The document discusses reaction order and methods for determining reaction order. It defines reaction order as the power dependence of the rate of reaction on the concentrations of reactants. Reaction order can be determined using the initial rates method, integral method, or differential method. Different reaction orders are also described, including zero-order reactions, where concentration does not affect rate; first-order reactions, where rate depends on one reactant; and second-order reactions, where rate depends on two reactants or one reactant squared. Pseudo-first order reactions are also discussed.
4. Reaction Order
The order of reaction can be defined as the
power dependence of rate on the
concentration of all reactants. For example,
the rate of a first-order reaction is
dependent solely on the concentration of
one species in the reaction
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6. “
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◇ Reaction order represents the number of
species whose concentration directly
affects the rate of reaction.
◇ It can be obtained by adding all the
exponents of the concentration terms in the
rate expression.
◇ The order of reaction does not depend on
the stoichiometric coefficients
corresponding to each species in the
balanced reaction.
7. “
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◇ The reaction order of a chemical reaction is
always defined with the help of reactant
concentrations and not with product
concentrations.
◇ The value of the order of reaction can be in
the form of an integer or a fraction. It can
even have a value of zero.
8. In order to determine the reaction
order, the power-law form of the
rate equation is generally used.
The expression of this form of
the rate law is given by r =
k[A]x[B]y.
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9. What is the Rate
Law?
◇ The rate law (also known as
the rate equation) for a
chemical reaction is an
expression that provides a
relationship between the
rate of the reaction and the
concentrations of the
reactants participating in it.
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10. In the expression described above, ‘r’
refers to the rate of reaction, ‘k’ is the
rate constant of the reaction, [A] and [B]
are the concentrations of the reactants.
The exponents of the reactant
concentrations x and y are referred to as
partial orders of the reaction. Therefore,
the sum of all the partial orders of the
reaction yields the overall order of the
reaction.
Terms used above
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11. How to Determine
Reaction Order
Initial Rates
Method
Integral Method
Differential
Method
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◇ There are several different
methods which can be
followed in order to
determine the reaction
order. Some of these
methods are as following.
12. Initial Rates Method
◇ If the partial order of A is being determined,
the power-law expression of the rate
equation now becomes ln r = x.ln[A] + C,
where C is a constant.
◇ A graph is now plotted by taking ‘ln r’ as a
function of ln[A], the corresponding slope is
the partial order, given by x.
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13. Integral Method
◇ The order of reaction obtained from the initial
rates method is usually verified using this method.
◇ The measured concentrations of the reactants are
compared with the integral form of the rate law.
◇ For example, the rate law for a first-order reaction
is verified if the value for ln[A] corresponds to a
linear function of time (integrated rate equation of
a first-order reaction: ln[A] = -kt + ln[A]0).
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14. Differential Method
◇ This method is the easiest way to obtain the
order of reaction
◇ First, the rate expression of the reaction is
written (r = k[A]x[B]y..)
◇ The sum of the exponents x+y+… gives the
final value of the reaction order.
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16. Zero Order Reactions
◇ The rate of reaction is independent of the
concentration of the reactants in these
reactions.
◇ A change in the concentration of the
reactants has no effect on the speed of the
reaction
◇ Examples of these types of reactions
include the enzyme-catalyzed oxidation of
CH3CH2OH (ethanol) to CH3CHO
(acetaldehyde).
17. First-Order Reactions
◇ The rates of these reactions depend on the
concentration of only one reactant, i.e. the
order of reaction is 1.
◇ In these reactions, there may be multiple
reactants present, but only one reactant
will be of first-order concentration while
the rest of the reactants would be of zero-
order concentration.
◇ Example of a first-order reaction: 2H2O2 →
2H2O + O2
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18. Pseudo-First Order
Reactions
◇ In a pseudo-first order reaction, the
concentration of one reactant remains
constant and is therefore included in
the rate constant in the rate
expression.
◇ The concentration of the reactant may
be constant because it is present in
excess when compared to the
concentration of other reactants, or
because it is a catalyst.
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19. ◇ Example of a pseudo-first order
reaction: CH3COOCH3 + H2O →
CH3COOH + CH3OH (this reaction
follows pseudo-first order kinetics
because water is present in
excess).
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20. Second-Order Reaction
◇ When the order of a reaction is 2, the
reaction is said to be a second-order
reaction.
◇ The rate of these reactions can be obtained
either from the concentration of one
reactant squared or from the concentration
of two separate reactants.
◇ The rate equation can correspond to r =
k[A]2 or r = k[A][B]
◇ Example of a second-order reaction: NO2 +
CO → NO + CO2
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