lecture21222
DESCRIPTION
a supplemental resource for studentsTRANSCRIPT
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Kinetics: the Rate of Chemical
ReactionLecture 21
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It is now time to learnHow fast the reaction is proceeding at a given moment;What the reactant and product concentrations will be when the reaction is complete;Whether the reaction will proceed by itself and release energy or it will require energy to proceed.
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Chemical equilibrium
in a chemical process is the state in which the chemical activities or
concentrations of the reactants and products have no net change
over time.
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Chemical thermodynamics
studies the effects of changes in temperature, pressure, and volume on chemical systems at
the macroscopic scale by analyzing the collective motion
of their particles.
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Chemical kinetics
is the study of rates of chemical processes, the
changes in concentrations of reactants (or products)
as a function of time.
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Key factor of reaction rate: concentrationMolecules must collide to react.Reaction rate is proportional to the concentration of reactants.Rate ~ collision frequency ~ concentration.
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Key factor of reaction rate: physical stateMolecules must mix to collide.The more finely divided a solid or liquid reactant, the greater its surface area per unit volume, the more contact it makes with the other reactant, and the faster the reaction occurs.
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Key factor of reaction rate: temperatureMolecules must collide with enough energy to react.At a higher temperature, more collisions occur in a given time.Raising the temperature increases the reaction rate by increasing the number and, especially, the energy of the collisions.Rate ~ collision energy ~ temperature.
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Key factor of reaction rate: catalysisA catalyst effects a lower activation energy, which in turn makes the rate constant larger and the rate higher.In more detail - on Wednesday.
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Rate
is a change in some variable per unit of
time.
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Rate of motion =
change in position x2 — x1
∆x
------------------------- = -------------- = --------change in time t2 — t1 ∆t
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Rate of growth =
change in height l2 — l1 ∆l
------------------------- = -------------- = --------change in time t2 — t1 ∆t
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Reactant concentrations
decreasewhile product concentrations
increase.Consider A B
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The change in concentration
of reactant A in A Bis always negative.
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The change in concentration
of product B in A Bis always positive.
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Rate of concentration =
change in concentration of A= — --------------------------------------- = change in time concentration A2 — concentration A1
= — ----------------------------------------------- = t2 — t1
∆(conc A) = — --------------
∆t .
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Use square brackets
to express concentration in moles per liter:
∆ [A] ∆ [B]
Rate = — ------------ or -------------
∆ t ∆ t
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The rate of the reaction itself
varies with time as the reaction proceeds.
The rate decreases during the course of the reaction.
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Reaction rates: different approachesAverage rate;Instantaneous rate;Initial rate.
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A sample problem
on expressing rate in terms of changes in
concentration with time.
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THE END