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Describing Reaction Rates
Kinetics
How fast does the reaction go?
Reaction Rate :
the change in the amount of
reactant or product per unit time.
•
•
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The word rate can be used as a synonym of speed
Rates measure the speed of any change that
occurs within a time interval
The interval of time may range from fractions of a
second to centuries
rusting
slow
baking
explosion
fast
very fast
Describing Reaction Rates
•The figure below illustrates the progress of a typical
reaction. Over time, the amount of reactant
decreases and the amount of product increases.
Graphing rates of reaction
Rates of chem rxns are related to the
properties of atoms, ions, and
molecules through a model called
Part 1: Collision Theory
• Reactions don’t happen
unless the substances are
in contact.
Collision Theory and Reaction
Rates
According to collision theory, atoms, ions,
and molecules can react to form products
when they collide if the particles have
enough kinetic energy.
Particles that do not have enough energy to
react bounce apart unchanged when they
collide.
Collision Theory
Describing Reaction Rates
The minimum energy that colliding
particles must have in order to react is
called the activation energy.
An activated complex is an unstable
arrangement of atoms that forms for a
moment at the peak of the activationenergy barrier. Very brief existence;
sometimes called the transition state.
The activation-energy barrier must be
crossed before reactants are converted
to products.
Collision
Theory
Enough energy: effective collision – product
formed
Not enough energy: ineffective collision – no
product formed
Particle orientation is also
important
The minimum energy colliding
particles must have in order to react is
called the…
a. activated complex
b. reaction rate
c. reaction energy
d. activation energy
The study of a reaction’s progress
over a period of time is called
A) thermochemistry
B) kinetics
C) stoichiometry
D) catalyst
What happens if atoms collide without
enough energy?
A) they stick together
B) they bounce off each other
C) they form products
D) they react
Which of the following is the theory
that says that atoms, molecules and
ions must bump into each other for a
chemical reaction to occur?
A) reaction theory
B) mechanism
C) collision theory
D) accident theory
The minimum energy colliding
particles must have in order to react is
called the…
a. activated complex
b. reaction rate
c. reaction energy
d. activation energy
The study of a reaction’s progress
over a period of time is called
A) thermochemistry
B) kinetics
C) stoichiometry
D) catalyst
What happens if atoms collide without
enough energy?
A) they stick together
B) they bounce off each other
C) they form products
D) they react
Which of the following is the theory
that says that atoms, molecules and
ions must bump into each other for a
chemical reaction to occur?
A) reaction theory
B) mechanism
C) collision theory
D) accident theory
Part 2: Rate Laws
It is not enough to say that a reaction is fast or
slow. Industries must know HOW fast or slow
and WHY the reaction is fast or slow.
H2SO4 Sulfuric acid uses
fertilizers
gelatin
Plastics
perfumes
Dyes
car batteries
Paper
glue
Explosives rayon
Detergent
paints
Steel
printing ink
Disinfectants
chemotherapy drugs
Rate laws express the rate of a reaction in
terms of the concentration [ ] of the reactants.
For example Rate = k[A]x[B]y
where [A] and [B] are concentration in molarity and x
and y are the powers of the concentrations of the
reactants.
The specific rate constant (k) for a reaction relates
the concentrations of reactants to the rate of the
reaction.
k is large if products form quickly
k is small if products form slowly
Part 4: What affects reaction rate?
1.
2.
3.
4.
5.
Nature of the reactants
Temperature
Concentration or pressure of a reactant
Surface area of a solid reactant
Presence/absence of a catalyst.
The Nature of the Reactants:
Individual properties of substances also affect
reaction rates. Some of the properties in this
category are state of matter, molecular size,
bond type and bond strength.
In general, gases react more rapidly than liquids
than solids.
Temperature and particle collisions
Temperature: Particles turn heat energy into
kinetic energy.
When they get hotter they move faster
Generally, ↑temperature ↑reaction rate.
↓temperature ↓reaction rate.
More collisions
More molecules with enough kinetic energy to slip
over the activation-energy barrier.
↑ temperature causes products to form faster.
The effect of temperature on rate
The higher the temperature,
the faster the rate
of a reaction.
In many reactions,
a rise in temperature
of 10°C causes the
rate of reaction to
approximately double.
Temperature and Reaction Rate
Concentration and particle collisions
Concentration
More particles means more collisions.
↑collision frequency ↑ reaction rate.
Surface area and particle collisions
Grind up Chemicals –
Smaller Particle Size
Large
particles
Small
particles
The smaller the pieces, the larger the surface
area. This means more collisions and a
greater chance of reaction.
Catalysts
• They don’t speed up the actual molecules but
the reaction happens a lot faster. How?
They change
the pathway
of the
reaction
Boone
lake
Your house
More About How Catalysts Work?
PRODUCTS
REACTANTS
CATALYST – probably a metal like platinum
The bonds in the reactants are weakened because
the metal atoms attract electrons away from the
"bonded" atoms. The bonds of the reactant break with
much less energy input.
Catalysts lower the
activation energy
How Do Catalysts Work?
Is a catalyst a reactant, since it is
involved in the reaction? No, because it is not
used up. Just like the bridge it will be there after
it is used.
Boone
Lake
Your house
Biological catalysts - enzymes
At normal body temperature (37C),
reactions in the body would be too slow
without catalysts.
• The catalysts that increase the rates of
biological reactions are called enzymes.
An inhibitor is a substance that
interferes with the action of a catalyst.
Why is it cost effective to use expensive
metals such as platinum and palladium in
chemical reactions?
It is a one time investment since they
are not used up in the reaction.
Which of the following factors could be
increased in order to decrease a reaction
rate?
A. Catalyst concentration
B. Concentration
C. Temperature
D. Particle size
A catalyst works by ____.
a.lowering the activation energy barrier
b. shifting the equilibrium position toward
the products
c. changing the temperature of the reactants
d. changing the particle size of the reactants
Which will have a faster reaction rate?
A. One reactant is
a solid; the other
dissolved.
B. Both reactants
are dissolved
Which of the following factors could be
increased in order to decrease a reaction
rate?
A. Catalyst concentration
B. Concentration
C. Temperature
D. Particle size
A catalyst works by ____.
a.lowering the activation energy barrier
b. shifting the equilibrium position toward
the products
c. changing the temperature of the reactants
d. changing the particle size of the reactants
Which will have a faster reaction rate?
A. One reactant is
a solid; the other
dissolved.
B. Both reactants
are dissolved