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NOTES: UNIT 8: KINETICS / EQUILIBRIUM (1) NAME _____________________________ I) Kinetics (DEFINITION): studies of the rate of a reaction the change over time of bond breaking &/or of bond making READ THIS! Too often, the concept of kinetics is viewed as how we may speed up a chemical reaction, only. But, chemists are interested in slowing down (inhibiting) chemical reactions as well as in speeding up reaction rates. Chemists work to control everything from the economically important to the biologically very important reactions. For example, chemists wish to learn how to control (and slow down) the rate of oxidation of metal bridges. They wish to speed the refractory period (the recharging time) of a digital camera’s screen image, so we can take another photo, in less time. Chemists wish to learn how to control the processes of aging. They want to know the kinetics as to how antioxidants (Vitamin A,C and E for instance) help to keep our cytoplasmic materials, DNA and proteins from damage, (if they do so at all). Scientists want to learn how to control the rates at which cancer cells reproduce and how to increase the rate of cancer cell contact inhibition. Scientists want to get a more predictable life span out of light bulbs, batteries, etc. All of these issues and many more are connected to the specialized field of kinetics. Kinetics is explained best by one of the six to eight big ideas in Regents chemistry: The Collision Theory. You want to know this theory – it comes in very handy... II) The Collision Theory: (TEXT READING: P.541-547) A) The Collision Theory is a series of ideas which help explain or predict the kinetics of a reaction. Any factor which increases (or decreases) the number of effective collisions between the reacting particles will help speed up (or slow down) a reaction. WE CAN CONTROL A REACTION, BY CONTROLLING THE NUMBER OF EFFECTIVE COLLISIONS. (MAJOR REASON # 6) 1) Effective Collision Any collision between two reactant species which results in a * a new bond being produced (products being made) 2) There are 2 necessary conditions for an effective collision: a) * sufficient kinetic energy b) * proper orientation 3) Complete the statement: The greater the number of * effective collisions the greater the rate of a chemical reaction. 4) Activation Energy: A compilation of ideas related to: The minimum amount of energy required to break the bonds of the reacting species, in preparation for new bonds to be made or * the extra energy needed to be added The bond energy of the reacting species AND The energy required to raise the energy levels of the species to a point of reaction. (called the activated complex) 503 III) There are 5 fundamental factors which affect the ability to achieve the requirements of effective collisions A)5 factors that affect the rate (the kinetics) of a chemical reaction Nature of the Reactants Surface Area Temperature Cut To The Chase: Ionic compounds in solution react at a greater rate than molecular substances …because the bonds of the ionic substances are already broken Cut To The Chase: An increase in temperature increases BOTH the exothermic and endothermic reaction …but an increase has a greater effect on an endothermic reaction. Cut To The Chase: Greater surface area leads to a greater availability of reactants effectively colliding and bonding. Molar Concentration (or Pressure for gaseous systems Catalysts (or Inhibitors) Cut To The Chase: A catalyst must be a substance …and it allows a reaction to proceed along a path of lower activation energy… Hence, reactant bonds are broken at lower energies that are reached sooner in time…. Cut To The Chase: Greater concentrations lead to more effective collisions and that means a faster reaction 1) nature of the reactants (dissolved ionic compounds vs. molecular (covalent) compounds) Basic Idea: Solutions of ionic compounds react * faster (form new bonds) combination than solutions of * molecular (covalent) upon substances. Recall: There are both molecular * inorganic and molecular organic compounds. Reason: Often, and in the simplest of reactions, WATER can disrupt the ionic bond and thus breaks the compound into ions (electrolytes), ready to make new bonds. Water can NOT break the bonds of most covalent compounds. The consequence is that ionic compounds (pre-dissolved in water) react faster, than dissolved covalent, compounds because the ionic bonds are * broken already (dissociated) Whereas, the bonds of dissolved covalent compounds must first * be broken and then the species can * react 504 Think about it: In the beakers = Pb(NO3)2(aq) + 2 KI(aq) Pb2+(aq)+ 2 NO31-(aq) 2 K1+(aq)+ I1-(aq) PbI2(s) + 2 KNO3(aq) ◘ Do you see how the ionic bonds of the reactants are broken already? ◘ Note: As a comparison to the Bosch-Haber Process, this ionic reaction occurs at room temperature and standard pressure… Think about it: Bosch-Haber Process: N2(g) + 3H2(g) 2 NH3(g) While not in aqueous solution, the gaseous reactants are bonded strongly enough that this reaction requires a catalyst of iron, (mixed with a potassium hydroxide promoter), a temperature of 400°C to 450°C and approximately 200. atmospheres of pressure. Think about it: C12H25OH(l) + H2SO4(aq) HC12H25SO4 + H2O lauryl alcohol dodecyl hydrogen sulfate Bonds are NOT yet broken Water disrupts the ionic bond, but the covalent bonds of the polyatomic ion are NOT broken In the beakers: = C12H25OH + 2 H+1(aq) + (SO42-)(aq) H H H H H H H H H H H H | | | | | | | | | | | | HCCCCCCCCCCCCO H | | | | | | | | | | | | H H H H H H H H H H H H O -1 │ O ═ S─ O -1 ║ O All of these covalent bonds must first be broken before atoms can recombine to produce the product & water can’t attack the covalent bonds. Thus before the reaction gets going, everything must be “taken apart”. 505 2) Temperature: Basic Idea: As a rule, an increase in temperature * increases the rate of both endothermic and exothermic reactions. However, endothermic reactions are increased a greater extent Reason: An increase in temperature, indicates that more energy has been added to the reacting species. This increase in energy is reflected in the faster movement of the species. In theory then, faster moving species have a statistically greater chance of colliding with with other reactive species with sufficient energy and orientation. a) The opposite of every exothermic reaction is a(n) *endothermic reaction b) endothermic reactions are affected MORE by changes in temperature than exothermic reactions, because changes in temperature may change indicate changes in the systems thermal energy. This thermal energy is a necessary part of endothermic reactions. c) Don’t forget, when you read: average kinetic energy you think of * temperature 3) Concentration (note: for gases, changes in concentration = changes in pressure for gases) a) symbol for concentration is a set of brackets: [ ] Thus the when discussing the concentration of dioxygen gas, a chemist may write the phrase as [O2] b) Recall our work on Molarity (see your independent packet) 506 c) How do changes in concentration affect collisions? i) With a greater concentration, the likelihood of appropriate orientation increases, statistically, as does the probable number of actual collisions. Each leads to a statistically greater number of effective collisions. d) Note: Increasing the pressure on a gas is synonymous with increasing its concentration, due to an increase in the number of moles per unit volume (Reference: Molarity: M = moles / Liter … the same # of species, in a smaller volume, effectively increases the molar concentration) 4) Surface area (particle size): Greater surface area allows for greater collisions and greater probability for proper orientation by exposing more of the reactive species to the other reactant(s) Action Sought On Dust Explosions The Chemical Safety & Hazard Investigation Board (CSB) recommended last week that federal agencies