Unit 7How do we analyze a complex chemical system? Through this course, we have learned to use thermodynamic and kinetic properties at the macroscopic level, and electronic and steric factors at the submicroscopic scaleto predict and control chemical reactions. Can you apply what you have learned to analyze a relevant complex system?
Unit 7How do we analyze a complex chemical system 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. Detect electron transfer among reacting species in a system. M1. Tracking Electron Transfer . Analyze electron sharing among reacting species in a system. M2. Detecting Electron Sharing Analyze processes occurring simultaneously in a system. M3. Analyzing Coupled Processes
NO2(g) +O2(g) NO(g) +O3(g) In which ways what we have learned can help us predict and control water acidification in our planet? 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? 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.
The central question is how to apply chemical concepts, ideas, and ways of thinking to predict and control relevant processes: Outcomes Mechanism Directionality Extent The Problem Many complex systems of interest, such as our body or our planet, are made of hundreds of substances in constant interaction.
Unit 7How do we analyze a complex chemical system? Module 1: Tracking Electron Transfer Central goal: To analyze charge distribution in chemical compounds to detect transfer of electron density among reacting species.
TransformationHow do I change it? How can we analyze the types of chemical processes in which these substances are involved? The Challenge 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.
For example, identifying whether the substances are molecular or ionic is very useful in predicting their physical and chemical properties. Substances • To face this challenge we need to analyze: • the chemical nature of the substances that comprise the system;
Let’s Think CH4 NO2 N2 H2O CO2 H2CO3 H2SO4 O2 SO2 SO3 HNO3 FeS2 NO O3 CaCO3 Al(OH)3 C6H12O6 This is a list of substances involved in or affected by water acidification in our planet.Classify them as molecular or ionic.
Properties determined by the charge and size of the ions in the ionic network. Properties determined by their molecular structure and charge distribution. The Substances Ionic: CaCO3, FeS2, Al(OH)3 Molecular: CH4, CO2, C6H12O6, H2CO3N2, NO, NO2, HNO3O2, O3, H2OSO2, SO3, H2SO4
Again, analysis of structure and charge distribution in reactants and products is crucial to make predictions about the types of processes that may occur. Substances and Reactions • In the analysis of complex chemical systems we also need to understand: • the chemical nature of the substances that comprise the system; • the characteristics of the chemical reactions in which they participate.
Acid Formationand Dissolution H2O(l) +CO2(g) H2CO3(aq) H2CO3(aq) +H2O(l) HCO3-(aq) +H3O+(aq) Acid Neutralization Al(OH)3(s) + 3H3O+(aq) Al3+(aq) + 6H2O(l) The Reactions For example, this set of interrelated chemical processes lead to water acidification in our planet: C6H12O6(g) +6O2(g) 6H2O(l) +6CO2(g) CO2 Production CH4(g) +2O2(g) 2H2O(l) +CO2(g) How do we differentiate these processes?
Thus, a chemical reaction tends to result in the transfer or redistribution of charged particles: Electrons Ions among the reacting species. d+ d- + - Chemical Reactions Most chemical reactions are driven by the interaction between positive and negative charge centers on different particles. How can we decide what is actually happening?
Let’s consider one of the triggering reactions for water acidification: CH4(g) +2O2(g) 2H2O(l) +CO2(g) Build the Lewis structure each substance. Predict their molecular geometry, and their bond and molecular polarity. Let′s think! 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.
d+ d- d- d+ d+ d- Tetrahedral Linear Bent Linear Polar Non Polar Non Polar Focus on Structure CO2 CH4 O2 H2O 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) +2O2(g) 2H2O(l) +CO2(g) d+ d+ d- d- d+ 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?
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 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.
Consider this distribution of electrons in the molecule of CO2. If the bonding electrons are assigned to the most electronegative atom: Oxidation Number How do we determine the oxidation number? 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?
FC(O) = 6 – 8 = -2 FC(C) = 4 – 0 = +4 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. 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).
d+ d+ d- d+ d- d+ d- d- -2 +4 -2 +1 0 0 +1 -2 +1 -4 Let’s Think CH4(g) +2O2(g) 2H2O(l) +CO2(g) 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. C is oxidized (loses e-density), O is reduced (gains e- density).
