Chemistry 330
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Date: KEY
§ 17.02a
Kinetics: Introduction & Energy Diagrams
Most Important Idea(s):
Purpose
The purpose of this activity is introduce you to the study of chemical kinetics, including the factors
affecting the rate of reaction, and the activation energy, Ea.
Reference
Read the Modern Chemistry textbook: pages 529, 532-535.
Videos. (1) Kinetics: Chemistry's Demolition Derby - Crash Course Chemistry #32
(https://youtu.be/7qOFtL3VEBc)
(2) Bozemant: The Rate of Reactions
(https://youtu.be/6mAqX31RRJU?list=PLYuEED1EUXeYOzF7WRIPHMK0jGJO54IA-)
Background
+
–
Some chemical reactions proceed to conclusion (e.g., HCl(aq) + H2O(l)  H3O (aq) + Cl (aq)), some
reactions are reversible (e.g., H2(g) + I2(g)
2HI(g)). Most chemical reactions occur at different rates.
The speed at which a chemical reaction occurs depends on the energy pathway: (Figure 1.4, p. 532).
What makes chemicals react is based on the collision theory. The collision theory is a set of assumptions
regarding collisions and reactions. Essentially, it states that in order for a reaction to occur, the
reactants must have at least a minimum amount of energy (Ea, below), and be oriented in the correct
positions. (See video 1, above). Note, very seldom do more than two reactions collide with each other in a
chemical reaction.
A plot of the frequency of collisions versus energy of collision. Notice that
the distribution curve changes at the temperature (kinetic energy) changes.
The number of collisions occurring with a high enough energy (at least
Ea) is less that the low temperature curve. Thus, as we expect, as
temperature increases so does the reaction rate.
Reaction Pathways: The difference between the
activation energies for the reverse and forward
reactions of a reversible reaction, equals the energy
change in the reaction, E. The quantity for E is
the same in both directions, but is negative for the
exothermic reaction and positive for the
endothermic reaction.
Ea: The energy required to reach the activated
complex is called the activation energy. (E.g.,
wood starts to burn only when the activation
energy (lit match) is introduced into the system.
Ea’: The energy required to activate an endothermic
reaction. (Note that it is larger than Ea.)
E: The energy released by an exothermic reaction
E:\2014-2015\330_ModChem\330_sections\330_17_Kinetics\330.17.02a.Kinetics.Intro.Energy Diagrams Rate Equation_KEY.docx (5/28/2015 7:11:00 AM)
Chemistry
Kinetics: Introduction & Energy Diagrams
p. 2
or absorbed in an endothermic reaction.
Energy Diagram: The energies for an endothermic
reaction (energy products > energy reactants) in
the figure at the right are as follows:
Eforward: energy products – energy reactants
50 kJ/mol – 0 kJ/mol = 50 kJ/mol
Ereverse: energy reactant – energy products
0 kJ/mol –50 kJ/mol = –50 kJ/mol
Ea:

Ea’:


energy activated complex – energy of reactants
0 kJ/mol - 0 kJ/mol = 80 kJ/mol

energy activated complex – energy products
0 kJ/mol - 50 kJ/mol = 30 kJ/mol
Note: Reactions can only occur when the reactants hit enough with sufficient energy (Ea), have the
correct orientations (see above video), and with sufficient collision frequency.
Factors That Can Influence Reaction Rate:
1. Nature of reactants. For example, hydrogen combines vigorously with chlorine under certain
conditions but may only react weakly with nitrogen. (This may seem obvious to you.)
2. Surface area. Gaseous mixtures and dissolved particles react relatively quickly because they mix and
collide frequently. Heterogeneous mixtures, where reactants are in two different phases, require that
the reactants come in contact and thus, the surface area of the solid is an important consideration.
For example, small pieces of wood kindling have a more surface area-to-volume ratio than a large
piece of wood. The wood can react with more oxygen and thus, burn faster.
3. Temperature. An increase in temperature increases the kinetic energy of the particles, producing an
increase in collision frequency and collision energy. This results in more particles forming an
activated complex, and the reaction rate increases.
4. Concentration. The more particles that are present, the higher the collision frequency, and the faster
the reaction rate.
5. Catalysts. A catalyst is a substance the changes the rate of a chemical reaction without itself being
permanently consumed or produced. They achieve this by lowering the activation energy, Ea. A
homogeneous catalyst is in the same phase as reactants (e.g., adding aqueous KI to the
decomposition of H2O2(aq)  H2O(l) + O2(g).) A heterogeneous catalyst is one that is in a different
phase than the reactants (e.g., adding ice to carbonated water to the decomposition of carbonic acid
H2CO3(aq)  H2O(l) + CO2(g).)
Chemistry
Kinetics: Introduction & Energy Diagrams
p. 3
Problems
1. According to the collision theory, what are the two conditions that must be met for a collision
between reactants to be effective in producing a product?
The reactants must collide in the correct orientation to react, and with sufficient energy to
overcome the activation energy.
2. What do each of the following terms represent?
a. Activation Energy (Ea)
The energy required to reach the activated complex is called the activation energy.
b. Energy change in a chemical reaction (E)
The difference between the activation energies for the reverse and forward reactions of a
reversible reaction, equals the energy change in the reaction, E.
c. Ea’:
The energy required to activate an endothermic reaction.
d. Which is larger in an endothermic reaction: Ea or Ea’?
Ea’
3. Draw an energy diagram for a typical reaction, showing the activated complex, energy & course of
reaction labels, E, Ea, and Ea’.
4. In a reversible reaction, how does the activation energy required for an exothermic reaction compare
with the energy required for the endothermic reaction?
The activation energy required for the endothermic change is greater by the amount of enthalpy
of the reaction system. [Ea’]
Chemistry
Kinetics: Introduction & Energy Diagrams
p. 4
5. For each of the below energy diagrams, label the reactants, the products, E, Ea, and Ea’. Also,
determine (1) E for the forward and reverse reactions, (2) the values of Ea and Ea’, and (3) is the
reaction exothermic or endothermic?
Eforward =
+80 kJ/mol
Ereverse =
–80 kJ/mol
100 kJ/mol
20 kJ/mol
Ea =
Ea’ =
Eforward =
–40 kJ/mol
Ereverse =
+ 40 kJ/mol
Ea =
Ea’ =
20 kJ/mol
60 kJ/mol
Eforward =
+10 kJ/mol
Ereverse =
–10 kJ/mol
Ea =
Ea’ =
70 kJ/mol
60 kJ/mol