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Unit 7 How do we analyze a complex chemical system? Through this course, we have learned to use thermodynamic and kinetic properties at the macroscopic level, and Chemistry XXI electronic and steric factors at the submicroscopic scale to predict and control chemical reactions. Can you apply what you have learned to analyze a relevant complex system? Unit 7 How do we analyze a complex chemical system Chemistry XXI The central goal of this unit is to apply and extend central concepts and ideas discussed in this course to the analysis of a complex chemical system. M1. Tracking Electron Transfer Detect electron transfer among reacting species in a system. M2. Detecting Electron Sharing Analyze electron. sharing among reacting species in a system. M3. Analyzing Coupled Processes Analyze processes occurring simultaneously in a system. Context To illustrate the power of the concepts, ideas, and ways of thinking discussed in the course, we will focus our attention on understanding the causes and effects of water acidification in our environment. NO2(g) + O2(g) NO(g) + O3(g) Chemistry XXI CO2(g) + H2O(l) H2CO3(aq) CO32-(s) + H3O+(aq) HCO3-(aq) + H2O(l) C(s) + O2(g) CO2(g) C8H18(l) + 12.5O2(g) 8CO2(g) + 9H2O(g) Why do we care? In which ways what we have learned can help us predict and control water acidification in our planet? The Problem Many complex systems of interest, such as our body or our planet, are made of hundreds of substances in constant interaction. The central question is how to apply chemical concepts, ideas, and ways of thinking to predict and control relevant processes: Chemistry XXI Outcomes Mechanism Directionality Extent Unit 7 How do we analyze a complex chemical system? Chemistry XXI Module 1: Tracking Electron Transfer Central goal: To analyze charge distribution in chemical compounds to detect transfer of electron density among reacting species. The Challenge Transformation How do I change it? Chemistry XXI We live in a complex environment made of hundreds of different substances in constant chemical interaction. Some of these interactions are crucial for the survival of life on Earth; others threaten several ecosystems. How can we analyze the types of chemical processes in which these substances are involved? Substances To face this challenge we need to analyze: Chemistry XXI a) the chemical nature of the substances that comprise the system; For example, identifying whether the substances are molecular or ionic is very useful in predicting their physical and chemical properties. Let’s Think CH4 H 2O NO2 N2 CO2 H2CO3 O2 FeS2 SO2 SO3 HNO3 NO Chemistry XXI H2SO4 CaCO3 C6H12O6 O3 Al(OH)3 This is a list of substances involved in or affected by water acidification in our planet. Classify them as molecular or ionic. The Substances Ionic: CaCO3, FeS2, Al(OH)3 Chemistry XXI Properties determined by the charge and size of the ions in the ionic network. Molecular: CH4, CO2, C6H12O6, H2CO3 N2, NO, NO2, HNO3 O2, O3, H2O SO2, SO3, H2SO4 Properties determined by their molecular structure and charge distribution. Substances and Reactions In the analysis of complex chemical systems we also need to understand: a) the chemical nature of the substances that comprise the system; Chemistry XXI b) the characteristics of the chemical reactions in which they participate. Again, analysis of structure and charge distribution in reactants and products is crucial to make predictions about the types of processes that may occur. The Reactions For example, this set of interrelated chemical processes lead to water acidification in our planet: Chemistry XXI CO2 Production Acid Formation and Dissolution C6H12O6(g) + 6O2(g) 6H2O(l) + 6CO2(g) CH4(g) + 2O2(g) 2H2O(l) + CO2(g) H2O(l) + CO2(g) H2CO3(aq) H2CO3(aq) + H2O(l) HCO3-(aq) + H3O+(aq) Acid Al(OH)3(s) + 3H3O+(aq) Neutralization Al3+(aq) + 6H2O(l) How do we differentiate these processes? Chemical Reactions Most chemical reactions are driven by the interaction between positive and negative charge centers on different particles. Chemistry XXI Thus, a chemical reaction tends to result in the transfer or redistribution of charged particles: Electrons Ions d+ d- + - among the reacting