Chemistry Week 10

Chemistry Week 10 - Chemical Processes and Reactions...

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Chemical Processes and Reactions TEXTBOOK READING : BLB-10 , Chapters 4; 16.1-2, 10, 11; 20. 1-2. Practice PROBLEMS: (Ch 4) 22, 26, 32, 40, 47, 50; (Ch 16) 1, 18, 26, 102; (Ch 20) 18, 20 (1) Thermodynamics vs. Kinetics Chemical reactions are typically spontaneous processes over which we try to exert some control – that is, we try to get them to go faster or slower, to release energy in the form of work or heat, etc. To understand the fundamental nature of chemical processes, we must utilize concepts in both thermodynamics and kinetics . The most general expression for a chemical reaction is Reactants Conditions ⎯⎯⎯⎯→ Products in which “Conditions” refers to certain characteristics external to the reaction we can control. In the thermodynamic sense, the chemical reaction is the “system,” the environment in which the reaction occurs is the “surroundings.” The conditions we can control include temperature, pressure, amounts of reactants and/or products (typically expressed either as moles or concentrations, when a reaction occurs in solution), catalysts present, etc. We can diagram how a reaction takes place by plotting the concentration of reactants and products vs. time: Time [n] Products Reactants Time [n] Products Reactants “Reactants Conditions Products” (Reaction “goes to completion”) [n] = concentration of chemical species (M = moles / liter) “Reactants Conditions ZZZZZX Z YZZZZZZ Products” (Reaction establishes chemical equilibrium) In both cases above, equilibrium is established when the concentrations (amounts) of products and reactants remain constant in time. There are two fundamental questions: (a) What drives a chemical reaction ? This question refers to the thermodynamics of the system, i.e., concepts including enthalpy , entropy , Gibbs free energy , chemical potential , which are critical for understanding chemical equilibrium (reversible processes). Reaction outcomes like precipitation of a solid or formation of a gas may lead to a spontaneous process.
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(b) How does a chemical reaction occur ? This addresses kinetics , which includes reaction rates (how fast?), mechanisms (paths), and activation energy (energies needed to break chemical bonds in the reactants). Throughout these two fundamental areas, always utilize concepts from chemical periodicity and bonding to help understand chemical behavior, e.g., intermolecular and interatomic (bonding) forces, electronegativity, size issues, molecular orbitals and relative energies, etc. How do you know when a chemical reaction has reached its completion or the state of equilibrium? In principle, one can never really know, but once we can no longer measure any changes, then we consider equilibrium to be established. Thus, assignment of equilibrium depends on the precision and accuracy of our instrumentation designed to study reactions. There are classes of reactions called oscillating reactions , which have complex kinetics (rates, mechanisms) and thermodynamics (energetic and entropic issues). The reaction shown in class uses Ce(IV) / Ce(III) ions to catalyze the reaction between malonic acid (CH 2 (COOH) 2
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This note was uploaded on 03/27/2008 for the course CHEM 201 taught by Professor Miller during the Fall '07 term at Iowa State.

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Chemistry Week 10 - Chemical Processes and Reactions...

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