Unit 4 Module 3 - Tracking Energy.pdf

Unit 4 How do we characterize chemical processes? THREE MAIN MODULES Identifying factors that affect M1. Modeling Chemical Reactions chemical proc...

Unit 4 How do we characterize chemical processes? THREE MAIN MODULES Identifying factors that affect M1. Modeling Chemical Reactions chemical processes. Chemical Thinking Determining the amount of M2. Understanding Proportions substance formed or consumed. Predicting the amount of energy M3. Tracking Energy absorbed or released.  Review Calculate the amount of CO2 produced when 1.0 kg of ethanol (C2H6O) reacts with O2 (the products are CO2 and H2O). C2H6O(l) + 3 O2 (g)  2 CO2(g) + 3 H2O(g) 1 mol C 2 H 6 O 2 mol CO 2 44.01 g CO 2 103 g C 2 H 6 O x x x 46.07 g C 2 H 6 O 1 mol C 2 H 6 O 1 mol CO 2  1.91 kg CO 2 Mass to Mole Mole to Mole to Mass Chemical Thinking Mole i  Review If 1.0 kg of C8H18 produces 3.08 kg CO2 and 1.0 kg of C2H6O generates 1.91 kg CO2, what percent reduction in CO2 emissions do we get by replacing octane with ethanol? 1.91 kg CO 2 x 100  62.0% 3.08 kg CO 2 Close to 100 % – 62 % = 38% reduction Chemical Thinking Does this mean that ethanol is “greener” than octane? Unfortunately, it’s not a simple question. You also have to consider the energy and environmental cost of generating the ethanol vs. octane, as well as the lower energy output of burning ethanol vs. octane.  Unit 4 How do we model chemical change? Central goal: Module 3: To make qualitative and Tracking Energy quantitative predictions about the amount of Chemical Thinking energy absorbed or released during a chemical reactions based on the nature of the chemical bonds in the molecules of reactants and products.  The Challenge We would like to generate models that allow us to answer questions such as this: What amounts of How much reactants and energy will be products are needed or involved? produced? Chemical Thinking How fast will the To what extent process go will the reactants and how can I be changed into control it? products?  Modeling Chemical Reactions Reminder from Unit 4 Module 1 Assumption 1 Chemical reactions are processes in which the atoms that make up the reactants are rearranged. As a result of these rearrangements, particles with a different composition and structure are formed Chemical Thinking REACTANTS PRODUCTS  Modeling Chemical Reactions Assumption 2 Rearrangement of atoms during a chemical reaction involves electron redistribution among different atoms. As a result of this process, the internal potential energy of particles in the system changes. Chemical Thinking now much mu now energy is ? isa energy absorbed  Energy Transfer A common signature of a chemical change is the release or absorption of energy. Surroundings Surroundings System System Tsurr Tsurr heat surroundings heat temp of gets Chemical Thinking get warmer surroundings colder Heat The system The system releases energy q < 0 absorbs energy q > 0 EXOTHERMIC ENDOTHERMIC · heat away pulling from you  Exothermic or Endothermic Combustion reactions are prototypical examples of exothermic processes: CH4 + 2 O2  CO2 + 2 H 2O a ener · d more + ENERGY I The decomposition of stable compounds, such as Chemical Thinking H2O, tends to be endothermic: 2 H 2O  2 H2 + O2 How can we a n explain the + ENERGY · producemore differences?  Review some fundamentals Answer these questions individually and then discuss them in your group: yes  Does it take energy to break a chemical bond? yes  Is energy released when a chemical Chemical Thinking bond is broken? yes  Does it take energy to form a chemical bond between two atoms? Yes  Is energy released when a chemical bond is formed?  A Chemical Model During a chemical reaction some bonds are broken and some new bonds are formed. a formed another broven o Chemical Thinking reactants TS products (transition State) The amount of energy released or absorbed in a formed chemical reaction is related to the types broken a bonds matter of number and numbers of bonds broken or formed type matter a bonds in the process.  Bond Energy Energy is needed to separate a pair of bonded atoms (“break” the bond). The energy provided is transformed into the potential energy of the separated atoms. Ep By convention, Chemical Thinking 0 Ep = 0 when the atoms are infinitely separated. add to energy raise potential atoms are most stable in energy a bond override that to break you must ADDING energy. by  Bond Energy Bond formation corresponds to a minimum in the potential energy Ep D to break energy required a bond - Dissociation Energy Chemical Thinking BOND ENERGY IBE)BDEdissociation energy dissapates Tthermal energy kinetic as energy Potential energy is transformed into kinetic energy when a bond is formed (energy is released).  