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University of Memphis - CHEM 1120 - Chem week 2 - Class Notes

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University of Memphis - CHEM 1120 - Chem week 2 - Class Notes

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background image CHAPTER 13:  CHEMICAL KINECTICS Introduction o Chemical kinetics examines the factors which can  influence the rate of a reaction o The rate at which a reaction progresses can be influenced by several factors: Temperature Pressure Reactant concentration The addition of a catalyst Mechanical force Collision Theory o Molecules A and BC must physically collide with each other
o The collision between A and BC must occur with sufficient energy
such that the bond between B and C breaks o Reactant molecule A must collide with the C side of the  compound BC. In other words, collisions must occur with the 
correct spatial orientation
Rates of Chemical Reactions o For any balanced chemical reaction:
o aA + bB à cC + dD 
o The average reaction rate is given by the equation:
¿ 1
a
[ A ] ∆ t = 1 b [ B ] ∆ t = 1 c [ C ] ∆ t = 1
d
[ D ] ∆ t o
o The reaction rate of a chemical species is proportional to its 
stoichiometric coefficient. Rate Laws o A rate law is a mathematical equation which relates the rate of a  reaction to the concentration of the reactant(s) Integrated Rate Laws o Integrated rate laws quantify the relationship between reactant  concentration and time. o If several concentrations are determined at different times, the  rate constant k can then be graphically determined o It is of interest to know the half-life (t 1/2 of a reaction o The time required for a reaction to go to half-completion
o In terms of radioactive isotopes, this is the time required for a 
sample to decay to half its initial amount
background image Reaction Mechanisms o A reaction mechanism represents the step-by-step reactions  which, when combined, add up to an overall net reaction o The overall reaction is only as fast as its slowest elementary  step. The slowest step is called the rate-limiting step.
o Recall that equilibrium is reached when the rate of the forward 
equals the rate of the reverse reaction.  We called this a dynamic
process.
Reaction Rates and Temperature o In general, as temperature increases, so does the reaction rate
o In order for a reaction to occur, reactant molecules must collide 
with sufficient energy AND in the proper orientation Catalysis o A catalyst lowers the activation energy of a reaction by assisting  reactant molecules to be in the proper orientation o Homogenous catalysts are in the same chemical state as the  compounds they will react with (the substrates) o Heterogeneous catalysts are in different chemical states than  their substrates do not appear in the overall balanced chemical equation can be reused many times over before losing efficacy are used very early on in a mechanism, and regenerated 
close to or during the last step
do not change ∆H rxn  (i.e., percent conversion) increase the rate of both the forward and reverse reactions
equally
often provide a different mechanism than the uncatalyzed 
reaction
Summary o Kinetics studies the factors that can influence the rate of  a chemical reaction o For a reaction to occur, reactant molecules must  physically collide with sufficient energy and in the proper 
orientation
o The method of initial rates and the use of integrated rate  laws can help solve for the rate orders and for the rate 
constant
o The slowest step of a reaction mechanism dictates the  overall reaction rate

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School: University of Memphis
Department: Chemistry
Course: General Chemistry II
Professor: Brewster
Term: Spring 2016
Tags:
Name: Chem week 2
Description: These notes cover chapter 13
Uploaded: 02/11/2018
3 Pages 32 Views 25 Unlocks
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