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Unformatted text preview: Chapter 21 Chemical Kinetics I. The Basic Ideas Chapter 21 “Laws, Experiments, and Measurements” what we do in the lab Chapter 22 “Theory and Mechanism” how we explain what was measured SUGGESTION: grab a general chem. textbook and read the kinetics chapter Time Rate Rate Laws General expression for a chemical reaction, “The Overall Chemical Reaction” aA + bB → cC + dD This reaction shows the stoichiometry given by the coefficients: a, b, c, and d This is DIFFERENT from the mechanism which is the detailed pathway from reactants to products. Rate of Reaction: dt E d dt C d dt B d dt A d Rate ] [ ] [ ] [ ] [ = = = = Let’s take an equally nondescript but simple reaction… 2 A + B → C + D Rate dt A d dt C d dt B d t t t t t t = = = = = = 1 1 1 ] [ 2 1 ] [ ] [ Rate Law is a formula that indicates the rate of change of the concentrations. It is usually a “Power” law depending on the concentrations of either reactants or products. An example rate law: [ ] [ ] [ ] [ ] B C A k dt B d Rate 2 / 1 ⋅ ⋅ = = 21 [ ] time Bo Ao [ C ], [ D ] [ B ] [ A ] Concentration Profile [A] at t = 0 is Ao [B] at t = 0 is Bo t = t 1 Note that the exponents may be nonintegral or even negative, but STOICHIOMETRY does NOT dictate exponents. We would also like to know how [ B ] varies with time DIRECTLY, [B](t). This is called the INTEGRATED RATE LAW. In the Iodine Clock lab we are set up to measure rates, but in the Ethyl Acetate Hydrolysis lab we measure something closer to the integrated rate. Simple rate laws: [ ] A k Rate ⋅ = simple first order rate law [ ] 2 A k Rate ⋅ = simple second order rate law [ ] [ ] B A k Rate ⋅ ⋅ = also a second order rate law [ ] [ ] [ ] C B A k Rate ⋅ ⋅ = more complicated…here C acts as an inhibitor “k” is called the rate constant. It has units that are consistent with the equation. What are the units of “Rate” ?? As time goes on, Rate generally decreases. That is because the Rate depends on concentrations that are decreasing. The Rate Law MUST be consistent with the mechanism and vice versa. The transition from rate law to integrated rate law. First Order: [ ] [ ] A k dt A d a Rate ⋅ = = 1 The rate constant, k, only changes if T (temp) changes. Otherwise it is a constant factor with a specified temperature. To make this a little easier for you, let x = [A] Then the rate law becomes x a k dt dx ⋅ ⋅ = We next separate variables to give dt a k x dx ⋅ ⋅ = We next integrate both sides of the equation between corresponding limits. For instance, assume that at t = 0, [ A ] = Ao, and therefore, x = Ao And later, t = t 1 when [ A ] = A 1 = x(t 1 ). ∫ ∫ ⋅ ⋅ = 1 1 t A A dt a k x dx 22 ( 29 1 1 ln t a k x A A ⋅ ⋅ = Which leads to 1 1 1 ln ln t a k A A A A ⋅ ⋅ = = If we apply the exponential function here we get 1 1 1 1 or t a k t a k e A A e A A ⋅ ⋅ ⋅ ⋅ = = Since concentrations are never negative, we don’t need the absolute value symbol any more Since the choice of ( t...
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 Spring '11
 LoBue
 Physical chemistry, pH, Kinetics, Rate equation

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