Consider the information presented in these graphs: Chemical Thinking Build a reasonable explanation to justify these results. the stronger the bond , the more bond required · energy  Construct PE diagrams for a C-C, C=C and a C≡C bond. Clearly justify your reasoning Ep Ep Ep C-C C=C C≡C Chemical Thinking 0 vvv 0 0  Let’s analyze this reaction: CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g) What is the net energy released or absorbed? endo exothermic and by nuch ? how Ep or 24H 48 1C + 20 4H + 20 0 ↑ ↑ ↓ ! energy released when forming bonds Chemical Thinking energy needed do break the bonds releasing 4 CH 4x 414 energe 20 = 2 498 = : =* 164645 = 99625 2C= 0 = 2 799 + 4) 0 H. = 4 464. motel 1598k5 1856k5 , per = = = - Ziond Zeronds mole) bonds 34 , per 2 CH4 + 2 O2  CO2 3 + 2 H 2O bonds balanced equation as all in breaking 1 02 molecules  Heat of Reaction Ep C+H+H+H+H+O+O+O+O 0 + 265225 & Dissociation CH4 + 2 O2 -2652 kJ · 3454k5 Formation Chemical Thinking CO2 + 2 H2O -3454 kJ enthalpy Net Energy Released = - 802 25 : Ah = gren head of negativever reaction ↓ carrie ↓  Given the information provided, build the “Energy Diagram” for the decomposition of water. How much energy is released or absorbed? & Doa = 2 H-O-H  2 H-H 2 436 + O=Oneedso 498 divul /energyove. = 872 Bond Dissoc. Energy Ep 0 928 or a molar -M ↓ Chemical Thinking (KJ/mol) - 2x436 - 498 O-H 464 "bondsclean He H-H 436 48645 O=O 498 SH = H20 · decomposing 2 ad What happens to the energy we put in? It is stored as Ep in the products  Given the information provided, build the “Energy Diagram” for the production of HCl. e How much energy is released or absorbed? Jasmine Bond Energy __H I + __Cl I  __HCl 2 Chemical Thinking 2 2 (kJ/mol) H-H 436 436 - + 248 Endo or Exo? 428 2 Cl-Cl 240 676. H-Cl 428 H2 + Cl2 2 HCl856 - 188 - - H = 436 + 240 – (2 x 428) = -180 kJ  Compare these two sets of bond energies: Bond Energy Bond Energy Which bonds tend (kJ/mol) (kJ/mol) to be stronger A-A H-H 436 C-H 414 or A-B? Why? not the same atom bonds tendto C-C 347 O-H 464 form stronger bonds O-O 142 C-O 360 N-N 163 N-H 389 Based on this trend, predict whether each of these Chemical Thinking reactions is likely to be exothermic or endothermic: stronger stronger bonds (+) weaker 1 bands- 2 bonds ~ _____________ ative W A-A + B-B  2 A-B -no pinds _____________ 2 A-B-C  2 A + 2 B + C-CI endothermic exothermic a nedoverall positive  Evaluating Energy Sources In daily life, we generate energy by using reactants with high potential energy (weaker bonds) to produce products with lower potential energy (stronger bonds). Chemical Thinking  Let’s Think Ethanol, C2H6O, is now used as an alternative or as a supplementary fuel in many vehicles. 1. Estimate the heat of reaction per mole of I C2H6O burned; 2. The PE of Chemical Thinking ethanol is lower than that of CO2 and H2O. Why is this process exothermic? need to consider # of bonds and type net change  Evaluating Energy Sources The ideas discussed in this module are of central importance in making decisions about what energy source may be best in different circumstances. For example, which substance is a better fuel? Chemical Thinking Methane, CH4 Formaldehyde, CH2O Methanol, CH4O  Start with the reaction Methane, CH4 Formaldehyde, CH2O Methanol, CH4O 1. Write the balanced chemical equation for the energy production reaction involving ONE mole Chemical Thinking of the fuel. CH4(g) + O2(g)  CO2(g) + H2O(g) CH2O(g) + O2(g)  CO2(g) + H2O(g) CH4O(g) + 32O2(g)  CO2(g) + 2 H2O(g) ↳ in order to have I mol of fuel  Consider the molecule structures I moloaf CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g) CH2O(g) + 1 O2(g)  CO2(g) + H2O(g) 1412 799 Bond Energy 211L7 12/12/ 21/L 1114414 (kJ/mol) Chemical Thinking C-C 347 · 88245/mol C-O 360 2. Estimate Hrxn for the C-H 414 combustion of one mole O-H 464 of formaldehyde; O=O 498 C=O 799  Heat of Reaction CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g) 4(414 2(498) 2(799) 41464) = -80225/d - AH - + = -80225/mol AH 2652 3454 = - = Chemical Thinking CH2O(g) + 1 O2(g)  CO2(g) + H2O(g) DH = 2(41415799 + 498 - 2(799) - 2(464) = - 401k5/ wol  CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g) 0 Ep Bond Energy (kJ/mol) C-C 347 C-O 360 Ep 0 C-H 414 ↑ + Chemical Thinking O-H 464 O=O 498 C=O 799 CH2O(g) + 1 O2(g)  CO2(g) + H2O(g)  Given the information we have, estimate the heat of reaction for the combustion of one mole of methanol CH-0 + 3202 +CO2 & 2HeO Bond Energy (kJ/mol) A = 3(414) + 360 + 464 + 3/21498) - C-C 347 Chemical Thinking 2 C-O 360. 