take action to protect workers from industrial dust explosions and fires. The recommendation grew from a two-year investigation of the hazards of combustible dust. In 2003, 14 workers were killed and another 81 were injured in three industrial accidents that were fueled by combustible dust. Between 1980 and 2005, CSB adds, 119 workers were killed and 718 were injured in nearly 300 dust-related accidents. Combustible dust accumulates in many chemically related manufacturing facilities, CSB notes, and once ignited, the dust leads to explosive accidents. Chemical and Engineering News 13 Nov 2006 p. 30 5) Catalysts: a) Catalysts work by allowing the reaction to proceed along an * alternate pathway that has a lower activation energy requirement (bond breaking thus occurs “faster). b) Catalysts are often written above the reaction arrow because * they are not consumed during the reaction (they are regenerated). They are neither reactants nor products. c) enzymes are biological catalysts (remember the names of enzymes often end in –ase) *eg. lactase vs. lactose, amylase vs amylose, acetylcholinesterase vs. acetylcholine d) many metals (Pt, Zn and even Na) and compounds may be effective catalysts. *A catalyst must be a substance e) Note: Thermal energy (okay, “heat”) IS NOT A CATALYST!! f) The opposite of a catalyst is an inhibitor g) Note: The addition of a catalyst does NOT change a reaction’s H (change in enthalpy) 507 QUESTIONS: Given the reaction: Zn(s) + 2HCl(aq) Molarity of HCl H(2)(g) + ZnCl2(aq) Description of Zn Temperature A 2M 10 grams in small lumps 26 C B 1M 10 grams of fine powder 30 C C 1M 10 grams in small lumps 26 C D 2M 10 grams of fine powder 30 C Which of the listed conditions will result in the fastest generation of hydrogen gas?______ Defend your answer: _____________________________________________________________________________________ ___________________________________________________________________________________ Which set of conditions probably results in the slowest generation of hydrogen? ________ Why? ________ ____________________________________________________________________________________ ISSUE: A kitchen grease fire can range (get it?) from a troublesome mishap to a home disaster. Grease can reach its “flash point”. The flash point is the temperature at which the vapors spontaneously combust in the presence of atmospheric oxygen. The combustion of grease produces combustion products and some nasty smelly once such as acrolein. But the take home message is that grease will ignite when there is sufficient oxygen gas present and when the grease is sufficiently vaporized at high temperature. Throwing baking soda (sodium hydrogen carbonate) onto the fire, often puts out the fire. Baking soda is absorbent and it decomposes under high heat according to the reaction equation: 683kJ + 2 NaHCO3 (s) Na2CO3(s) + H2O(l) + CO2(g) Using the ideas of collision theory and the factors that help control the kinetics of a reaction, please explain how the kinetics of the combustion reaction are controlled (diminished in this case) by the addition of baking soda? (That is… how does its absorbency and decomposition help?) * Baking soda can absorb liquid grease, preventing the vaporization process. This limits one of the reactants. Baking soda decomposes under high energy and produces a denser carbon dioxide, which is present just long enough to displace (push out) dioxygen gas (thus limiting another reactant). 