species. How can we decide what is actually happening? Focus on Structure To distinguish types of reactions and make predictions about reactivity we need to have a good idea of the structural features of reactants and products. Let’s consider one of the triggering reactions for water acidification: Chemistry XXI CH4(g) + 2O2(g) 2H2O(l) + CO2(g) Let′s think! Build the Lewis structure each substance. Predict their molecular geometry, and their bond and molecular polarity. Focus on Structure CH4 O2 d+ CO2 H2O d+ d- d- d+ Chemistry XXI d- Tetrahedral Linear Bent Linear Non Polar Non Polar Polar Non Polar We can gain insights about chemical reactions by analyzing the changes they induce in the electron density around each atom. Charge Transfer CH4(g) d+ + 2O2(g) 2H2O(l) d+ d- + CO2(g) d- d+ Chemistry XXI d- Combustion reactions belong to an important type of chemical processes characterized by the transfer of electron density from one atom to another. C goes from having d- to d+ (loses electron density). O goes from neutral to d- (gains electron density). How can we better characterize this charge transfer? Oxidation Number Chemistry XXI The extent to which a reaction leads to electron density transfer can be assessed by analyzing changes in the oxidation number (or state) of each atom. The oxidation number is defined as the partial charge that an atom in a molecule would have if all of the bonding electrons were assigned to the most electronegative atom in the bond (molecule seen as fully ionic). H+ H+ C-4 H+ H+ Oxidation Number How do we determine the oxidation number? Consider this distribution of electrons in the molecule of CO2. Chemistry XXI If the bonding electrons are assigned to the most electronegative atom: C has 0 valence e- in the molecule. O has 8 valence e- in the molecule. Central Question: How does these numbers compare with what they would have in their elemental form? Oxidation Number To calculate the oxidation number (ON) we compare the number of valence electron each atom has with those that it would have in its elemental form: ON = # of valence e- of the elemental atom – # valence e- in fully ionic molecule. Chemistry XXI FC(O) = 6 – 8 = -2 FC(C) = 4 – 0 = +4 Notice that SON = charge of molecule We say carbon in the molecule is in a highly oxidized state (largest positive ON), while oxygen is in a highly reduced state (largest negative ON). Let’s Think CH4(g) 2O2(g) + 2H2O(l) + CO2(g) d+ d+ Chemistry XXI d- d- d+ d- d+ d- Assign ONs to all of the atoms in CH4, O2, and H2O. Identify which atoms are “oxidized” (its oxidation number increases) and which ones are “reduced” (its oxidation number decreases) in this process. +1 0 0 +1 -2 +1 -2 -4 C is oxidized (loses e- density), O is reduced (gains e- density). +4 -2 Redox Reactions Combustion reactions are typical examples of oxidation-reduction (redox) reactions in which the oxidation number of the atoms involved changes, signaling a transfer of electron density. CH4(g) + 2O2(g) 2H2O(l) + CO2(g) Chemistry XXI Assigning oxidation numbers is useful in: Identifying electron-rich and electron-poor centers in molecules; Tracking electron transfer or redistribution during a chemical reaction; Making predictions about reaction directionality. Redox Reactions In general, we may expect that compounds with highly electronegative atoms in high oxidation states ( 0 ) will be good oxidizing agents (they can oxidize other substances). Chemistry XXI 0 On the other hand, compounds with weakly eletronegative atoms in low oxidation states ( 0 ) will be good reducing agents (they can reduce other substances). 