Transcript Unit V Kinetics & Equilibrium

Unit V
Kinetics &
Equilibrium
Textbook Chapters 16,10,14, 13.3,11.3 &11.4
Chemical Kinetics
• Branch of chemistry
concerned with
reaction rates and
mechanisms by which
chemical reactions
occur.
Image taken from http://departments.ozarks.edu/msc/chemistry/Molecules.JPG on 8/31/09.
Rate of Chemical Reactions
• How fast or slow a
reaction occurs
• Measured by moles of
reactant used or
moles of product
formed in a given
amount of time.
KClO3 + Gummy Bear
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Effective Collisions (Collision Theory)
• Reactant particles must collide with sufficient
energy at the proper orientation (correct angle).
Image taken from http://www.wiley.com/legacy/college/boyer/0470003790/reviews/kinetics/collision_theory.gif on 8/31/09.
Effective Collision
Role of Orientation in Effective Collisions
Factors Affecting Reaction Rate
1.
2.
3.
4.
5.
Nature of Reactants
Concentration
Temperature
Surface Area
Catalysts
Image taken from http://www.reachoutmichigan.org/funexperiments/agesubject/lessons/other/antacid.gif on 8/31/09.
Nature of Reactants
• A rxn that involves the
smallest (least) amount of
bond rearrangement
(breaking and making new
bonds) is fast.
• Ionic substances in water
(aq) react fast, covalent
substances react slow,
WHY???
Fast Ionic Reaction
Slow Covalent Reaction
Image taken from http://www.nursce.com/x_courses/1071/iony.jpg on 8/31/09.
Concentration
• When concentration of one or more
reactants increases, rate of rxn increases.
• Direct relationship.
• Why?
• More rxnt particles  more collisions more
effective collisions.
• With gases, ↑ pressure will ↑concentration and
also ↑ the rxn rate.
Zinc + H2SO4
Images taken from http://www.chem4kids.com/files/react_rates.html on 8/31/09.
Temperature
• Increasing
temperature,
increases rate of rxn.
Why?
• ↑ temp makes
particles move faster
(↑KE) causing
↑collisions thus
↑effective collisions.
Glowsticks
Animation of Particles at
Different Temperatures
Image taken from http://apollo.lsc.vsc.edu/classes/met130/notes/chapter2/graphics/temp_molec.free.gif on 8/31/09.
Surface Area
• ↑surface area, ↑rxn rate. Why?
• For solid rxnts, ↑surface area means ↑exposed
particles to collide which ↑effective collisions.
Decomposition of H2O2
Image taken from http://www.catskillhouse.us/blog/files/paulthurst41_Jotul_F100_with_kindling.jpg on 8/31/09.
Image taken from http://coffeeteawarehouse.com
files/coffee/coffee-grind-size-large.jpg on 8/31/09.
Catalysts
• A substance that speeds up the rate of a reaction
without being altered or consumed itself.
• Catalysts lower activation energy needed for a reaction
to proceed by providing an alternate pathway.
• Common examples: enzymes, platinum, MnO2
Image taken from http://www.chem4kids.com/files/art/reaction_catalyst2.gif on 8/31/09.
Role of Catalysts
Image taken from http://www.aa1car.com/library/converter.gif on 8/31/09.
Activation Energy
• The minimum
energy needed to
start or initiate a
chemical rxn.
Collisions lacking Activation Energy
Image taken from http://www.chem.ufl.edu/~itl/2045/matter/FG14_013.GIF on 8/31/09.
Image taken from http://www.umdnj.edu/biochweb/education
/bioweb/PreK/EnzymeActivationEnergy2.gif on 8/31/09.
Activation Energy Cont.
• Also characterized as the energy required to transform the
rxnts into the intermediate form (activated complex).
Image taken from http://www.chem.ufl.edu/~itl/2045/lectures/lec_m.html on 8/31/09.
Mechanisms of Chemical Reactions
• Reaction Mechanism- sequence
of stepwise reactions by which
the overall change occurs, *all these
View of Piano Mechanism
steps may not be observable, often only the net reaction
is observable
• Net reaction- summation of all
the changes that occur
• Example: H2(g)+I2(g) ⇆ 2HI(g)
I2 ⇆ 2I
I + H2 ⇆ H2I
H2I + I ⇆ 2HI
Above is one possible rxn
mechanism
• H2I is possible activated complex
•
•
•
•
Image taken from
http://upload.wikimedia.org/wikipedia/commons/1/19/Upright_pian
o_inside_mechanism.jpg on 8/31/09.
