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Chapter 17 - Kinetics

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Chapter 17 - Kinetics Reaction Rates Section 17.1 – PowerPoint PPT presentation

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Title: Chapter 17 - Kinetics


1
Chapter 17 - Kinetics
  • Reaction Rates
  • Section 17.1

2
What is the reaction rate?
Kinetics - The branch of chemistry that is
concerned with the rates of change in the
concentration of reactants in a chemical
reaction. The reaction rate is a measure of the
change in the concentration of reactants or
products over time in a chemical
reaction. Reactions can happen very quickly or
extremely slow. Examples fast and furious
chemical rxn slower rxn rate
millions of years rxn rate
3
Expressing Reaction Rates
Fast and slow helps us to visualize the speed of
chemical reactions, but chemists, doctors and
engineers need to be more specific about how fast
reactions occur. For example, a knowledge of the
speed at which radioactive tracking devices break
down in the human body enables doctors to take
x-rays to diagnose diseases.
4
Expressing Reaction Rates
In medicine, specific isotopes are used to
observe the condition of specific organs. A
common procedure is the injection of iodine-131
for the observation of the thyroid gland. A
healthy thyroid will accumulate any iodine
entering the body. Because the body cannot
distinguish between stable iodine and its
radioactive isotope, iodine-131 will also be
accumulated. When a physician scans the patient,
if iodine-131 is present in the thyroid, the
gland is working properly. However, if the trace
element has not collected in the thyroid, the
physician knows the gland is failing. Thyro
id gland Showing cancer
5
Expressing Reaction Rates
Related technology is often used in industrial
settings. Using trace elements, engineers can
follow the path of a coolant or lubricant
throughout a system. It is also used to identify
inclusions, cracks, areas of porosity, and other
flaws. When used in test situations,
radioisotopes can help to detect areas of
abnormal wear and corrosion.
6
Exothermic Reactions

An exothermic reaction is a chemical reaction
accompanied by the release of heat. In other
words, the energy needed for the reaction to
occur is less than the total energy released. As
a result of this, the extra energy is released,
usually in the form of heat.
7
Endothermic Reactions

An endothermic reaction is a chemical reaction in
which heat is absorbed. The energy needed for
the reaction to occur is more than the amount of
energy released. Therefore, high energy products
are formed.
8
Factors that Affect Reaction Rate - Concentration
Four main external conditions affect reaction
rate. The first is the concentration of
reactants. Generally speaking, if we increase the
concentration of one or more reactants, the
reaction will go more quickly. This is simple
because the more molecules, the more collisions
between molecules, and the faster the reaction
will go.
9
Factors that Affect Reaction Rate - Temperature
The second factor that influences reaction rate
is temperature. The higher the temperature of the
reaction, the more quickly it will proceed. At
higher temperatures, the molecules are moving
around more quickly (they have more kinetic
energy) this means they will collide with each
other with more energy, and its more likely that
they will overcome the activation energy needed
to start the reaction. Its a general rule of
thumb that a 10C increase in temperature will
double the reaction rate.
10
Factors that affect Reaction Rate - Catalyst
The addition of a catalyst will also speed up a
chemical reaction. A catalyst speeds up the rate
of reaction by lowering the activation energy.
Biological catalysts are known as enzymes. The
only other important thing you need to remember
about catalysts is that they are not consumed in
the course of the reaction.
11
Factors that Affect Reaction Rate Physical
State of Reactants
The final factor that affects certain reactions
is the physical state of the reactants. For
example, if you mix two gases or two liquids,
this represents a homogenous reaction, but if
reactants are in different phases, for example,
if one is a gas and one is a liquid, then the
reaction area is limited to the area where they
touch each other, and the larger this area, the
faster the reaction will proceed. For example,
consider a teaspoon of salt dissolving in water.
If you were to dump the salt into the beaker of
water and let it float to the bottom without
stirring it, it would take much longer for it to
dissolve than if you stirred the solution.
12
Reaction Rate Calculations
The speed at which a process works is measured
against time. For example, a sprinter moves in
meters per second, the speed of a car in miles
per hour, and the speed of light is measured by
the distance it travels in a year. The rate of a
chemical reaction is measured as the change in
concentration (mol/L) per second. CO (g)
NO2 (g) -gt CO2 (g) NO (g) NO _at_ t2 NO
_at_ t1 ?NO t2 t1
?t molar concentration t
time ? change
13
Reaction Rate Calculation Example
t1 0.00 sec t2 2.00 sec NOt1 0.00
M NOt2 0.01 M 0.01 M 0.00 M
.01 M .0050 M/L s 2.00 sec 0.00 sec
2 sec
14
Practice Problems
  • 1. Determine the rate of reaction of a 1.0 M
    concentration of chlorine when it changes during
    a reaction to a 0.0 M concentration in 10
    seconds.
  • Determine the rate of reaction of a 2.4 M
    concentration of hydrogen when it changes during
    a reaction to a 0.00 M concentration of hydrogen
    in 6 seconds.
  • 3. Determine the rate of reaction of a 1.6 M
    concentration of calcium when it changes during a
    reaction to a 0.2 M concentration of calcium in
    40 seconds.

15
Questions you should be able to answer . . .
  • What does the reaction rate indicate about a
    particular chemical reaction?
  • How is the rate of a chemical reaction usually
    expressed?
  • What is chapter 17 about?
  • What factors affect the reaction rate?
  • What is the formula for average rate?
  • How are reaction rates expressed?
  • Are reaction rates positive or negative?
  • What is the difference between M and mol/(L s)
  • What do brackets around a formula represent?
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