0 O2 is a good oxidizing agent. + +1 -4 DG < 0 Hydrocarbons are good reducing agents. Favored Processes Simple Rules The assignation of the oxidation numbers of atoms in chemical compounds can be greatly facilitated by applying these basic rules: 1) ON = 0 for all atoms in elemental substances: O2, O3, Al, Cu. Chemistry XXI 2) ON = charge for monoatomic ions: Cl-1 ON = -1 O-2 ON = -2 3) Some atoms, when combined, USUALLY have the same oxidation number: O usually is -2 F always is -1 H usually is +1 Simple Rules 4) SON = 0 for a neutral polyatomic formula. CH4 ON(C) + 4ON(H) = 0 If ON(H) = +1 ON(C) = - 4 CO2 ON(C) + 2ON(O) = 0 If ON(O) = -2 ON(C) = + 4 Chemistry XXI 5) SON = charge for a charged polyatomic ions. NH4+ ON(N) + 4ON(H) = + 1 If ON(H) = +1 ON(N) = - 3 SO4-2 ON(S) + 4ON(O) = - 2 If ON(O) = -2 ON(S) = + 6 Let’s Think Chemistry XXI Most of the reactions that generate the substances that are ultimately responsible for water acidification in our planet are redox reactions: The relevant substances are oxides of non metallic elements. C6H12O6(g) + 6O2(g) 6H2O(l) + 6CO2(g) N2(g) + O2(g) 2NO(g) 2NO(g) + O2(g) 2NO2(g) 2SO2(g) + O2(g) 2SO3(g) Identify the oxidized and reduced atoms as well as the oxidizing and reducing species. Let’s Think 0 +1 -2 0 C6H12O6(g) + Reducing Oxidizing + 2NO(g) Chemistry XXI +2 -2 2NO(g) O2(g) +2 -2 Agents 0 + Reducing O2(g) Oxidizing +4 -2 Reducing Agents Oxidizing 0 + O2(g) Oxidizing +4 -2 6H2O(l) + 6CO2(g) 0 Reducing 2SO2(g) 6O2(g) 0 N2(g) +1 -2 +4 -2 2NO2(g) Agents +6 -2 2SO3(g) Agents Chemistry XXI Let′s apply! Assess what you know Natural Cycles Chemistry XXI Compounds of sulfur and nitrogen play a central role at various levels in our planet. They participate in natural cycles that have been altered by human activities. Compounds of sulfur and nitrogen, for example, not only are the cause of “acid rain” but also contribute to Global Warming and to the depletion of the Ozone Layer. N 2O NO2 HNO3 SO2 H2SO4 C 2H 6S (DMS) Sulfur Cycle Chemistry XXI SO2, H2S, DMS, and SO42- are the main chemical species in this cycle. 72 SO2 leads to the formation of acids. In million of tons/year 9 FeS2 Main Sources Biogenic Sources: SO42-(aq) + 2CH2O(aq) + 2H3O+(aq) H2S(g) + 4H2O(l) + 2CO2(g) Anaerobic bacteria CH3 S H 3C DMSP + CH2 CH2 O Enzyme H 3C S C OH DMS H 3C + O H 2C C CH OH + H+ Chemistry XXI Plankton Anthropogenic Sources: Combustion of coal, which typically contains 1% to 3% sulfur in the form of pyrite (FeS2): 4FeS2(s) + 11O2(g) 2Fe2O3(s) + 8SO2(g) Analyze Let′s apply! SO42-(aq) + 2CH2O(aq) + 2H3O+(aq) CH3 S H 3C DMSP + CH2 CH2 O DMS S C OH Chemistry XXI Enzyme H 3C H2S(g) + 4H2O(l) + 2CO2(g) H 3C + O H 2C C CH OH + H+ 4FeS2(s) + 11O2(g) 2Fe2O3(s) + 8SO2(g) Are these redox reactions? If they are, identify the oxidized and reduced atoms, and the oxidizing and reducing agents. Further Oxidations Further oxidation of sulfur compounds in the gas phase in the atmosphere tends to follow reaction mechanisms involving free radicals. Chemistry XXI hn The hydroxyl radical HO, constantly formed and destroyed in the atmosphere, plays a central role in these processes. O3 O2 + O O + H2O 2HO H ON = +1 O ON = -1 Let′s apply! Analyze Consider the transformations of H2S to SO2, and of SO2 to H2SO4: H2S to SO2 1) H2S + HO HS + H2O Chemistry XXI 2) HS + O3 O2 + HSO SO2 to H2SO4 1) SO2 + HO HOSO2 3) HSO + O3 HSO2 + O2 2) HOSO2 + O2 HO2 + SO3 4) HSO2 + O2 HO2 + SO2 3) SO3 + H2O H2SO4 a) In which of these steps are sulfur atoms being? Oxidized; b) How is their ON changing in each step? c) Is this change due to the actual transfer of e-?. Chemistry XXI Working in pairs, summarize the important chemical information that can be derived from the analysis of the oxidation numbers of atoms in reactants and products. Tracking Electron Transfer Summary Oxidation-Reduction (Redox) are an important type of chemical processes characterized by the transfer of electron density from one atom to another. Chemistry XXI The extent to which a reaction leads to electron density transfer can be assessed by analyzing changes in the oxidation number (or state) of each atom. +1 0 0 +1 -2 +1 -2 -4 C is oxidized (loses e- density), O is reduced (gains e- density). +4 -2 Oxidation Numbers The oxidation number is defined as the partial charge that an atom in a molecule would have if all of the bonding electrons were assigned to the most electronegative atom in the bond (molecule seen as fully ionic). Chemistry XXI Assigning oxidation numbers is useful in: Identifying electron-rich and electron-poor centers in molecules; Tracking electron transfer or redistribution during a chemical reaction; Making predictions about reaction directionality. Chemistry XXI For next class, Investigate what is a Lewis acid and a Lewis base. Why is that CO2 is considered a Lewis acid while H2O is a Lewis base?