508 IV) Potential Energy Diagrams: LEARNING GOALS : As a first year chemistry student you should be able to: identify a potential energy diagram as representing either an exothermic or an endothermic reaction. label any portion of a potential energy diagram. judge, using a diagram, as to whether the reactants are more or less stable than the products. sketch a rough potential energy diagram for any given chemical equation. The Potential Energy Diagram is a qualitative visual means of representing the potential energy changes and the progress of a chemical reaction, with potential energy as a function of time. The Change in Enthalpy (new term) is the difference in the potential energy of the products and reactants. Its symbol is ∆H or q Since the graph is concerned with the "time for a reaction", it has a direct link to the study of kinetics. Each diagram is divided into: potential energy of the reactants activation energy (the energy gained by the reactants needed to break the existing bonds) the potential energy of the activated complex the energy lost due to bond formation the potential energy of the products. the change in enthalpy also called the heat of reaction (∆H, change in enthalpy) 509 A) Putting all the pieces together: PE Reaction Coordinate PE Reacton Coordinate NOT: PE Reaction Coordinate 510 V) Potential Energy Diagrams B) Change in Enthalpy and Generalized /Labeling Potential Energy Diagrams 1) Every chemical reaction is a combination of exothermic and endothermic processes. The reaction is classified according to whichever process is greater. There is a "competition of sorts" between the acts of bond breaking and bond formation. 2) The H or the "q" of a reaction is equal to the difference in energy possessed by the products and reactants...it is the * change in enthalpy or heat of reaction the net change in energy of a reaction a) H= Hproducts - Hreactants Table I: Heats of Reaction at 101.3 kPa and 298 K Reaction ∆H (kJ)* CH4(g) + 2 O2(g) CO2(g) + 2 H2O(𝓁) C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(𝓁) 2 C8H18((𝓁) + 25 O2(g) 16 CO2(g) + 18 H2O(𝓁) 2 CH3OH (𝓁) + 3 O2(g) 2 CO2(g) + 4 H2O(𝓁) C2H5OH(𝓁) + 3 O(g) 2 CO2(g) + 3 H2O(𝓁) C6H12O6(s) + 6 O(g) 6 CO2(g) + 6 H2O(𝓁) 2 CO(g) + O2(g) 2 CO2(g) C(s) + O2(g) CO2(g) 4 Al(s) + 3 O2(g) 2 Al2O3(s) N2(g) + O2(g) 2 NO(g) N2(g) + 2 O2(g) 2 NO2(g) 2 H2(g) + O2(g) 2 H2O (g) 2 H2(g) + O2(g) 2 H2O (𝓁) N2(g) + 3 H2(g) 2 NH3(g) 2 C(s) + 3 H2(g) C2H6(g) 2 C(s) + 2 H2(g) C2H4(g) 2 C(s) + H2(g) C2H2(g) H2(s) + I2(g) 2 HI(g) 𝑤𝑎𝑡𝑒𝑟 KNO3(s) → 𝑤𝑎𝑡𝑒𝑟 NaOH(s) → 𝑤𝑎𝑡𝑒𝑟 NH4Cl(s) → K+(aq) + NO3- (aq) -890.4 -2219.2 -10943 -1452 -1367 -2804 -566.0 -393.5 -3351 +182.6 +66.4 -483.6 -571.6 -91.8 -84.0 +52.4 +227.4 +53.0 +34.89 Na+(aq) + OH- (aq) -44.51 NH4+(aq) + Cl- (aq) +14.78 𝑤𝑎𝑡𝑒𝑟 NH4NO3(s) → 𝑤𝑎𝑡𝑒𝑟 NaCl(s) → NH4+(aq) + NO3- (aq) Na+(aq) + Cl- (aq) +25.69 P E Reaction Coordinate P E +3.88 𝑤𝑎𝑡𝑒𝑟 -48.83 LiBr(s) → Li+(aq) + Br- (aq) -55.8 H+(aq) + OH- (aq) H2O(𝓁) *The ∆H values are based on molar quantities represented in the equations. A minus sign indicates an exothermic reaction. Reaction Coordinate 511 3) Exothermic Reaction: The products of an exothermic reaction are * more stable (less likely to react) The products are * lower than the reactants in energy The change in enthalpy of an exothermic reaction is written with a *negative ∆H heat of reaction e.g) 2 NO(g) + O2(g) 2 NO2(g) + 112.86 kJ sign. ∆H = * - 112.86 kJ 2 C7H5N3O6(s) 3 N2(g) + 12 CO(g) + 5 H2(g) + 2C(s) + 2,092kJ ∆H =* -2,092 kJ 4) ENDOthermic Reaction: The products of an endothermic reaction are * less stable The products are * higher (greater) than the reactants in energy The change in enthalpy of an endothermic reaction is written with a * positive ∆H heat of reaction 53 kJ + H2(g) + I2(g) 2 HI(g) sign. ∆ H = * + 53 kJ 512 5) For synthesis reactions we can speak of “heat of formation”. This is the energy released or absorbed for the production of ONLY 1 mole of compound from its elements. This is the critical attribute of the definition 53 kJ + H2(g) + I2(g) 2 HI(g) ∆H = +53 kJ N2(g) + 3H2(g) 2 NH3(g) + 91.8 kJ ∆H = -91.8 kJ but ∆Hf = *-45.9 kJ C(s) + O2(g) CO2(g) + 393.5 kJ ∆H = -393.5 kJ and ∆Hf = *-393.5 kJ 66.4 kJ + N2(g) + 2 O2(g) 2 NO2(g) ∆H = +66.4 kJ but ∆Hf = *+ 33.2 2 Al(s) + 3 O2(g) 2 Al2O3(s) + 3351 kJ ∆H = -3351 kJ but ∆Hf = *-1,675.5 kJ 2 C(s) + 3 H2(g) C2H6(g) + 84 kJ ∆H = -84 kJ and ∆Hf =*-84 kJ but ∆Hf = *+26.5 kJ C) Labeling Potential Energy Diagrams (continued) 1) Activation Energy * the energy required to break the bonds of the reactants and raise their potential energy to the activated complex (The endothermic portion of a reaction) a) Activation Energy of the Reverse Reaction: * Represented by the line from the products to the activated complex. 