CHEMISTRY KEYWORDS:
Rate Constant: Proportionality constant, k, in a rate
equation.
Rate Equation: An equation showing the relationship
between the rate constant and the concentrations of the species that affect the
rate of reaction.
Order of Reaction: Power to which the concentration
of the reactant is raised in the rate equation. If a rate is directly
proportional to concentration, it is first order. If the rate is directly
proportional to the square of the concentration, it is a second-order reaction.
Limiting reactant: Reactant which is not in excess.
The reaction will stop when the limiting reactant is all used up.
Rate-determining Step: Slowest step in a reaction
mechanism.
Adsorption (in catalysis): First stage in
heterogeneous catalysis, where reactant molecules form bonds with atoms on the
catalyst surface.
Desorption: Last stage in heterogeneous catalysis.
The bonds holding the molecules of products to the surface of the catalyst are
broken, and the product molecules diffuse away from the surface of the
catalyst.
CHEMISTRY DEFINITIONS:
Half-life, t1/2: Time taken for the amount
(or concentration) of the limiting reactant in a reaction to decrease to half
its initial value.
Homogeneous Catalysis: A type of catalysis in which
the catalyst and reactants are in the same phase.
Heterogeneous Catalysis: A type of catalysis in which
the catalyst is in a different phase to the reactants.
CHEMISTRY IMPORTANT NOTES:
The rate of reaction is measured by a
decrease in concentration of a particular reactant or an increase in
concentration of a particular product over a period of time.
The rate is directly proportional to the
concentration of cyclopropane.
The order of a reaction shows how the
concentration of a reactant affects the rate of reaction. Order of reaction
with respect to a particular reactant is the power to which the concentration
of that reactant is raised in the rate equation.
Identify the order of a reaction:
·
Plot a graph of reaction rate against
concentration of reactant.
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Zeroth-order reaction = Horizontal straight
line.
>
First-order reaction = Inclined straight line
going through the origin.
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Second-order reaction = Upwardly curved line.
·
Plot a graph of the concentration of reactant
against time.
>
Zeroth-order reaction = descending straight
line.
>
Curve for the second-order reaction is much
deeper than for the first-order reaction.
·
Deduce successive half-lives from graphs of
concentration against time.
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Zeroth-order reaction = Successive half-lives
which decrease with time.
>
First-order reaction = Half-life, which is
constant.
>
Second-order reaction = Successive half-lives
which increase with time.
Half-life, t1/2 is the time taken
for the concentration of a limiting reactant to fall to half of its initial
value.
The progress of the reaction can be followed
by measuring the initial rate of formation of iodine.
While studying the Boltzmann Distribution
curve, we learned that at higher temperatures, a greater proportion of the molecules
have energy greater than the activation energy. The rate of reaction increases
at higher temperatures. Therefore rate of reaction is proportional to the
fraction of molecules with energy equal to or greater than the activation
energy. So the rate constant increases as the temperature increases.
Analysing the data:
1.
Plot a graph to show how the concentration of a
particular reactant (or product) changes with time.
2.
Take tangents at various points along the curve
which correspond to particular concentrations of the reactant.
3.
Calculate the slope (gradient) at each
concentration selected. The rate of reaction is calculated from the slope of
the graph.
4.
Plot a graph of the rate of reaction against
concentration.
The initial rates method is often used when
the rate of reaction is slow.
A reactant that appears in the chemical
equation may not affect the reaction rate.
A substance that is not a reactant in the
chemical equation can affect the reaction rate.
Reactions that occur in a number of steps
are known as reaction mechanisms. The overall rate of reaction depends on the
slowest step, also known as the rate-determining step. If the concentration of
a reactant appears in the rate equation, then that reactant appears in the
rate-determining step. If a substance does not appear in the overall rate
equation, it does not take part in the rate-determining step.
Steps that follow the slow step are
relatively fast and do not affect the reaction rate.
If there's only a single species in the
rate-determining step, we call it unimolecular. If 2 species are involved in
the rate-determining step, we call it bimolecular. Mechanisms that involve a
trimolecular step are rare.
Homogeneous catalysis occurs when the
catalyst is in the same phase as the reaction mixture.
Heterogeneous catalysis occurs when the
catalyst is in a different phase from the reaction mixture. It often involves
gaseous molecules reacting at the surface of a solid catalyst.
Chemical adsorption, also called
chemisorption, occurs when molecules become bonded to atoms on the surface of a
solid. Nickel is particularly good at chemisorbing hydrogen gas.
Adsorb means to bond to the surface of a
substance. Absorb means to move right into the substance, rather like a sponge
absorbs water.
The stages of adsorption of hydrogen onto
nickel are:
·
Hydrogen gas diffuses to the surface of the
nickel.
·
Hydrogen is then physically adsorbed onto the
surface: weak forces link the hydrogen molecules to the nickel.
·
Hydrogen becomes chemically adsorbed onto the
surface: this causes stronger bonds to form between the hydrogen and the nickel.
·
This causes weakening of the hydrogen-hydrogen
covalent bond.
When a reaction on the catalyst surface is
complete, the bonds between the products and the catalyst weaken so much that
the products break away from the surface. This is called desorption.
Iron in the Haber Process:
1. Diffusion: Nitrogen and hydrogen gas diffuse to the surface of the iron.
2. Adsorption: Reactant molecules are chemically adsorbed onto the surface of the iron. The bonds formed between the reactant molecules and the iron are:
· Strong enough to weaken the covalent bonds within the nitrogen and hydrogen molecules so the atoms can react with each other.
· Weak enough to break and allow the products to leave the surface.
3. Reaction: Adsorbed nitrogen and hydrogen atoms react on the surface of the iron to form ammonia.
4. Desorption: Bonds between the ammonia and the surface of the iron weaken and are eventually broken.
5. Diffusion: Ammonia diffuses away from the surface of the iron.
Possible steps in the catalytic process
include:
· Adsorption of nitrogen oxides and carbon monoxide onto the catalyst surface.
· Weakening of the covalent bonds within the nitrogen oxides and carbon monoxide.
· Formation of new bonds between
§ Adjacent nitrogen atoms (to form nitrogen molecules)
§ Carbon monoxide and oxygen atoms to form carbon dioxide.
· Desorption of nitrogen molecules and carbon dioxide molecules from the surface of the catalyst.
SUMMARY:
The general form of the rate equation is: rate = k[A]m[B]n.
k is the rate constant, [A] and [B] are the concentrations of those reactants
that affect the rate of reaction, m is the order of the reaction with respect
to A, and n is the order of the reaction with respect to B.
The order of reaction can be determined from graphs of
reaction rate against concentration.
The half-life of a first-order reaction may be used in
calculation to find the first-order rate constant using the relationship: t1/2
= 0.693/k
The rate-determining step is the slowest step in a reaction
mechanism. The rate-determining step determines the overall rate of reaction.
The order of reaction with respect to a particular reactant
shows how many molecules of that reactant are involved in the rate-determining
step of a reaction.
The order of a reaction can be predicted from a given
reaction mechanism, knowing the rate-limiting step.
Homogeneous catalysis occurs when a catalyst and the
reactants are in the same phase.
Heterogeneous catalysis occurs when a catalyst is in a
different phase from the reactants.
The mechanism of heterogeneous catalysis involves the
processes of adsorption, reaction and desorption.
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