Rate Determining Step
• The slowest step in a reaction mechanism that
determines the overall rate for the reaction.
• “A chain is only as strong as its weakest link”
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/macvittie/WindowsLiveWriter/FullStackSecurity_6C69/weak%20link-8x6.JPG on 8/31/09.
map2003/innovat/funnels.gif on 9/19/09.
Heat of Reaction (ΔH)
What is the value of ΔH in the diagram below?
Image taken from
http://www.saskschools.ca/curr_content/chem30_05/graphics/2_graphics/practice/endo_va
lues_2.gif on 9/19/09.
• Heat energy released
or absorbed during a
chemical rxn.
• Difference in heat
content (aka enthalpy)
between products and
reactants.
• ΔH= Hproducts – Hreactants
• Always final minus initial
• Reference Table I
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http://cache.valleywag.com/assets/resources/2007/10/campfire1.jpg on 9/19/09.
Types of Enthalpys
Ref Table I
• Heat of Formation (∆Hf)
Energy released or absorbed when a molecule
is created from its constituent elements.
o
• Standard Heat of Formation(∆H f)
• Since the quantity of heat absorbed/liberated
for a reaction varies with temperature,
o
scientists use 25 C and 1atm as standards for
comparing enthalpies
• Heat of Combustion(∆Hc)
• Heat released by the complete combustion of
1 mole of a substance.
Exothermic Rxn
Rxn of Iron with Chlorine
• Energy is released.
• ΔH is negative. (Ref Table I)
• Example: 2H2(g) + O2(g)  2H20(l) +571.6kJ
• ΔH= -571.6kJ
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9/19/09.
Image taken from http://www.files.chem.vt.edu/RVGS/ACT/notes/activation-energy.gif on 8/31/09.
Endothermic Rxn
• Energy is absorbed.
• ΔH is positive. (Ref Table I)
• 2H2O(l) + 571.6kJ  2H2(g) + O2(g)
• ΔH = +571.6kJ
Melting Ice
Melting Ice
Image taken from http://www.uniongas.com/images/
meltingIcetechnology.jpg on 9/19/09.
Exothermic vs. Endothermic
• Notice exothermic and endothermic rxns can be the
reverse of one another. Beach Ball.
• This can be represented with a double arrow.(see below)
• 2H2(g) + O2(g) ⇆ 2H2O(l) + 571.6kJ
Exo and
Endo PE
or
diagrams
narrated
• 2H2(g) + O2(g) ↔ 2H2O(l) + 571.6kJ
Image taken from http://www.totallyatomic.org/bigchem/200things/pe_diags.gif on 9/19/09.
PE Diagrams
How does PE diagram
change if a catalyst is added?
• Label the above diagrams with double arrows to identify the
following segments (PErxnts, PEproducts,PEactivated complex, Activation
Energyforward, Activation Energyreverse, Heat of RxnΔH)
www.lacs-ny.org/webpages/bmarks/photos/183197/3PE%20Diagrams.gif on 9/19/09.
Spontaneous Reactions
• Rxn that having once been initiated will continue
under the existing conditions.
• Once activation energy is added, rxn goes.
• Example: Burning paper w/ match.
Image taken from http://wwwdelivery.superstock.com/WI/223/
1491/PreviewComp/SuperStock_1491R-1040281.jpg on 9/19/09.
Spontaneous rxns depend on balance
between 2 fundamental tendencies in nature.
1. Toward a lower energy state.
2. Toward randomness.
.
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9/19/09.
Image taken from http://9
www.ineedmotivation.com/blog/wpcontent/uploads/2007/08/laziness.jpgon
9/19/0
Energy/Enthalpy Changes (ΔH)
• Nature favors rxns
that move from high
energy to low energy.
Why?
• Lower energy states
are more stable.
• Exothermic rxns are
favored.
• ΔH is negative.
Image taken from
http://amadeo.blog.com/repository/1434082/3671375.jpg on
9/19/09.
Image taken from http://www.lazyenvironmentalist.com/3%20lazy
%20polar%20bears.jpg on 9/19/09.
Entropy/Randomness Changes (ΔS)
• Entropy is measure of
disorder, randomness or lack
of organization in a system.
• ↑disorder, ↑entropy
• Examples:
• At constant temp, nature
favors rxns that move from
less entropy to more
entropy.(2 Law of Thermodynamics)
Image taken from http://hyperphysics.phyastr.gsu.edu/hbase/therm/imgthe/timarr.gif on 9/19/09.
nd
Entropy animation
Free Energy Change (ΔG)
• So what if only one of nature’s tendencies is being
followed, will rxn be spontaneous?