2) Activated Complex A point at which intermediate associations are made ... the species are at the highest potential energy ... in that bond breaking has been achieved and intermediate species exist temporarily... just before, all out new bond formation occurs. This is a point of transition.The species are not isolated easily and are often hypothesized. See: http://en.wikipedia.org/wiki/Sulfur_dichloride for an application of the concept of the “intermediate” (HONORS) 3) Energy Lost during Bond Formation (There is no "technical" name – but it is a useful concept) a) ALL bond formation * releases regardless of the overall reaction. energy (it is *exothermic ) 513 D) Effects of a catalyst and P.E. Diagrams The Effect of a Catalyst on Reaction Rate* 1400 uncatalyzed 1300 1200 potential energy 1100 (kJ) 1000 catalyzed 900 800 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Time (minutes) 4.5 5.0 5.5 (should be reaction coordinate ... see *) A catalyst works by allowing the reaction to occur via an alternate pathway, one which has * a lower activation energy. The level of activation energy and the activated complex are lessened … but H and the heat of the reactants and heat of the products remain unchanged. *Please note that I have taken liberties with the interpretation of the reaction coordinate by equating it with time. I have done this only for the illustrative purpose of demonstrating a lesser amount of time in order to initiate a reaction, with the use of a catalyst. The concept of the reaction coordinate is a far more complex issue and as a rule should not be simplified as time. Again, I do so here only to emphasize the impact of the catalyst on the rate of reaction (HONORS). 514 E) Practice Labeling P.E. Diagrams 1) Given the following reaction equation and potential energy diagram answer the questions. An answer may be used once, more than once or not at all. 4Al(s) + 3O2(g) 2 Al2O3(s) + 3,351 kJ 2 3 Potential Energy 4Al + 3O2 4 1 2 Al2O3 5 Reaction Coordinate _____ a) Which arrow represents the energy required to initiate the reaction by breaking the bonds of the reactants? _____ b) Which arrow represents the heat of reaction (∆H or change in enthalpy)? _____ c) Which arrow represents the chemical energy possessed by the reactants? __&__ d) Which arrows would be most affected by the addition of a catalyst? _________e) Which type of reaction is represented: exothermic or endothermic? ______ f) Which arrow represents the value: 3,351 kJ? ______ g) Which is / are more stable? Al & O2 or Al2O3 ___________h) According to the diagram, was more energy absorbed from the surroundings or released into the surroundings, as the reaction occurred? 2) Given the following reaction equation , answer each question. 53.0 kJ + H2(g) + I2(g) 2 HI(g) a) Sketch the potential energy diagram. b) Label Hreactants and the Hproducts c) Label the area which represents the 53.0 kJ d) Identify the area representing the activation energy 515 3) a ) The reaction is Exothermic / Endothermic 5 4 2 Elost by new bonding b) Ereactants is represented by arrow c) E products is represented by arrow Ep 3 1 d) Eactivated complex is rep. by arrow e) Eactivation is represented by arrow 6 f) Heat of reaction (H) is rep. by arrow Reaction Coordinate 4) 2 a ) The reaction is Exothermic / Endothermic 3 4 E lost by new bonding b) Ereactants is represented by arrow c) E products is represented by arrow Ep 1 d) Eactivated complex is rep. by arrow 5 6 Reaction Coordinate e) Eactivation is represented by arrow f) Heat of reaction (H) is rep. by arrow 516 NAME _________________________________________ KINETICS DIRECTIONS: Study first and then attack the worksheet. All of the answers are given at the end. For 1-4 