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Chem 6c chap 12 week 2

Chem 6c chap 12 week 2 - 6C Chapter 13 Week 2 Bill Trogler...

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6C Chapter 13 Week 2 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]
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