799-21464 C-H 414 2 813-. 1 717. : O-H 464 O=O 498 C=O 799  Let’s Make Sense of the Data 3. Identify and explain overall trends in the value of Hrxn with increasing degree of oxidation (Hint: Compare bond energies for reactants and products). CH4O Bond Energy (kJ/mol) Chemical Thinking C-C 347 C-O 360 C-H 414 Hrxn = Hrxn = Hrxn = O-H 464 ______ ______ ______ kJ/mol kJ/mol kJ/mol O=O 498 C=O 799  Important Trends The more oxygen is used during combustion of burned fuel, the more energy will be released. In that sense, “oxygenated” Chemical Thinking fuels will produce less energy per mole. However, ½[ ] they normally generate less CO and other pollutants. There are always costs and benefits!  Now consider other fuels Most of the energy our body needs is supplied by the combustion of carbohydrates and lipids. The most common carbohydrate is glucose. Most lipids result from the combination of fatty acids, such as oleic acid. C6H12O6 Chemical Thinking C18H34O2 Predict which combustion reaction will produce more energy per mole of each substance.  Estimate Verify your prediction by estimating the amount of energy produced by the combustion of one mole of each substance. C6H12O6 Bond Energy (kJ/mol) C-C 347 Chemical Thinking C=C 611 C-H 414 O-H 464 O=O 498 C18H34O2 C-O 360 C=O 799  Remember, energy is needed to break bonds; energy is released when bonds form. C6H12O6 + 6 O2  6 CO2 + 6 H2O C-C C-H C=C C-O O-H C=O O=O B 5x347 7x414 0 5x360 5x464 1x799 6x498 12540 F 0 0 0 0 -12x464 -12x799 0 -15156 Hmodel = -2.62 x 103 kJ/mol Hexp = -2.80 x 103 kJ/mol Chemical Thinking C18H34O2 + 51/2 O2  18 CO2 + 17 H2O C-C C-H C=C C-O O-H C=O O=O B 16x347 33x414 1x611 1x360 1x464 1x799 25.5x498 34147 F 0 0 0 0 -34x464 -36x799 0 -44540 Hmodel = -1.04 x104 kJ/mol Hexp = -1.12 x 104 kJ/mol  It is common to express the energy produced during fuel combustion in amount of energy per unit mass (energy density; kJ/g). Glucose C6H12O6 Hexp = -2.80 x 103 kJ/mol Oleic Acid C18H34O2 Hexp = -1.12 x 104 kJ/mol Determine and compare the energy density, Chemical Thinking in kcal/g = Cal/g , for these two substances. Discuss why this quantity may be more useful for purposes of fuel analysis and evaluation than its equivalent in kJ/mol. 1 calorie = 1 cal = Energy required to raise the temperature of 1.00 g of H2O by 1.0 oC = 4.184 J  Compare Glucose C6H12O6 Hexp = -2.8 x 103 kJ/mol M = 180.2 g/mol Oleic Acid C18H34O2 Hexp = -11180 kJ/mol M = 282.5 g/mol Chemical Thinking Chemical Thinking Are You Ready?  The Quest for Ammonia Ammonia, NH3, is one of the most important industrial chemical substances. It is widely used in the production of fertilizers, pharmaceuticals, refrigerants, Chemical Thinking explosives, and cleaning agents. It ranks as one of the 10 top chemicals substances produced annually in the world.  The Synthesis Ammonia is mainly produced via this simple chemical reaction: N2(g) + H2(g) NH3(g) Balance this chemical reaction, complete the following particulate representation of the process, and identify the limiting reactant. Chemical Thinking Hydrogen Nitrogen  Energy Transfer The optimization of the industrial production of ammonia requires a good understanding of energy transfer during the process: N2(g) + 3 H2(g) 2 NH3(g) Estimate the heat of reaction for this process and determine whether the Chemical Thinking reaction is exo or endothermic. Bond Dissoc. Energy (kJ/mol) N-H 389 H-H 436 N≡N 945  Let’s Think N2(g) + 3 H2(g) 2 NH3(g) N≡N H-H N-H B 1x945 3x436 0 2253 F 0 0 -6x389 -2334 Hmodel = Hexp = -92 kJ/mol Chemical Thinking Sketch the energy profile for this process and discuss whether the forward or backward processes can be expected to be favored.  Competing Factors Ep N2(g) + 3 H2(g) 2 NH3(g) The forward process has a lower Ea. The backward process is N2 + 3 H2 expected to have a higher Chemical Thinking % of effective configurations. 2 NH3 Consider the effect of increasing T and P on both reaction rate and reaction extent.  The Decision The reaction is run at higher T with a lower yield (10-20%) to make it go fast, extracting NH3 as it forms to keep the process going. Haber Process Catalysts, such Chemical Thinking as powdered iron, are used to reduce the Ea.