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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.

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Unit V Kinetics & Equilibrium

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  1. Unit V Kinetics & Equilibrium Textbook Chapters 16,10,14, 13.3,11.3 &11.4

  2. 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.

  3. 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 Image taken from http://www.dkimages.com/discover/previews/888/30037892.JPG on 8/31/09.

  4. 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

  5. Factors Affecting Reaction Rate • 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.

  6. 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 Image taken from http://www.nursce.com/x_courses/1071/iony.jpg on 8/31/09. Slow Covalent Reaction

  7. 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.

  8. 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.

  9. 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.

  10. 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.gifon 8/31/09.

  11. Activation Energy Collisions lacking Activation Energy • The minimum energy needed to start or initiate a chemical rxn. 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.gifon 8/31/09.

  12. 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.

  13. Mechanisms of Chemical Reactions View of Piano Mechanism • Reaction Mechanism- sequence of stepwise reactions by which the overall change occurs, *all these 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 Net= Reactants-->Products Image taken from http://upload.wikimedia.org/wikipedia/commons/1/19/Upright_piano_inside_mechanism.jpg on 8/31/09.

  14. 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” Image taken from http://www.m1creativity.co.uk/ map2003/innovat/funnels.gif on 9/19/09. Image taken from http://devcentral.f5.com/weblogs/images/devcentral_f5_com/weblogs /macvittie/WindowsLiveWriter/FullStackSecurity_6C69/weak%20link-8x6.JPG on 8/31/09.

  15. Heat of Reaction (ΔH) What is the value of ΔH in the diagram below? • 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 Image taken from http://www.saskschools.ca/curr_content/chem30_05/graphics/2_graphics/practice/endo_values_2.gif on 9/19/09. Image taken from http://cache.valleywag.com/assets/resources/2007/10/campfire1.jpg on 9/19/09.

  16. Types of Enthalpys Ref Table I • Heat of Formation (∆Hf) Energy released or absorbed when a molecule is created from its constituent elements. • Standard Heat of Formation(∆Hof) • Since the quantity of heat absorbed/liberated for a reaction varies with temperature, scientists use 25oC and 1atm as standards for comparing enthalpies • Heat of Combustion(∆Hc) • Heat released by the complete combustion of 1 mole of a substance.

  17. 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 Image taken from http://learn.sdstate.edu/deb_pravecek/chem106l/footwarmer.JPG on 9/19/09. Image taken from http://www.files.chem.vt.edu/RVGS/ACT/notes/activation-energy.gif on 8/31/09.

  18. Endothermic Rxn Melting Ice Melting Ice Image taken from http://www.uniongas.com/images/ meltingIcetechnology.jpg on 9/19/09. • Energy is absorbed. • ΔH is positive. (Ref Table I) • 2H2O(l) + 571.6kJ  2H2(g) + O2(g) • ΔH = +571.6kJ

  19. 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 or • 2H2(g) + O2(g) ↔ 2H2O(l) + 571.6kJ Exo and Endo PE diagrams narrated Image taken from http://www.totallyatomic.org/bigchem/200things/pe_diags.gif on 9/19/09.

  20. How does PE diagram change if a catalyst is added? PE Diagrams • Label the above diagrams with double arrows to identify the following segments (PErxnts, PEproducts,PEactivatedcomplex, Activation Energyforward, Activation Energyreverse, Heat of RxnΔH) www.lacs-ny.org/webpages/bmarks/photos/183197/3PE%20Diagrams.gif on 9/19/09.

  21. 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.

  22. Spontaneous rxns depend on balance between 2 fundamental tendencies in nature. • Toward a lower energy state. • Toward randomness. . Image taken from http://www.graphicsfactory.com/clip-art/image_files/image/2/1329142-autumn.gif on 9/19/09. Image taken from http://9 www.ineedmotivation.com/blog/wp-content/uploads/2007/08/laziness.jpgon 9/19/0

  23. 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.

  24. 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.(2nd Law of Thermodynamics) Image taken from http://hyperphysics.phy-astr.gsu.edu/hbase/therm/imgthe/timarr.gif on 9/19/09. Entropy animation

  25. 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

  26. 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/Balance_scale.jpg on 9/19/09.

  27. 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.

  28. 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_over-p137834164602600933q53o_400.jpg on 9/19/09.

  29. 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)

  30. ↔ or ⇆ Equilibrium • 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/chemical-equilibrium/dynamic-equilibrium.jpeg on 9/19/09.

  31. Open vs. Closed Systems • Equilibrium occurs in a closed system. A closed system is isolated from its surroundings. OpenClosed Image taken from http://www.avogadro.co.uk/chemeqm/i2_eqm.gif on 9/19/09. Image taken from http://www.plf.net/htmaccessories/mdyahoo.jpg on 9/19/09. Image taken from http://www.get-vending.com/img/venidng-20%20oz%20pepsi%20bottle.gifon 9/19/09.

  32. Types of Equilibrium • Phase Equilibrium • Solution Equilibrium • Chemical Equilibrium Image taken from http://1.bp.blogspot.com/_ydUOoRJIwWQ/Sgs6oqXpyFI/AAAAAAAAABk/XLu3OtYz9qo/s320/PhaseEquilibriumCartoon-1.gif on 9/19/09.

  33. 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) Animation Image taken from http://zedomax.com/blog/wp-content/uploads/2007/10/cooler.jpg on 9/19/09. Image taken from http://www.chem.ufl.edu/~itl/2045/lectures/FG11_020.GIF on 9/19/09.

  34. 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 Image taken from http://www.chem.ubc.ca/courseware/pH/section17/content.html on 9/19/09.

  35. 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.

  36. 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

  37. 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/112LectWks3and4_files/image002.jpg on 9/19/09.

  38. Le Chatelier’s Principle Henry Louis Le Chatelier • 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 Taken from http://i.ehow.com/images/GlobalPhoto/Articles/5112083/237728-main_Full.jpg on 9/19/09.

  39. 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.

  40. 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 equal moles of gas Animation showing effect of pressure on equilibrium

  41. 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.

  42. 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 Image taken from http://www.lifeplusvitamins.com/images/captain-enzyme-3.jpg on 9/19/09.

  43. 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.

  44. 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.

  45. 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

  46. 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

  47. Image taken from 2012books.lardbucket.org on 8/8/13. Image taken from chemguide.co.uk on 8/8/13.

  48. 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/MY-1518096/silver-chloride_10992746_250x250.jpg on 9/19/09.

  49. Reactions Go To Completion • Some reactions are not reversible (one directional and go to completion). • These rxns may include a gas, precipitate or un-ionized 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.

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