Download Section 16.1 A Model for Reaction Rates

Section 16.1 A Model for Reaction Rates
• Calculate average rates of chemical reactions from
experimental data.
• Relate rates of chemical reactions to collisions
between reacting particles.
Section 16.1 A Model for Reaction Rates (cont.)
reaction rate
collision theory
activated complex
activation energy
Collision theory is the key to
understanding why some reactions are
faster than others.
Expressing Reaction Rates
• The reaction rate of a chemical reaction is
stated as the change in concentration of a
reactant or product per unit of time.
• the reaction rate of a chemical reaction is
generally stated as the change in
concentration of a reactant or product per
unit of time, expressed as mol/(L·s).
Brackets around the formula for a
substance denote its molar concentration.
For example, [NO2 ] represents the molar
concentration of NO 2 .
Expressing Reaction Rates (cont.)
• Reaction rates are determined
experimentally.
Collision Theory
• Collision theory states that atoms, ions,
and molecules must collide in order to
react.
Collision Theory (cont.)
• An activated complex “ sometimes is
called transation state” is a temporary,
unstable arrangement of atoms in which
old bonds are breaking and new bonds are
forming.
• Q: What might prevent the occurrence of a
reaction per each collision?
• 1-incorrect orientations
• 2- insufficient energy.
Collision Theory (cont.)
• The minimum amount of energy that reacting
particles must have to form the activated complex
and lead to a reaction is called the activation
energy ”Ea”.
•A high Ea means that relatively few collisions have the
required energy to produce the activated complex, and the
reaction rate is slow. A low Ea means that more collisions
have sufficient energy to react, and the reaction rate is
faster.
•Think of this relationship in terms of a person pushing a
heavy cart up a hill. If the hill is high, a substantial amount
of energy is required to move the cart, and it might take a
long time to get it to the top. If the hill is low, less energy is
required and the task might be accomplished faster
Collision Theory (cont.)
Collision Theory (cont.)
• When an exothermic reaction occurs,
molecules collide with enough energy to
overcome the activation energy barrier.
They form an activated complex, then
release energy and form products at a
lower energy level.
Collision Theory (cont.)
• In the reverse reaction, which is
endothermic, the reactant molecules are at
a lower energy than the products. To
react, the reactants must absorb enough
energy to overcome the activation energy
barrier and form higher-energy products.
• Q1: Relate collision theory to reaction rate.
• Collision theory explains why reactions
occur and how reaction rates can be
modified.
• Q2: Explain what the reaction rate
indicates about a particular chemical
reaction.
• The reaction rate indicates the rate of
change of the concentration of a reactant or
product in mol/(L·s).
• Q3: Compare the concentrations of the
reactants and products during the course
of a chemical reaction (assuming no
additional reactants are added )
• The concentrations of the reactants
decrease, and the concentrations of the
products increase at the same rate .
• Q5: Explain why the average rate of a
reaction depends on the length of the time
interval over which the rate is measured.
• The rate of change of a reactant or
product in a chemical reaction is not linear
in time
• Q6: Describe the relationship between
activation energy and the rate of a reaction
• The higher the activation energy, the
slower the rate of the reaction.
• Q7: Summarize what happens during the brief
existence of an activated complex.
• Bonds in the reactants are in the process of
breaking, while new bonds are beginning to
form to produce the products.
• Q8: Apply collision theory to explain why
collisions between two reacting particles do not
always result in the formation of a product.
• The collision must be in a correct orientation
and have sufficient energy to form the activated
complex.
• Explain the negative sign for the reaction
rate of reactants
• Explain why reaction rate for product is
positive?
• As concentration of reactant decreases, a
negative sign is used to express the rate of
reaction in terms of reactants. As the
concentration of products increases, a
positive sign is used in terms of products.
Section 16.1 Assessment
Which of the following is NOT a requirement
for a reaction to occur, according to the
collision theory?
A. Reacting substances must collide.
B. Reacting substances must be
in an exothermic reaction.
A. A
C. Reacting substances must
B. B
collide in the correct orientation.
C. C
D. Reacting substances must collide
with sufficient energy to form an
D. D
activated complex.
Section 16.1 Assessment
A temporary, unstable arrangement of
atoms in which old bonds are breaking
and new bonds are forming is called ____.
A. reaction complex
B. reaction substrate
C. activated complex
D. activated molecule
A.
B.
C.
D.
A
B
C
D
The energy required to initiate a reaction
is called ____.
A. initiation energy
B. activation energy
C. complex energy
D. catalyst energy
A.
B.
C.
D.
A
B
C
D
Which of the following is an acceptable
unit for expressing a rate?
A. mol/L ● s
B. L/s
C. M
D. mL/h
A.
B.
C.
D.
A
B
C
D
End of Section 16.1