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) • 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.
In general, we may expect that compounds with highly electronegative atoms in high oxidation states ( ) will be good oxidizing agents(they can oxidize other substances). O2is a good oxidizing agent. On the other hand, compounds with weakly eletronegative atoms in low oxidation states ( ) will be good reducing agents (they can reduce other substances). + Hydrocarbons are good reducing agents. +1 0 0 -4 Favored Processes DG < 0 Redox Reactions
Simple Rules The assignation of the oxidation numbers of atoms in chemical compounds can be greatly facilitated by applying these basic rules: • ON = 0 for all atoms in elemental substances: O2, O3, Al, Cu. • ON = charge for monoatomic ions: • Cl-1ON = -1O-2ON = -2 • 3) Some atoms, when combined, USUALLY have the same oxidation number: • O usually is -2F always is -1 H usually is +1
Simple Rules 4)SON = 0 for a neutral polyatomic formula. CH4ON(C) + 4ON(H) = 0 If ON(H) = +1 ON(C) = - 4 CO2 ON(C) + 2ON(O) = 0 If ON(O) = -2 ON(C) = + 4 5)SON = charge for a charged polyatomic ions. NH4+ ON(N) + 4ON(H) = + 1 If ON(H) = +1 ON(N) = - 3 SO4-2ON(S) + 4ON(O) = - 2 If ON(O) = -2 ON(S) = + 6
Let’s Think 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.
0 +1 -2 0 +1 -2 +4 -2 0 +2 -2 0 +2 -2 0 +4 -2 +4 -2 0 +6 -2 Let’s Think C6H12O6(g) + 6O2(g) 6H2O(l) + 6CO2(g) Reducing Oxidizing Agents N2(g) + O2(g) 2NO(g) Reducing Oxidizing Agents 2NO(g) + O2(g) 2NO2(g) Reducing Oxidizing Agents 2SO2(g) + O2(g) 2SO3(g) Reducing Oxidizing Agents
Let′s apply! Assess what you know
N2O 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. NO2 HNO3 SO2 C2H6S(DMS) H2SO4 Natural Cycles 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.
Sulfur Cycle SO2, H2S, DMS, and SO42- are the main chemical species in this cycle. 72 SO2 leads to the formation of acids. 9 In million of tons/year FeS2
SO42-(aq) + 2CH2O(aq) + 2H3O+(aq) H2S(g) + 4H2O(l) + 2CO2(g) Anaerobic bacteria Plankton Anthropogenic Sources: DMSP DMS Enzyme Combustion of coal, which typically contains 1% to 3% sulfur in the form of pyrite (FeS2): + + H+ 4FeS2(s) + 11O2(g) 2Fe2O3(s) + 8SO2(g) Main Sources Biogenic Sources:
Let′s apply! DMSP DMS Enzyme + + H+ Analyze SO42-(aq) + 2CH2O(aq) + 2H3O+(aq) H2S(g) + 4H2O(l) + 2CO2(g) 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.
H O ON = +1 ON = -1 Further Oxidations Further oxidation of sulfur compounds in the gas phase in the atmosphere tends to follow reaction mechanisms involving free radicals. 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
Let′s apply! Analyze Consider the transformations of H2S to SO2, and of SO2 to H2SO4: • H2S to SO2 • H2S + HO HS + H2O • HS + O3 O2 + HSO • HSO + O3 HSO2 + O2 • HSO2 + O2HO2 + SO2 • SO2 to H2SO4 • SO2 + HO HOSO2 • HOSO2 + O2 HO2 + SO3 • 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-?.
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.
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. -2 +4 -2 +1 0 0 +1 -2 +1 -4 C is oxidized (loses e-density), O is reduced (gains e- density). 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.
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). • 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.
For next class, Investigate what is a Lewis acid and a Lewis base. Why is that CO2is considered a Lewis acid while H2O is a Lewis base?