Transcript Rates of Reactions

Rates of Reactions
Rates

Speed or rate
is measured
by the
amount of
change that
occurs in a
given time
interval
REACTION RATE
The amount of time it takes to use up
the reactants (decrease in reactant
concentration)
OR
The amount of time it takes to make
products (increase in product
concentration)
I.A. Reaction Rates
Reaction Rate-change
in concentration of
reactants and products
in a given time period
 Consider the following
reaction:

2 NO2  2 NO + O2
time (sec)
0
5.0
10.0
15.0
20.0
[NO2]
4.0 M
3.1 M
2.5 M
2.1 M
1.8 M
Sketch a graph with t
on the x-axis and M on
the y-axis.
I.A. Reaction Rates
2 NO2  2 NO + O2
time (sec)
 How much NO is
0
present at the
5.0
beginning of the
10.0
reaction?
15.0
20.0
[NO2]
4.0 M
3.1 M
2.5 M
2.1 M
1.8 M
[NO]
I.A. Reaction Rates
2 NO2  2 NO + O2
time (sec)
 How much NO2
was used up in the 0
5.0
first 5 seconds?
 How much NO was 10.0
15.0
produced?
20.0
Calculate
[NO2]
4.0 M
3.1 M
2.5 M
2.1 M
1.8 M
[NO]
0M
the moles of NO produced for each
value of t (always use the total moles of NO2
used up to that point).
Collision Theory

Reaction rate depends on the collisions between
reacting particles.

Effective collision Reaction occurs

Elastic collisions  No reaction occurs

Effective collisions occur if the particles...

collide with each other

have the correct orientation

have enough kinetic energy to break bonds
Particle Orientation
Required Orientation
Unsuccessful
Collisions
Successful Collision
I.B. Collision Theory
Elastic Collisions
Effective Collisions
Activation Energy (Ea)
Even though the
boulder will be more
stable at the bottom of
the hill, it needs a push
to get up and over the
bump first.
Ea
depends on reactants
 low Ea = fast rxn rate

Activation Energy (Ea)

Minimum energy required for a reaction to occur

Transition state-complex that is neither reactant nor product
Activated Complex
Energy of products
Activation
Energy
Energy of reactants
∆H - Difference
between energy
of reactants and
products
Draw an energy diagram for an
exothermic reaction. Label the axes,
reactants, products, activated complex,
activation energy, and change in
enthalpy.
Factors Affecting Reaction Rate
 1.
Nature of the Reactants
 2. Concentration of Reactants
 3. Surface Area of Reactants
 4. Temperature
 5. Catalysts
Factors Affecting Reaction Rate
1. Nature of the Reactants
Certain bonds are easier to break than others
Ca(s) + 2H2O(l)  H2(g) + Ca(OH)2(aq)
 2Na(s) + 2H2O(l)  H2(g) + 2NaOH(aq)


Na reacts with water faster than Ca because Na
is more reactive.
2. Concentration
high concentration =
the higher the
concentration → the
more molecules there
are → the more
collisions take place
creating product
3. Surface Area

the more surface a
molecule can touch
→ the faster the
reaction

Increase surface
area by…


using smaller
particles
dissolving in water
Temperature
4. Temperature
 high
temp = fast reaction rate
the hotter the molecules → the more
they move → the more they collide &
change into product
• Lower temp, more
reactant particles have
low amts of kinetic
energy
• As temperature
increases, more
reactant particles have
higher kinetic energy
• Only a small # of
particles will have
enough energy to form
an activated complex.
Catalysts
5. Catalyst



substance that increases
reaction rate without being
consumed in the reaction
lowers the activation
energy, more collisions
will have enough energy
to pass over the energy
barrier
reaction is faster because
a greater fraction of
collisions is effective at
any given point

Catalysts in body

Enzymes

Breakdown sugars, proteins, etc
Reaction Mechanisms
Many reactions occur in several
steps.
2 O3 (g)  3 O2 (g)
The decomposition of ozone to
oxygen is catalyzed by Cl.
Reaction Mechanisms
Cl + O3  ClO + O2
O3  O + O2
ClO + O  Cl + O2
2 O3  3 O2
Elementary
step
Intermediates
Add steps
Reaction Mechanisms
2 NO  N2O2
N2O2 + H2  N2O + H2O
N2O + H2  N2 + H2O
Fast
Slow
Fast
Determine the original equation.
Rate-determining step.
Reaction Mechanisms
HBr + NO2  HOBr + NO Slow
HBr + HOBr  H2O + Br2
Fast
What is the overall reaction?
How many elementary steps?
What are the intermediates?
What is the rate-determining step?