7
How Unit 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 Chemistry XXI 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
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. Analyze electron sharing among . reacting species in a system. Analyze processes occurring simultaneously in a system.
Chemistry XXI
M2. Detecting Electron Sharing M3. Analyzing Coupled Processes
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.
2 2 3
NO (g) + O (g) NO(g) + O (g)
2 3
CO (g) + H O(l) H CO (aq)
323 + 32
Chemistry XXI
CO (s) + H O (aq) HCO (aq) + H O(l)
2 2
8
18
C(s) 2+ O (g) CO (g) 2
2
C H (l) + 12.5O (g) 8CO (g) + 9H O(g)
In which ways can we use what we have learned to help us predict and control water acidification in our planet?
Why do we care?
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: Outcomes Chemistry XXI Mechanism Directionality Extent
Unit 7
How 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.
Chemistry XXI
The Challenge
Transformation How do I change it?
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?
Chemistry XXI
Substances
To face this challenge we need to analyze: a) the chemical nature of the substances that comprise the system;
Chemistry XXI
For example, identifying whether the substances are molecular or ionic is very useful in predicting their physical and chemical properties.
Lets Think
NO
Chemistry XXI
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: Properties determined by the charge and size of the ions in the ionic network. Molecular: Chemistry XXI
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; 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.
Chemistry XXI
The Reactions
For example, this set of interrelated chemical processes lead to water acidification in our planet:
Chemistry XXI
Acid Formation and Dissolution Acid Neutralization
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. Thus, a chemical reaction tends to result in the transfer or redistribution of charged particles: Electrons Chemistry XXI Ions + -
-
+
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. Lets consider one of the triggering reactions for water acidification:
Chemistry XXI
Lets think!
Build the Lewis structure each substance. Predict their molecular geometry, and their bond and molecular polarity.
Focus on Structure
Chemistry XXI
We can gain insights about chemical reactions by analyzing the changes they induce in the electron density around each atom.
Charge Transfer
Combustion reactions belong to an important type of chemical processes characterized by the transfer of electron density from one atom to another. Chemistry XXI C goes from having - to + (loses electron density). O goes from neutral to - (gains electron density). How can we better characterize this charge transfer?
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) each of atom.
Chemistry XXI
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+ -4 H+ H+ C
Oxidation Number
How do we determine the oxidation number? Consider this distribution of
If the bonding electrons are assigned to the most electronegative atom:
Chemistry XXI
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:
FC(O) = 6 8 = -2 Chemistry XXI FC(C) = 4 0 = +4
Notice that ON = 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).
Lets Think
+ + + +
Chemistry XXI
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.
Assigning oxidation numbers is useful in: Chemistry XXI 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 ( ) will be good oxidizing agents 0 (they can oxidize other substances).
0 0
Chemistry XXI
On the other hand, compounds with weakly eletronegative atoms in low oxidation states ( ) 0 will be good reducing agents (they can reduce other substances).
+1 -4
+
Hydrocarbons are good reducing agents.
G < 0
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:
Chemistry XXI
Simple Rules
4) ON = 0 for a neutral polyatomic formula.
Chemistry XXI
Lets 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.
Chemistry XXI
Identify the oxidized and reduced atoms as well as the oxidizing and reducing species.
Lets apply!
Chemistry XXI
Assess what you know
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. 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.
Chemistry XXI
Sulfur Cycle
72
Chemistry XXI
In million of tons/year
9
FeS2
Main Sources
Biogenic Sources:
Anaerobic bacteria
CH3 S H3C
+
DMSP
CH2 CH2 O C OH
Enzyme
H3C H3C
DMS
S
O H2C CH C OH
+
Plankton
Chemistry XXI
Anthropogenic Sources: Combustion of coal, which typically contains 1% to 3%
Lets apply!
Analyze
CH3 S H3C
+
DMSP
CH2 CH2 O C OH
Enzyme
H3C H3C
DMS
S
O H2C CH C OH
+
Chemistry XXI
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.
h
Chemistry XXI
The hydroxyl radical HO, constantly formed and destroyed in the atmosphere, plays a central role in these processes.
H
ON = +1
O
ON = -1
Lets apply!
Analyze
Chemistry XXI
a) In which of these steps are sulfur atoms being? Oxidized; b) How is their ON changing in each step?
3 2
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. 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. Chemistry XXI
+1 -4 0 0 +1 -2 +1 -2 +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). Assigning oxidation numbers is useful in: Chemistry XXI 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,
Chemistry XXI Investigate what is a Lewis acid and a Lewis base.