one or more of the responses given is (are) correct. Using your understanding of chemistry decide which of the responses is (are) correct. Then choose : a) b) c) d) e) if only I is correct if only II is correct if only I and II are correct if only II and III are correct if I, II, and III are correct ___4)Given: 80.0 kJ + X2 + 2Y + 8Z 2 XYZ4 According to the above reaction equation, I) the ∆H of the forward reaction is + 80 kJ II) the reaction is exothermic III) products are more stable than reactants ___1) Given the following reaction, which statement(s) is (are) true ? ****************** For questions 5 - 18, only a single answer per question is most correct. C(s) + O2(g) CO2(g) + 393 kJ ___5) The "heat of reaction", ∆H, is equal to I) The forward reaction is exothermic II) The product is more stable that the reactants III) The heat of reaction (∆H) for the forward reaction is +393 kJ ___2) Which of the following statements, regarding the role of catalysts is (are) true? I) A catalyst reduces the activation energy of a reaction II) A catalyst reduces the value for the heat of reaction (∆H) III) A catalyst gets used up and converted into product ___3) Given: 2 Pb(s) + O2(s) 2 PbO(s) + 438 kJ According to the above thermochemical equation, I) the forward reaction is exothermic II) the heat of reaction equals -438 kJ a) Hproducts - Hreactants b) Hproducts + Hreactants c) Hproducts / Hreactants d) (Hproducts) (Hreactants) ___6) According to Table I, which of these compounds is "most stable" ? a) NO2(g) b) HI(g) c) CO2(g) d) NH3 (g) ___7) Given the reaction: Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g) The reaction occurs more slowly when a single lump of zinc is used when compared to the rate of the reaction when the same mass of powdered zinc is used. This is best explained because the powdered zinc: a) b) c) d) is more concentrated has a greater surface area requires a lower activation energy generates more heat III) products are more stable than reactants 517 ___8) Given the reaction : For questions 13 -17 use the given reaction and potential energy diagram. A + B C + D N2(g) + O2(g) + 66 kJ 2 NO2(g) The reaction most likely will occur at the greatest rate if A and B represent: a) metal elements and nonmetal elements in the solid phase B D b) ionic compounds in the solid phase c) organic compounds in the solid phase C E A d) solutions of ionic compounds ___9) Given A + B C + 100 kJ the potential energy of the products when compared to the potential energy of the reactants is: a) less, so the reaction is exothermic b) more, so the reaction is endothermic c) less, so the reaction is endothermic d) more, so the reaction is exothermic ___10) According to Table I which compound forms via an endothermic reaction ? a) carbon dioxide b) aluminum oxide c) nitrogen monoxide d) ammonia ___11) The minimum energy required to break bonds and initiate a chemical reaction is a) activated complex c) change in enthalpy b) heat of reaction d) activation energy ___12) Assume that the potential energy of the products in a chemical reaction is 60. kJ. The reaction which formed these products would be exothermic if the potential energy of the reactants were equal to : a) 50. kJ b) 20. kJ ___13) Which arrow(s) is/are the equivalent(s) of the energy of activation for the forward reaction? a) A + E b) B + A c) C + D d) just arrow E ___14) The heat of formation is the heat of reaction when only 1 mol is produced. According to the given equation, the heat of formation of NO2 is equal to : a) -33 kJ b) -132 kJ c) +132kJ d) +33 kJ ___15) According to the diagram, the heat of reaction is best represented by arrow : a) A b) B c) C d) D ___16) The activation energy of the REVERSE reaction is represented by arrow a) A b) B c) C d) D c) 30. kJ d) 80. kJ 518 17) According to the diagram, the reaction is ____________ and the products are _____ stable. a) b) c) d) endothermic, more exothermic , more endothermic, less exothermic, less For question 18 use