boiling water bath
• Decreasing the volume of the cylinder

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Homework 8b

• p.204 CYU all
• p. 205ff 13, 14, 16, 18, 20, 22, 29, 30, 31

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Water and its properties 

• Good solvent for ionic and polar substances
• Melting and boiling point high for its MM
• Solid density less than liquid state
• Surface tension very high
• High heats of fusion and vaporization
• High specific heat
• Hydrogen bonding in water explains most of these

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Suspensions

• Suspensions- mixtures which have particles too
  large to be supported by the solvent and which
  settle out of the mixtureriver water

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Colloids

• Colloids- mixtures where the particles are small
  enough to be supported by the solvent, but which
  are not truly dissolved
• Classes of colloids-depend on what phase
  dispersing medium and dispersed medium aresee
  table 9.3
• Properties i. Brownian movement the continual
  motion of colloid particles due to bombardment by
  solvent particles
• ii. Tyndall effectthe scattering of light so
  as to see a beam of light passing through a
  transparent colloidal suspension

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Solutions

• Solutethe component which is dissolved
• Solventthe component which dissolves the solute
• Process of solvation i. In water, water
  molecules pull substance particles into
  solution ii. water molecules surround substance
  particles (hydration)

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A Solute

• dissolves in water (or other solvent)
• changes phase (if different from the solvent)
• is present in lesser amount (if the same phase as
  the solvent)

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A Solvent

• retains its phase (if different from the solute)
• is present in greater amount (if the same phase
  as the solute)

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Electrolytes

• Strong - conduct current efficiently
• NaCl, HNO3
• Weak - conduct only a small current
• vinegar, tap water
• Non - no current flows
• pure water, sugar solution

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Dissolving of Electrolytes

• Fe2Fe(CN)63 ---gt 2 Fe3 3 Fe(CN)6-2
• H2SO4 --gt 2H SO4-2
• BaCl2 ---gt Ba2 2 Cl-
• All strong electrolytes break up completely into
  their respective ions.

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Power Source
04_43

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(a)
(b)
(c)
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Solubility

• The extent to which a solute will dissolve in a
  solvent
• Factors which affect solubility
• Naturelike dissolves likepolar vs nonpolar
• Temperature
• Solids usually more in warm
• Gases more in cold
• PressureHenrys Law from gases
• Ionic substances solubility Table page 222

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Simple Rules for Solubility

• 1. Most nitrate (NO3?) salts are soluble.
• 2. Most alkali (group 1A) salts and NH4 are
  soluble.
• 3. Most Cl?, Br?, and I? salts are soluble (NOT
  Ag, Pb2, Hg22)
• 4. Most sulfate salts are soluble (NOT BaSO4,
  PbSO4, HgSO4, CaSO4, Ag2SO4)
• 5. Most OH? salts are only slightly soluble
  (NaOH, KOH are soluble, Ba(OH)2, Ca(OH)2 are
  marginally soluble)
• 6. Most S2?, CO32?, CrO42?, PO43? salts are only
  slightly soluble.

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Net Ionic Equations

• In reactions between soluble ionic substances,
  not all of the ions get involved in the reaction.
  Those which do not change between reactant and
  product sides of the reaction are called
  spectator ions and should be eliminated from the
  equation. An equation where only active ions are
  shown is called a net ionic equation

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Examples
sodium phosphate calcium chloride?
potassium chloride lead(II) nitrate?
lithium carbonate strontium iodide?
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Homework-9

• p223 CYU
• p225ff 8, 13, 14, 17, 18, 23, 24, 25, 26