CHAPTER 12 notes - Welcome to CHEMISTRY 1B Chemical...

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1/27/2009 1 Organic Chemistry Welcome to CHEMISTRY 1B!! Chemical Kinetics Nuclear Chemistry Electrochemistry Chemical Equilibrium Transition Metal Chemistry Chemical Thermodynamics Chemical Reactions: 1. What Happens? Three Fundamental Questions CH 4 + 2O 2 CO 2 + 2H 2 O 2. How fast does it happen? 3. To What Extent Does it Happen? Chemical Kinetics Reaction Rate may be expressed as: Chemical Kinetics : Involves Reaction rates and the sequence of steps by which reactions occur. The INCREASE in the concentration of a PRODUCT per unit time. The DECREASE in the concentration of a REACTANT per unit time. Factors that Affect the Rate of Reactions Concentrations The Physical State of Reactants and Products Temperature Catalysts
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1/27/2009 2 2N 2 O 5 ( g ) 4NO 2 ( g ) + O 2 ( g ) Reaction Rates Table 12.1: Concentration as a function of Time at 55 0 C Time(s) N 2 O 5 NO 2 O 2 Concentration (M) 0 100 200 300 400 500 600 700 0.0200 0.0169 0.0142 0.0120 0.0101 0.0086 0.0072 0.0061 0 0.0063 0.0115 0.0160 0.0197 0.0229 0.0256 0.0278 0 0.0016 0.0029 0.0040 0.0049 0.0057 0.0064 0.0070 Decrease Increase Concentration (M) Time(seconds) 2N 2 O 5 ( g ) 4NO 2 ( g ) + O 2 ( g ) Reaction Rates 0 0.01 0.02 0.03 0 200 400 600 800 NO 2 O 2 0 0.01 0.02 0.03 0 100 200 300 400 500 600 700 800 Concentration (M) Time(seconds) 2N 2 O 5 ( g ) 4NO 2 ( g ) + O 2 ( g ) NO 2 O 2 N 2 O 5 D T = 100 s D [NO 2 ] = 0.0036M D T = 100 s D [NO 2 ] = 0.0009M D T = 100 s D [N 2 O 5 ] = -0.0018M
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1/27/2009 3 2N 2 O 5 ( g ) 4NO 2 ( g ) + O 2 ( g ) Reaction Rates Rate of O 2 Formation: Rate = D t D [O 2 ] = (0.0049 M - 0.0040 M) (400 s - 300 s) = 9.0 x 10 -4 s Rate = D t D [NO 2 ] -(0.0197 M - 0.0016 M) (400 s - 300 s) = 3.6 x 10 -3 M s Rate of NO 2 Formation: = The Rate of NO 2 is four times the Rate of O 2 formation! Reaction Rates 2N 2 O 5 ( g ) 4NO 2 ( g ) + O 2 ( g ) Rate of N 2 O 5 Decay Rate = D t -D [N 2 O 5 ] Rate of O 2 Formation Rate = D t D [O 2 ] Rate = D t D [NO 2 ] Rate of NO 2 Formation 2N 2 O 5 ( g ) 4NO 2 ( g ) + O 2 ( g ) Reaction Rates D [NO 2 ] D t 4 D [O 2 ] D t = - D [N 2 O 5 ] D t 2 D [O 2 ] D t = The rate of NO 2 FORMATION is four times as fast as the FORMATION of O 2 . The rate of N 2 O 5 DECAY is two times as fast as the FORMATION of O 2 . D [NO 2 ] D t D [O 2 ] D t - D [N 2 O 5 ] D t Rate = = = 1 2 1 4 General rate of reaction: aA + bB dD +eE Reaction Rates General rate of reaction: - D [B] D t - D [A] D t Rate = = = 1 a 1 b D [D] D t = 1 d D [E] D t 1 e Write a General rate of reaction expression for: The combustion of C 2 H 6 .
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1/27/2009 4 0 0.01 0.02 0.03 0 200 400 600 800 [NO 2 ] Time(seconds) Reaction Rates Let’s Develop an Equation that relates the CONCENTREATION of each REACTANT to the RATE of the REACTION. Try this at home!! Rate Law : An equation that shows the dependence of the reaction rate on the concentration of each reactant. a A + b B products Rate [A] m [B] n Rate Laws and Reaction Order Rate k [A] m [B] n = k is known as the rate constant Rate Laws and Reaction Order The values of the exponents in the Rate Law MUST be determined by EXPERIMENT ; they cannot be deduced from the stoichiometry of the reaction. (CH 3 ) 3 CBr(aq) + H 2 O( l ) (CH 3 ) 3 COH(aq) + H + (aq) +Br - (aq) Rate = k[CH 3 ) 3 CBr] Table 12.2: Balanced Chemical Equations and Experimentally Determined Rate Laws for Some Reactions.
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