the following table and equation. The concentration and surface area of X remain constant, but the concentration and surface area of Y may be changed in each trial. X(l) + Y(s) + heat Z(g) Trial a b c d Concentration of Y (M) 1 1 2 2 Surface area of Y cube powdered powdered cube Temp (◦C) 100 50 150 100 ___18) Which Trial, most probably has the fastest rate of reaction ? ____19) The following potential energy diagram represents the formation of which compound? P.E Reaction Coordinate a) C2H4(g) c) NO2(g) b) C2H6(g) d) NO(g) 519 ANSWERS : 1) c the heat is to the right of the , so it's an exothermic reaction. Since it is exothermic carbon dioxide would be more stable than the reactants (if you draw a quick p.e. diagram you can see that the products of exothermic reactions are lower in energy thus more stable) However, the change in enthalpy is NOT a POSITIVE 393 kJ. The ∆H must be a NEGATIVE 393 2) a catalysts affect the activation energy and therefore the activated complex - but nothing else. See your Potential Energy notes under catalysts. Catalysts are not "consumed" or used up. 3) e The energy (kJ) is to the right of the -----> , so it's an exothermic reaction. Since it is an exothermic reaction, the products are lower (more stable) in energy 4) a The total heat is +80 kJ, but that means it is an endothermic reaction 5) a definition : see your notes 6) c check Table I. The compound with the MOST negative value is the most stable (This negative illustrates that the most energy was lost in the formation of the compound, thus producing a compound that is lower in energy and hence, more stable, and thus more difficult to react, in its own right. 7) b 8) d the bonds are already broken and you have free moving ions. Reactions of solid organic compounds are just slower. Even reactions of predissolved organic compounds are slower than reactions of dissolved ionic compounds. Reactions between ionic compounds in aqueous solution just tend to be faster, due to the already broken bonds. 9) a 10) c it has a + value 11) d not a bad definition 12) d this is the only way to get an exothermic reaction. An exo. means a -∆H. Using ∆H = Hp -Hr , only 80 kJ gives you a negative value. 13) c Combine the two arrows, their sum extends from the reactants of to the pinnacle of the graph. Yes, arrow B is the activation energy of the forward, but I wanted you to analyze the relationships between the other energies, and so, I did not give it to you as a choice. 14) d The reaction is endothermic, so the ∆H and the ∆Hƒ must be positive. Since 2 moles of product are produced the ∆H must be divided by 2. Note how the heat of formation is defined in the question. Be prepared to flex your brain like that ... according to the Regents rules ... it is perfectly acceptable to ask a question like this as long as the concept is defined first. 15) 16) 17) 18) 19) d it is the difference between the products and the reactants c c c the molarity is the greatest, the surface area is the greatest and the temperature is the greatest b the PE Diagram represents an exothermic reaction. According to Table I, C 2H6 is the only choice offered which is produced exothermically 520 NAME ____________________________________ KINETICS: TRY THIS DIRECTIONS: Use everything you know ... Go in groups, go into your notes, go big... 1) Given: 4 Al(s) + 3 O2(g) 2 Al2O3(s) + 3,351 kJ For the above reaction: a) what is the change in enthalpy? * -3,351 kJ b) what is the heat of formation for aluminum oxide? * -1676 kJ c) what would be the change in enthalpy were 8.0 moles of aluminum oxide produced? * -13408 kJ d) sketch the potential energy diagram for the production of aluminum oxide. potential energy Reaction Coordinate e) sketch the effect of a catalyst on the production of the aluminum oxide, using the above potential energy diagram. f) the reaction is endothermic / exothermic g) the products are less stable than the reactants (select one) / more stable than the reactants (select one) 521