• Depends on which tendency is dominant.
• Need to compare tendencies in mathematical
expression called….
Free Energy Change or the Gibbs Equation
ΔG= ΔH – TΔS
T is temperature measured in Kelvin
ΔG= ΔH – TΔS
• Negative ΔG means rxn will be spontaneous.
• Positive ΔG  rxn will NOT be spontaneous.
• If ΔG= zero, then 2 tendencies balance each other
out. System will be in equilibrium (balance).
Image taken from
http://upload.wikimedia.org/wikipedia/commons/7/7e/B
alance_scale.jpg on 9/19/09.
Will rxn be spontaneous and what sign
will ΔG have under these conditions?
• ↑ΔS and ↓ΔH
• Both tendencies in nature are followed.
• ΔG= ΔH – TΔS
•
(neg) - (pos)
•
(small) - (big)
• Always spontaneous, ΔG is negative.
Image taken from http://farm1.static.flickr.com/230/504782571_0d87cb7958.jpg on 9/19/09.
Will rxn be spontaneous and what sign
will ΔG have under these conditions?
• ↓ΔS and ↑ΔH
• Both tendencies in nature are NOT followed.
• ΔG= ΔH – TΔS
•
(pos) - (neg)
•
(big) - (small)
• Never spontaneous, ΔG is positive.
Image taken from http://rlv.zcache.com/funny_graduation_card_hell_freezes_overp137834164602600933q53o_400.jpg on 9/19/09.
What about when ↑ΔH and ↑ΔS or ↓ΔH and ↓ΔS ?
• When both factors are not favorable, depends on
which one is more dominant at that temp.
• Ex #1: H2O(l)  H2O(s)
• Exothermic (↓ΔH favorable)
• Forming a solid, more orderly (↓ΔS unfavorable )
• Spontaneity depends on temp.
• In this room (above freezing pt.), water does not freeze
spontaneously because ΔS dominates.
• Below freezing pt. (in a freezer or outside on cold day), water
does freeze spontaneously because ΔH dominates.
• Ex #2: H2O(g) +C(s) + 131.3kJ  CO2(g) + H2(g)
•
•
•
•
Endothermic (↑ΔH unfavorable)
(g)&(s)  two gasses (↑ΔS favorable)
At low temps.(25oC/298K) not spontaneous (ΔH dominates)
At high temps.(900oC/1173K), spontaneous rxn (ΔS dominates)
Equilibrium
↔ or ⇆
• When rates of 2 opposing reactions are equal.
• Equilibrium is dynamic because 2 opposite
reactions are occurring and change is happening,
even though it may look overall like nothing is
changing.
• Rxns must be reversible for equilibrium to occur.
Image taken from http://img.tfd.com/wn/3C/6C2BE-acid-base-equilibrium.jpg on 9/19/09.
Image taken from http://image.tutorvista.com/content/chemicalequilibrium/dynamic-equilibrium.jpeg on 9/19/09.
Open vs. Closed Systems
• Equilibrium occurs in a closed system. A closed
system is isolated from its surroundings.
Open
Closed
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dyahoo.jpg on 9/19/09.
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Types of Equilibrium
• Phase Equilibrium
• Solution Equilibrium
• Chemical Equilibrium
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on-1.gif on 9/19/09.
Phase Equilibrium
-Balance (equilibrium) in a closed system
between 2 reversible phase changes.
• Examples:
• Equilibrium between freezing and
melting. (liquid↔solid)
• Equilibrium between vaporization
and condensation. (liquid↔gas)
Image taken from http://zedomax.com/blog/wp-content/uploads/2007/10/cooler.jpg on 9/19/09.
Animation
Image taken from
http://www.chem.ufl.edu/~itl/2045/lectures/FG11_020.GIF on 9/19/09.
Solution Equilibrium
• Balance (equilibrium) in a closed system between a
dissolved and undissolved form within a solution.
• Occurs in saturated solutions.
• 2 types:
• Gases dissolved in a Liquid
• Solids dissolved in a Liquid
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Gases Dissolved in Liquids
• Equilibrium occurs when rate of gas dissolved in
liquid going up into the air space above the
liquid EQUALS the rate of the undissolved gas on
top dissolving back into the liquid.
• Effect of temp & pressure?
Image taken from
http://www.scienceinschool.org/repository/images/issue2fizz1.jpg on
9/19/09.
Solids Dissolved in a Liquid
• Equilibrium occurs when rate of solid dissolving
into a liquid EQUALS the rate of the
crystallization of the solid solute from the liquid
solvent.
• Saturated solutions.
