Chapter13 week 4

Chapter13 week 4 - 6C Chapter 13 Week 2 and 3 Bill Trogler Kinetics The study of how fast reactions occur A topic distinct from thermodynamics

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6C Chapter 13 Week 2 and 3 Bill Trogler
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Kinetics The study of how fast reactions occur. A topic distinct from thermodynamics, which only tells whether a reaction is energetically favorable overall. Overhead 1. The speed of a chemical reaction is called the rate and is defined for the reaction: a A + b B + … … + x X + y Y + z Z Rate of rxn = (1/x) d[X]/dt = (1/z) d[Z]/dt = - (1/a) d[A]/dt = - (1/b) d[B]/dt It has units of conc/t or M/s
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Consider the Simple Combustion of Hydrogen Reaction time
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Rate of Diappearance of a Reactant at times of 5 and 10 Weeks Estimated from Tangent to Conc vs. t curve at these Times
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Rate from Tangents: At 5 weeks 0 M - 0.076 M = - 0.00633 M/wk for rate of disappearance 12 wk 0 wk Rate of rxn = 0.00633 M/wk At 10 weeks 0 M - 0.045 M = - 0.00264 M/wk for rate of disappearance 17 wk 0 wk Rate of rxn = 0.00264 M/wk
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In general: Rate(fn of T, P, Conc, Specific rxn, Phase, Solvent, Catalyst,…) For homogeneous phase reactions(e.g. all species in soln or all in gas phase), the concentration dependence of the rate is expressed in terms of the Rate Law = product of concentrations of reactants (and rarely products), each raised to an experimentally determined power. e.g. for the rxn 2 A + 3 B P Rate Law : Rate = k[A] x [B] y [P] z x, y, z are determined by expt, often z = 0 and x = 0, 1, or 2 and y = 0, 1, or 2; however, any value (e.g. ½, 0.2, 1.3) may be found. k is a constant characteristic of a rxn at a specific temperature, which is called the rate constant.
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Reaction Order Reaction Order = sum of the powers of reactants in the Rate Law For the previous example = x + y + z The order in a reactant, A, is simply its power in the Rate Law For the previous example the reaction is x order in A Most common rate laws: Zero order reaction : Rate of rxn = k First order reactions : Rate = k[A] or Rate = k[B] or rarely Rate = k[A] 0.5 [B] 0.5 Second order reactions : Rate = k[A] 2 or Rate = k[B] 2 or Rate = k[A][B]
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Units of the Rate Constant Note that k has units, which depend on the rate law, since the rate must always have units of conc/time (e.g. M/s for soln or atm/s for gas phase reactions). Zero order rate constant units are M/s First order rate constant units are 1/s or s -1 Second order rate constant units are M -1 s -1
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How do you experimentally measure the rate law and rate constant? Two methods are commonly used: 1) Method of initial rates. Measure the initial values of the rate of reaction at various concentrations of reactants and observe how the rate varies as one concentration changes while the others held constant. 2) Integrated rate law. Assume a first or second order dependence in the rate law and see if the concentration vs. time curve of a reactant obeys a first or second order
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This note was uploaded on 06/12/2008 for the course CHEM 6C taught by Professor Hoeger during the Spring '08 term at UCSD.

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Chapter13 week 4 - 6C Chapter 13 Week 2 and 3 Bill Trogler Kinetics The study of how fast reactions occur A topic distinct from thermodynamics

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