Chapter 16 - Kinetics

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Chapter 16 - Kinetics

• Honors Chemistry
• Reaction Rates

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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
(mol/Ls) Reactions can happen very quickly or
extremely slow. Examples fast and furious
chemical rxn slower rxn rate
millions of years rxn rate
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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.
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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
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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.
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Rate of change At what rate is this car traveling?   If the rate is 50, what is this in the metric system?    
How is rate of a reaction expressed? What is happening to the reactants and products over time?      
Reaction Rate and units Definition           Units        
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Reaction Rates are determined experimentally   CO NO2 ? CO2 NO       Note negative reaction rate values indicate that the concentration of a compound decreases as the reaction proceeds. When the rate is measured by consumption of a reactant, a negative sign is applied to the calculation to get a positive reaction rate.   Average Rate Example If the concentration of NO is 0.000M at time t10.00 s and 0.010M two seconds after the reaction begins, the following calculation gives the average rate of the reaction expressed as moles of NO produced per liter per second.     Answer    
                   
                       
Why is my answer negative?
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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 (s) ? change
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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 mol/L s 2.00 sec 0.00 sec
2 sec
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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.

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Collision Theory
Reaction rates are actually calculated from
experimental data. However, looking at chemical
reactions on a molecular level provides us with a
clear picture of what reaction rate
measures. Have you ever seen a demolition derby
in which the competing vehicles are constantly
colliding? Each collision results in the
demolition of one or more vehicles. In a
chemical reaction, reactants must also come
together in order to form products.
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The Collision Theory
The Collision Theory states that atoms, ions, and
molecules must collide in order to react. The
more collisions that occur between atoms, ions,
and molecules the faster the reaction. However,
in reality, detailed calculations of the number
of molecular collisions that actually occur in a
reaction yield a puzzling result only a small
fraction of collisions produce reactions. For a
collision of atoms, ions, and molecules to
produce a reaction a collision must have
1. Correct Orientation 2.
Sufficient Energy
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Orientation and Activated Complex
Correct orientation requires that molecules,
ions, and atoms strike each other with the
correct atoms in the molecules coming into
contact. Animation Collision Theory
If this occurs, an activated complex is formed
which is a temporary unstable arrangement of
atoms that may break apart to form products. The
activated complex is also called the transition
state.
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Activated Complex
Another critical factor that will determine if an
activated complex is formed is the presence of
enough activation energy (Ea).
Animation
Even though molecules, atoms and ions collide in
the correct orientation, if there is not enough
activation energy (Ea) the atoms, molecules and
ions will rebound and not form an activated
complex and new product.
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What Factors Affect the Rate of a Reaction?
In order for something to be produced, what must the molecules do? Crash Course
What are the two criteria that must be met when particles collide?         For example, how is water produced? What must collide? 1.       2.          
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Mini Lab Activity Im Feeling Dizzy To see how important energy is to controlling a reaction, you are going to design an experiment using marbles and a petri dish. 1. You must find the relationship between temperature, the frequency of collision, and the energy needed to cause a change from reactants to products.   Results                            
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  What you have just modeled from the activity was called the activation energy, Ea, of a reaction.   What you have just modeled from the activity was called the activation energy, Ea, of a reaction.
What is activation energy (Ea)?               What can we compare activation energy to?            
What is an activated complex and where does it form?                
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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.
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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.
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What is the difference between the diagrams? Label them as endothermic and exothermic.       How does Ea affect a reaction?                  
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Factors Affecting Reaction Rates

• Besides correct orientation and sufficient
  energy, there are other factors that affect the
  speed of a chemical reaction. These include
• The nature of reactants
• 2. Concentration
• 3. Surface Area
• 4. Temperature
• Catalyst
• Inhibitors

