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Chemical kinetics is essential for understanding and controlling reaction rates. Learn about spontaneous reactions, rate laws, and reaction mechanisms to predict and enhance the speed of reactions.
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For a rxn to occur we need to know two things • Our reactants and products • If our reaction is going to occur!!!
SPONTANEOUS • Inherent tendency for the process to occur • DOES NOT MEAN FAST!!!!!
Spontaneous Rxn • Spontaneous reactions can take weeks to occur • Example: graphite turns to diamond spontaneously!
To be useful • Rxns must occur at a reasonable rate • Must understand: • stoichiometry • thermodynamics and...
Chemical Kinetics • What governs the rate of reactions
Reaction mechanism • The series of steps by which a reaction takes place
2NO2(g) -->2NO(g) + O2(g) • Decomposition of NO2 (the gas in air pollution) • Start with flask of NO2 and measure the concentration of 3 gases as NO2 decomposes
What will happen? • NO will increase • O2 will increase • NO2 will decrease • we need to look at speed at which this occurs
Chemical Kinetics • Rate = change in given quantity/time • For chem rxns- the change is the amt or concentration of a reactant or product
For reaction • Rate = (conc of A at time t2- conc of A at time t1) (t2- t1) Rate =[A] t
2NO2(g) -->2NO(g) + O2(g) • NO will have negative slope of NO2 • Rate O2 is half as fast as NO
Rate Laws... • Rxns are reversible! • NO2 in empty container • Forward rxn dominant • After time, there are products and reverse rxn is important
Now... • [NO2] depends on the difference in the rates of the forward and reverse rxns
To eliminate problems • Study the problem where reverse rxn is negligible (right after start of rxn) • Choose conditions where reverse rxn is neglected
Rate = k[NO2]n • Rate depends on • conc • k = rate constant • n = order of reactant (+,-, integer or fraction) • Determined experimentally!!
Types of Rate Laws • Rate is a fcn of concentration • Rate = -[NO2]/ t = k[NO2]n
Differential Rate Law • Rate law that expresses how rate depends on the concentration (usually called rate law)
Why bother with rate laws? • To find steps that make rxn occur (series) • speed up rxns if we know slowest step