Chapter 11 - Chapter 11 Chemical Kinetics Dr. M. P. Soriaga...

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Unformatted text preview: Chapter 11 Chemical Kinetics Dr. M. P. Soriaga Department of Chemistry Texas A&M University Thermodynamically and kinetically favored reaction: 2HCl (aq) + Mg(OH) 2(s) MgCl 2(aq) + 2H 2 O (l) G o = 97 kJ/mol Thermodynamically favored but kinetically hindered: C (diamond) + O 2(g) CO 2(g) G o = 397 kJ/mol Thermodynamics and Kinetics Thermodynamics: Is a reaction favorable? Kinetics: Will a reaction actually occur within a given period of time? Thermodynamics and Kinetics Initial State Final State Thermodynamics Heat Content Progress of a Reaction Activated State Kinetics Chemical Kinetics Study of rates of chemical reactions: how fast reactants are consumed and how fast products are formed Reaction: Reaction rate: Change of concentration of reactant or product per unit time Chemical Kinetics Measurement of reaction rate : Plot the increase in concentration of product or the decrease in concentration of reactant as a function of time Study of reaction mechanisms: atomic-level sequence of elementary events that lead to formation of product(s) Chemical Kinetics + + + + Overall Reaction: O 3 + NO NO 2 + O 2 Possible Sequence of Events: Reaction Mechanism Ozone-to-Dioxygen Decomposition 2O 3(g) 3O 2(g) The Rate of a Reaction For a reaction such as 2N 2 O 5(g) 4NO 2(g) + O 2(g) the rate of reaction may be written as: Rate of decrease of concentration of reactants } Rate of increase of concentration of products } } Rate M s = 1 2 t N 2 O 5 [ ] = 1 4 t NO 2 [ ] = 1 1 t O 2 [ ] M = mol rxn/L solution Rates of Reaction Initial rate: The negative of the slope of the tangent to the concentration- vs.-time curve at t = 0 Rates of Reaction Instantaneous rate: The negative of the slope of the tangent to the concentration- vs.-time curve at time t Rates of Reaction Average rate: The negative of the slope of the concentration- vs.-time curve for some time interval t Factors Affecting Reaction Rates: Physical Nature of Reactants The more finely-divided the reactant particles, the greater the surface area, the faster the reaction Factors Affecting Reaction Rates: Chemical Nature of Reactants The more chemically-active the reagent, the faster the reaction Factors Affecting Reaction Rates: Concentration of Reactants An increase in reactant concentration generally increases the rate of a reaction An increase in temperature increases the rate of any reaction Factors Affecting Reaction Rates: Temperature Factors Affecting Reaction Rates: Presence of a Catalyst Catalysts increase the rates of reactions The reactions must be thermodynamically favored The Rate Law The Rate Law relates the rate to reactant concentration Reaction: a A + b B c C + d D Rate = k [A] x [B] y 1 a dA dt = 1 b dB dt = 1 c dC dt = 1 d dD dt =k[A] x [B] y k specific rate constant x and y reaction orders x + y overall reaction order x and y are not necessarily related to a and b Determined from experiment, not from the net...
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This note was uploaded on 02/23/2011 for the course CHEM 101 taught by Professor Williamson during the Spring '08 term at Texas A&M.

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Chapter 11 - Chapter 11 Chemical Kinetics Dr. M. P. Soriaga...

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