Module 5
Chemical Kinetics
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describe how various factors influence the rate of a reaction;
write the mathematical relationship between the rate of a reaction, rate
constant, and concentration of the reactants;
differentiate zero, first-, and second- order reactions;
write the rate law for first-order reaction;
discuss the effect of reactant concentration on the half-time of a first-order
reaction;
explain the effect of temperature on the rate of a reaction;
explain reactions qualitatively in terms of molecular collisions;
explain activation energy and how a catalyst affects the reaction rate; and
cite and differentiate the types of catalysts.
OBJECTIVES
Lesson 1. The Rate of a Reaction
Rate of Reaction
A measure of how fast a reaction takes place.
Often expressed as a change in amount or concentration of a
substance (reactant or product) per unit time.
Chemical kinetics is the study of rates of reactions and
factors that affect them.
Lesson 1. The Rate of a Reaction
There are two fundamental issues in chemical kinetics.
• How is the rate of a reaction defined?
• How is the rate of a reaction measured?
Lesson 1. The Rate of a Reaction
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Many chemical industries make use of chemical reactions whose rates should
be fast enough to be economically viable but slow enough to allow some
control.
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Action of drugs or medicines is an important consideration in medicine.
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Prevention of food spoilage.
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Photosynthesis in plants.
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Ripening of fruits.
Lesson 1. The Rate of a Reaction
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The rate of a chemical reaction tells how fast a given amount of a reactant or product
changes with time.
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It can be expressed either as the disappearance of a reactant or the appearance of the
product.
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The reaction rate is the ratio of the change in concentration to the elapsed time.
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Concentration is measured in M, or mol L-1, and designated with square brackets, [ ].
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Time is measured in s.
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The unit for rate is mol L-1 s-1.
Lesson 1. The Rate of a Reaction
● As a reaction proceeds, the rate of the reaction
can be measured by monitoring the concentrations of
products and reactants.
● As a reaction proceeds, the concentration of the
reactants decreases and the concentration of the
products increases.
Lesson 1. The Rate of a Reaction
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While
measuring
the
rate
of
increase
in
concentration, the rate of the reaction is a
number.
product
positive
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While
measuring
the
rate
of
decrease
in
reactant
concentration, the change in concentration will have a
negative sign. A negative sign is included in the rate
statement to obtain a positive value for the rate.
Lesson 1. The Rate of a Reaction
Lesson 1. The Rate of a Reaction
The change in the concentrations of the product and reactant
are not necessarily equal.
To ensure that the same reaction rate is obtained when using
either the reactants or the products, the stoichiometric
coefficient, v, is included in the denominator of the rate
expression.
Lesson 1. The Rate of a Reaction
Lesson 1. The Rate of a Reaction
Write the expression for the rate of the reaction of the following:
Lesson 1. The Rate of a Reaction
Sample Problem 1:
Lesson 1. The Rate of a Reaction
Sample Problem 1:
Lesson 1. The Rate of a Reaction
Sample Problem 2:
Lesson 1. The Rate of a Reaction
Sample Problem 3:
For the reaction 4NH3 + 5O2 --> 4NO + 6H2O, what
is the rate of formation of water vapor when the
rates of reaction of oxygen is 0.001 mol/L.s?
Lesson 1. The Rate of a Reaction
Average Rate and Instantaneous Rate
Lesson 1. The Rate of a Reaction
Average Rate and Instantaneous Rate
Lesson 1. The Rate of a Reaction
Average Rate and Instantaneous Rate
Lesson 2. Factors that influence reaction rate
1)
Nature of reactants – state that substances vary in their chemical reactivity. Chemical
reactivity is a major factor that determines the rate of a reaction. Give examples of
substances of varying chemical reactivity and reactions where these are involved.
2)
Surface area of reactants - Ability of reactants to meet/more area for reactants to be in
contact.
3)
Concentration of reactants - As the concentration of reactants increases, so does the
likelihood that reactant molecules will collide.
4)
Temperature of reaction system - At higher temperatures, reactant molecules have more
kinetic energy, move faster, and collide more often and with greater energy.
5)
Presence of a catalyst - Speed up reactions by lowering activation energy.
Lesson 3. Rate Law and its components
Lesson 3. Rate Law and its components
A rate law is an expression which relates that rate of a reaction to
the rate constant and the concentrations of the reactants.
A rate constant (k) is a proportionality constant for a given reaction.
The general rate law is usually expressed as:
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k is the rate constant
[A] and [B] are the reactant concentrations
m and n are typically either integers or half integers and must be
determined experimentally
Lesson 3. Rate Law and its components
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The experimentally determined exponents are referred to as the
order of the reaction.
❖ If m = 1, the reaction is said to be first order.
❖ If m = 2, the reaction is said to be second order.
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Exponents greater than 2 are unusual.
For reactions where the rate depends on more than one reactant
concentration:
❖ The exponent on each reactant is the order with respect to
that reactant.
❖ The sum of the exponents is the overall order of the reaction:
Reaction Order = m + n
Note: It is important to note that although the reaction order can be determined from the rate law, there is in general, no
relationship between the reaction order and the stoichiometric coefficients in the chemical equation.
Lesson 3. Rate LAw and its components
Sample Problem 4:
In the following rate laws, determine the orders with respect to each
substance and the overall order of the reaction.
1) rate = k[A]2[B]
2) rate=k[CH3OH]
3) rate=k[NO2]2[CO]0=k[NO2]2
4) rate = k[A][B]1/2
Resources
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https://www.chem.purdue.edu/gchelp/howtosolveit/Kinetics/
IntegratedRateLaws.html
https://chem.libretexts.org/Bookshelves/Physical_and_Theo
retical_Chemistry_Textbook_Maps/Supplemental_Modules_(Phy
sical_and_Theoretical_Chemistry)/Kinetics/02%3A_Reaction_
Rates/2.05%3A_Reaction_Rate
https://chem.libretexts.org/Bookshelves/Physical_and_Theo
retical_Chemistry_Textbook_Maps/Exercises%3A_Physical_and
_Theoretical_Chemistry/Exercises%3A_Chang/09.E%3A_Exercis
es