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8/12/2019 Chapter 8 Reaction Kinetics
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8/12/2019 Chapter 8 Reaction Kinetics
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explain and use the terms: rate of reaction, activation energy, catalysis.
explain qualitatively, in terms of collisions, the effect of
concentration changes on the rate of a reaction
show understanding, including reference to the Boltzmann
distribution, of what is meant by the term activation energy
explain qualitatively, in terms both of the Boltzmann distribution
and of collision frequency, the effect of temperature change on the
rate of a reaction
(i) explain that, in the presence of a catalyst, a reaction has a
different mechanism, i.e. one of lower activation energy
(ii) interpret this catalytic effect in terms of the Boltzmann
distribution
describe enzymes as biological catalysts (proteins) which may have
specific activity
8/12/2019 Chapter 8 Reaction Kinetics
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Chemical kinetics is the study of the rates
of chemical reactions.
Rate of reaction is the change in
concentration of reactants or products per
unit time.
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Collision theory
a set of three statements that give the
conditions necessary for a chemical reaction to
occur.
a) Molecular collisions reactants particles
(molecules, ions or atoms) must interact(collide) with one another before any reaction
can occur
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b) The activation energy
Is the minimum energy needed for a reaction to
take place upon proper collision of reactants.
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c) A chemical reaction occurs when
Collisions between molecules have sufficient
energy to break the bonds in the reactants.
Molecules collide with the proper orientation.
Bonds between atoms of the reactants (N2and
O2) are broken, and new bonds (NO) form.
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8/12/2019 Chapter 8 Reaction Kinetics
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More energy stored in product molecule bonds than in
reactant molecule bonds.
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Chemical reaction rate
The rate at which reactants are consumed or
products produced in a given time period.
Affected by these factors:
i) physical nature of the reactants
ii) reactant concentration
iii) reaction temperatureiv) presence of catalyst
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Physical nature of the reactants
Breaking a solid into smaller pieces increases the surface area
exposed to reacting chemicals in a gas or in solution.
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8/12/2019 Chapter 8 Reaction Kinetics
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As the temperature of a system increases
Average kinetic energy of the reacting
molecules increases and more collisions takeplace in a given time.
Larger fraction of collision result in reaction.
Every 10o
C increase, rate of chemicalreaction doubles.
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A catalyst
Speeds up the rate of a reaction.
Is not used up during the reaction.
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The area under the curve gives the total number of molecules.
This area does not change as the temperature rises.
The peak height falls as the temperature rises and the curve spreads tothe right
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At 310 K, more molecules have enough energy to react
when they collide with other molecules.
Thus, the reaction goes faster.
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A catalyst often changes the mechanism of a reaction and makes a
reaction more productive by increasing the yield of the desired
product.
The proportion of molecules able to react increases when a catalystprovides a pathway with lower activation energy.
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Enzymesare proteins that
Catalyze nearly all the chemical reactions taking place in the cells ofthe body.
Increase the rate of reaction by providing alternative pathway withlow energy of activation.
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The function of enzymes as catalysts in biological
systems are based on the concepts of enzyme
active site and enzyme-substrate complex
formation.
The active site
Is a region within an enzyme that fits the shape of the
reacting molecule called a substrate.
Contains amino acid R groups that bind the substrate. Releases products when the reaction is complete.
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In an enzyme-catalysed
reaction
A substrate attaches to
the active site.
An enzyme-substrate
(ES) complex forms.
Reaction occurs and
products are released.
An enzyme is used over
and over.
E+ S ES E+ P
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In the lock-and-key model of enzyme action,
The active site has a rigid shape.
An enzyme only binds substrates that exactly fit the active site.
Only substrates with the matching shape can fit.
The substrate is the key that fits that lock.
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Enzymes
Are most active at an
optimum temperature
(usually 37C inhumans).
Show little activity at
low temperatures.
Lose activity at high
temperatures as
denaturation occurs.
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Enzymes
Are most active atoptimum pH.
Lose activity in lowor high pH as tertiarystructure is disruptedand denaturation
occurs.