• Effect of temperature?
• Ref. Table G.
Image taken from http://chem12teacher.tripod.com/soleqdiagram1.JPG on
9/19/09.
AgCl solution equilibrium
Chemical Equilibrium
• Attained when the concentration of reactants
and products remains constant.
• Forward chemical rxn rate = reverse rxn rate.
• No observable changes (color, temp., phase
changes, etc.) once established.
Equilibrium animation
Image taken from
http://spinner.cofc.edu/~martine/112LectWks3
and4_files/image002.jpg on 9/19/09.
Le Chatelier’s Principle
• If a stress is applied to a
system at equilibrium, the
equilibrium is shifted in a
way that tends to reduce
or relieve the effects of
the stress.
• Types of stress:
• Concentration
• Pressure (gases only)
• Temperature
Jenn Wilder’s “Equilibrium” poem
Henry Louis Le Chatelier
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Concentration (Le Chatelier)
• Increase one substance in a reaction.
• Favors the reaction that uses this additional
substance as a reactant to reduce this stress.
• Removal or reduction of one substance in a
reaction.
• Favors the reaction that forms this substance
as a product to make up for the stress.
Pressure (Le Chatelier)
• Only a factor when gases are involved.
• Increase in pressure will shift equilibrium to favor
the formation of fewer moles of gas.
• Decrease in pressure favors the opposite.
Animation showing effect
of pressure on equilibrium
Animation showing effect
of pressure on equal
moles of gas
Temperature (Le Chatelier)
• Increasing temp. shifts equilibrium to favor the
endothermic rxn.(tries to use up the energy just like increasing a rxnt).
• Decreasing temperature favors the exothermic rxn.(tries
to make energy just like removing a rxnt).
Video of Temp Effect
On Equilibrium
Taken from http://www.daviddarling.info/images/Le_Chateliers_principle.jpg on 9/19/09.
Catalysts (Le Chatelier)
• Adding a catalyst will not cause the equilibrium
point to shift.
• It only causes the equilibrium point to be reached
more quickly.
4min demo on Youtube or
Realplayer library
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Law of Chemical Equilibrium
• When a reversible rxn has attained equlibrium at
a given temp., a mathematical relationship
exists equilibrium constant (Keq)
• Write Keq for example below:
• 2CH3OH(l) + 3O2(g) ↔ 2CO2(g) + 4H2O(l)
• Keq= [CO2]2
[O2]3
• Keq values only change
with temperature.
Image taken from scienceaid.co.uk on 8/8/13.
• Several types of Keq
•
•
•
•
Ka for ionization of acids
Kb for dissociation of bases
Kw for ionization of water
Ksp for solubility of solids
Image taken from glastonburyus.org on 8/8/13.
Solubility Product Constant (Ksp)
• Type of Keq
• Deals with solution equilibrium.
• Ksp- measure of the concentration of slightly soluble
salts in water.
• Equilibrium exists between dissolved ions in
saturated solution and excess solid phase.
• Example: CaCl2(s)  Ca2+(aq) + 2Cl-(aq)
• Write equilibrium expression! Remember do not
include solids.
• Ksp= [Ca2+] [Cl-]2
Size of Ksp 
Used in comparing solubilities of salts
• Example:
• At room temperature, Ksp of CaSO4 =2.4 X 10-5 and
Ksp of BaSO4 =1.6 X 10-9
• Ksp of BaSO4 < Ksp of CaSO4
• BaSO4 is less soluble than CaSO4 and would
precipitate at a lower concentration.
• Remember only changes in temp. can change Ksp values.
Ref Table F
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Image taken from chemguide.co.uk on 8/8/13
.
Common Ion Effect
• AgCl(s) ⇆ Ag+(aq) + Cl-(aq)
• Adding another soluble chloride salt (NaCl) will
increase chloride ion concentration.
• ↑Cl- concentration causes shift in equilibrium (Le
Chatelier’s Principle)
• Equilibrium shifts to left and decreases solubility.
Image taken from http://imghost.indiamart.com/data/W/2/MY1518096/silver-chloride_10992746_250x250.jpg on 9/19/09.
Reactions Go To Completion
• Some reactions are not reversible (one directional and
go to completion).
• These rxns may include a gas, precipitate or unionized product such as water. These types of
products are unable to react and reverse the reaction.
• H2CO3  H2O + CO2 ↑ ( ↑ can mean gas)
• NaCl + AgNO3  NaNO3 + AgCl ↓ ( ↓ means precipitate)
• HCl + NaOH  NaCl + H2O
• Use Ref Table F to find insoluble precipitates.