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Factors that Affect Reaction Rate - Concentration
Five 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.
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Concentration
Reactions speed up when concentrations of
reacting particles are increased because there
are more atoms, molecules, and ions present to
react. This produces more activated complexes
and more product.
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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.
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Temperature
In most cases, increasing the temperature will
increase the rate of reaction because atoms,
ions, and molecules move faster and collide more
often. By heating a gas from 20C to 30C, the
frequency of collisions increases by 10, but the
rate of reaction increases 100.
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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.
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Presence of a Catalyst
Catalysts are substances that change the rate of
a chemical reaction without being consumed by the
reaction.
A catalytic converter changes harmful CO and NO
to safer CO2 and N2 using platinum as the
catalyst. 2CO 2NO -gt 2CO2 N2
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Inhibitors
Inhibitors are catalysts that slow down a
chemical reaction. To keep apples from rotting
before they are sold, growers add inhibitors to
slow the effect of acetylene gas which causes
apples to ripen.
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Factors that Affect Reaction Rate Physical
State of Reactants
Another 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.
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Nature of Reactants
The nature of reactants strongly affects how
quickly a reaction will occur, because some
substances react more readily than others. Some
substances, depending on their position in the
Periodic Table, release electrons easier than
others. Elements in families 1A and 2A are much
more reactive than elements in families 3A and
4A.
Potassium in water
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Factors that Affect Reaction Rate Surface Area
The greater the surface area the more atoms,
ions, and molecules available to react which
causes a faster reaction.
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Surface Area
The greater the surface area, the more atoms,
ions, and molecules available to react and the
faster the reaction.
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Factors That Affect Reaction Rates
Factors That Affect Reaction Rates   Name the five factors that affect the reaction rate     1. What happens to group 1A elements when they are placed in water?         2. How does concentration affect the rate of a reaction?              3. What is surface area and how does it affect reaction rate?               4. Popping the Top activity how does temperature affect the rate of reaction?             5. Mini Lab Activity Jeanie in a Bottle How does a catalyst affect the rate of a reaction?        
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Popping the Top How Temperature Relates to
Rates of Reaction   Purpose You will
investigate how temperature can affect the rate
of a reaction. Materials film canisters, warm
water, ice water, room temperature tap water,
Alka Seltzer tablets Procedure 1. Break one
Alka Seltzer tablet in half. 2. Fill the
film canister about ½ full of tap water. 3.
Very quickly put the tablet in the canister and
put the lid on the canister. 4. Try not to
scream!!!!!
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• Mini Lab Activity Jeanie in a Bottle
• Purpose You will investigate a catalyst and
  write down your observations of a reaction.
• Materials 3 Hydrogen Peroxide, small beakers,
  yeast packets, manganese dioxide, glass stirring
  rod
• Procedure
• Pour about 10 mL (the exact amount is not
  important) into a small beaker.
• Add a pinch of yeast to the beaker and observe
  while stirring with the stirring rod.
• Pour out the beaker in the sink.
• Repeat steps 1 and 2 with the manganese dioxide.
• Pour out the contents of the beaker in to the
  sink.
• Data and Questions

What is equilibrium and how is it measured?   1.    
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This graph represents the energy of particles at two different temperatures. T1 is at a lower temperature than T2. A. What do you notice about the height of the hills? Which one is higher?           B. Notice that curve T2 is shifted relative to T1. What happened to the number of particles as the temperature increased relative to the activation energy?           C. If you wanted to increase the rate of reaction, how would you do it according to the chart?     D. Based on what you wrote, why would your guess work?          
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Look at the chart below. Can you figure out how
a catalyst works? Answer
What can we use to slow down the rate of
reaction? ________________ Inhibitors
What is the difference between a homogeneous catalyst and a heterogeneous catalyst? What is the difference between a homogeneous catalyst and a heterogeneous catalyst?
homogeneous catalyst           Example heterogeneous catalyst           Example      
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Please answer the following questions on a
separate sheet of paper

• 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)
• 9. What do brackets around a formula represent?