Karlin-Week of Feb.13-lectures-3pp - Chemical Kinetics What...

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2/12/12 1 Chemical Kinetics Intro Chem II, Section 2 030.102.02, Spring 2011 Kenneth D. Karlin Text Chapter 18 1 Chemical Kinetics Pertains to All chemical reactions: Biochemical & chemical Inorganic, organic Gas, solution and solid phases Nuclear chemistry aA + bB –––> Products What Factors Matter in Chemical Reactions ? When it comes to chemical reactivity… 1. What constitutes a chemical reaction? 2. What factors matter during a reaction? 3. What has to happen for chemicals to react? 4. How do you visualize a chemical reaction on the molecular scale? 5. What factors govern reactivity? Chemical Kinetics Chemical Kinetics : study of reaction rates Reaction rates : the change in concentration of reactants or products as a function of time
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Biology: The Dynamic Science
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2/12/12 2 Factors that Influence Reaction Rate ** Under a specific set of conditions , every reaction has its own characteristic rate , which depends upon the chemical nature of the reactants. Four factors can be controlled during the reaction: 1. Concentration - molecules must collide to react; 2. Physical state - molecules must mix to collide; 3. Temperature - molecules must collide with enough energy to react 4. The use of a catalyst . Collision Energy and Reaction Rate Molecules Must collide with enough energy to react. Expressing the Reaction Rate - Reaction rate - changes in the concentrations of reactants or products per unit time - Reactant concentrations decrease while product concentrations increase rate of reaction = - for A B change in concentration of A change in time = - conc A 2 -conc A 1 t 2 -t 1 Δ (conc A) - Δ t rate of reaction = - Or Rate = - [A] units mol/L·sec t Stop Feb. 9
2/12/12 3 Table 16.1 Concentration of O 3 at Various Time in its Reaction with C 2 H 4 at 303K C 2 H 4 ( g ) + O 3 ( g ) C 2 H 4 O( g ) + O 2 ( g ) Time (s) Concentration of O 3 (mol/L) 0.0 20.0 30.0 40.0 50.0 60.0 10.0 3.20x10 -5 2.42x10 -5 1.95x10 -5 1.63x10 -5 1.40x10 -5 1.23x10 -5 1.10x10 -5 Δ (conc A) Δ t Reaction rate varies as a reaction proceeds The concentrations of O 3 vs. time during its reaction with C 2 H 4 C 2 H 4 ( g ) + O 3 ( g ) C 2 H 4 O( g ) + O 2 ( g ) Δ [C 2 H 4 ] Δ t rate = - Δ [O 3 ] Δ t = Average Rate line (line b) Instantaneous Rate (line d) -slope of a line tangent to curve at a particular point Initial Rate (line a) rate at the moment reactants are mixed Plots of [C 2 H 4 ] and [O 2 ] vs. time Product [O 2 ] Reactant [C 2 H 4 ] Concentration Vs Time Note: Ethylene disappears at same rate that O 2 forms
2/12/12 4 Expressing Rate in Terms of Concentrations C 2 H 4 ( g ) + O 3 ( g ) C 2 H 4 O( g ) + O 2 ( g ) Rate= - [C 2 H 4 ] t = - [O 3 ] t = + [C 2 H 4 O] t = + [O 2 ] t Expressing Rates from Balanced Chemical Equations In general, for the reaction: aA + bB cC + dD rate = 1 a = Δ [A] Δ t 1 b Δ [B] Δ t 1 c Δ [C] Δ t = + 1 d Δ [D] Δ t = + The numerical value of the rate depends upon the substance that serves as the reference. The rest is relative to the balanced chemical equation. Expressing Rates from Balanced Chemical Equations Example: Because it has a nonpolluting product (water vapor), hydrogen gas is used for fuel aboard the space shuttle and may be used by Earth-bound engines in the near future.

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