Chemical Kinetics - Kinetics Rates of Chemical Reactions...

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Kinetics: Rates of Chemical Reactions Rate of a reaction: change in concentration / unit time can relate to any product or reactant [X] = "molar concentration of X" [reactant] decreases with time [product] increases with time product concentration reactant time Measuring reaction rates: appearance of a product disappearance of a reactant Detection of change: direct detection of a chemical species Indirect methods: change in color, pH, pressure measuring a product: rate = [product] concentration is increasing (+) time measuring a reactant: rate = - [reactant] concentration is dereasing ( - ) time Use (+) and (-) signs on concentration terms to make rate (+) Different ways to measure rate: 1
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average rate: over a specified time interval, from a specified starting time to a specified ending time. instantaneous rate: slope of line tangent at a specified time. initial rate: linear approximation of the first few moments. The effect of stoichiometry on rate: N 2 (g) + 3 H 2 (g) 2 NH 3 (g) to compare rates: H 2 disappears 3 times as fast as N 2 Their absolute rates are NOT THE SAME ! Reaction rate with respect to each compound: rate (N2) = -∆ [N 2 ] rate (H2) = -∆ [H 2 ] time time rate (NH3) = [NH 3 ] THESE ARE ALL time DIFFERENT RATES! To establish a common rate of reaction, divide H 2 rate by 3 and NH 3 rate by 2 (their coefficients in the balanced chemical eqn.) reaction rate = -∆ [N 2 ] = - 1 [H 2 ] = + 1 [NH 3 ] time 3 time 2 time Example: The concentration of a reaction product was measured spectroscopically during the course of a reaction. The concentration 2
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was 0 at time t = 0, and the concentration was 0.0350 M at time t = 15.0 minutes. What was the rate of the reaction during this interval, in M/sec ? Collision theory of reactions: particles must collide must collide with sufficient energy must collide with proper orientation Things that affect rate: concentration of the reactants: more molecules means more collisions more collisions means more chances to "get it right" temperature : kinetic energy T v 1/2 mv 2 as v number of collision per time as v energy of collisions
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