Reaction Rates

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Lecture 47

• Reaction Rates

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Bio

• Ehrich Weiss.

Harry Houdini
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Came from a poor family

• When 13 worked in the New York garment district

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"The Memoirs of Robert-Houdin,

• Had no money but read a great deal

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First illusions involved card tricks
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Later he met a Beatrice Rahner he called Bess
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"challenge escape artist,"

• He became great because he challenged himself and
  others challenged him

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He was a pioneer aviator

• In 1916 Houdini began a film career.

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Houdini did not die in an escape or fail in
somefinal escape as many believe.

• his appendix was ruptured.

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Who challenges you

• If you can not challenge yourself find someone
  who can propel you forward and set a great mark

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Read pages 576 to 585

• Problems 1 to 10 on page 585

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the first question I hear every morning is are we
going to blow something up??
You want excitment
WHY
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do you see a chemical reaction taking place?

• a piece of steel
• a glass of milk
• your hand
• felt tip markers

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Some chemical reactions are fast and some are
slow
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Rusting of metal is slow and even though it is
happening on a grand scale you dont always
notice it
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Other reactions are fast but small

• bacterial infection

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Back in 1993 Jack in the box had some tainted
meat that it obtained from a slaughter house.

• The state regulations were such that hamburgers
  must be cooked for 5 min. with a throughout temp
  of 140 degrees or 2 to 2.5 min at a temperature
  of 155 degrees. Jack in the box had their
  burgers checked and it was found that the burgers
  were at 130 degrees of 2.5 min
•  

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Show demoand how reactions occur at different
speeds

• Mg metal Zn metal tin metal
• 1 molar HCl

Alka-Seltzer Match burning
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Reaction rates are important they tell us a
great deal about what is going on in the chemical
reaction.

• How can we gauge how fast and how complete a
  chemical reaction is progressing?

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In a car what kind of gauge do we use to tell us
how fast we are going?

• Speedometer
• distance over time
• Miles per hour
• Km per hour

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When dealing with spinning propellers what gauge
do we use for the speed of the propeller?

• revolutions per min.

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How can you quantitate the rate of a chemical
reaction

• The Rate of something is always the unit of
  something per time
• Miles per hour
• Revolutions per second
• Liters per second

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• To measure the reaction rate for our car we use a
  stop watch and note the distance traveled and
  calculate the distance/time
• for the fan we count the revolutions after a
  particular amount of time

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In a chemical reaction we gauge a reaction rate
by measuring the change in the amount of either
a reactant or the product over time
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In order to accurately measure the rate of a
reaction it is critical to measure some
observable factor or physical property.

• 1)     Volume of a gas
• 2)     temperature
• 3)     color
• 4)     mass
• 5)     acidity
• 6)     evolution of a gas

We graph the change vs time
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by noting how fast the acidity changes, the color
changes. or the temperature changes then the
overall reaction rate can be determined.
Graph the change vs the time
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CH3OCH3(g) ? CH4(g) CO(g) H2(g)
You do not use a rate you just say it is
decreasing The same way you dont say -
acceleratoin

• The CH3OCH3(g) decreases with time until it is
  all decomposed

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2N2O5(s) ? 4NO2(g) O2(g)

• What about the coefficients 2N2O5
• It is like two fires burning, how do you
  calculate the rate of burning
• You dont want the rate of 2N2O5 just one N2O5

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For these problems we will give you the change in
a particular reactant or product over a
particular time and you can calculate the rate of
the reaction
The amount of NaCl went from 3.5 molar to 4.7
molar 5 seconds. What is the rate of creation of
NaCl?
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But wait a second

• When you drop an object does the rate of the fall
  consistent?

When our car accelerates or when it stops is the
rate of change consistent
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Like falling objects and momentum Chemical
reactions dont progress at a constant ratethey
are more complex then that
OH what a pain
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Graph of change
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Besides using a graph you can get your data from
a table

• Here we can see that the change in H3O from 85
  to 95 seconds (?T10s) you get a change in
  Molarity of .0018moles/liter

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Reaction rates are expressed asMoles/liter per
second
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Just remember that the reaction rate expression
includes the change in molarity on the numerator
and the coefficient and change in timeon the
denominator
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Reaction rates are dependent on many factors

• Why do you keep food in refrigerator?
• Why does your car run better when it is warmed
  up?
•  

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Factors that effect reaction rate can be
explained by studying the collision theory
which states that a factor that increases the
number of "effective collisions between two
reacting elements will increase the reaction
rate.
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An effective collision is one that is powerful
enough to cause a reorganization of an element
into a lower energy level
The reaction will continue to progress if the
energy released is enough to energize another
effective collision
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• Remember we talked about the 5 kinds of reactions
• some involve the coming together of
  atoms
• some involve the exchange of atoms
• some involve breaking apart

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• with the exception of decomposition most
  reactions involve the coming together of atoms
  and therefore the reaction rates of most chemical
  reactions involve collisions. And this is
  called the collision theory.
• More collisions more rapid the reaction.

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I used this theory to meet girls

• But the key word is effective collision.
• There must be enough energy in the collision to
  actually make a change in the reactants

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the number of collisions is a critical factor in
establishing the rate of reaction.

• What characteristic can increase the number of
  collisions?
• 1)     surface area of the reactants
• 2)     Concentration of the reactants
• 3)     nature of the reactants themselves
•  

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1)     surface area of the reactantsthe
greater the surface area the greater the number
of collisions

• Demo steel wool vs steel
• powdered sugar vs sugar

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2) Concentration of the reactantsThe greater
the concentration the greater the number of
atoms colliding

• Demo of concentrated acid vs dilute

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3)    nature of the reactants themselves

• Some reactions
• like endothermic or double replacement ones
• are inherently slow

Which takes longer Double replacement or
synthesis?
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Temperature is also a determining factor. The
more heat in a system the more rapidly the
molecules are moving and the more collisions that
are occurring.
And with added heat there is an increase in the
kinetic energy in the collision making the
reaction more likely to occur.
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Hot water and cold for alka-seltzer
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Just because two atoms or molecules collide this
does not mean the atoms or molecules will react.

• A molecular or atomic collision must be of great
  enough force as to dislodge or rearrange the
  molecules involved

It takes a particular amount of force to break a
bond
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That particular amount of force necessary to
crack a bond and form a new compound

• is called the Activation energy

Not all reactions are spontaneous
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Which takes more energy, to decompose a compound
or to synthesize compound?
Exothermic or endothermic have greater reaction
rate

• Remember Gibbs free energy is a quantitative
  value on whether of not a reaction was spontaneous

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Activation energy is the energy necessary to
start a reaction
Compounds go from a higher energy to a lower
energy but need a push to get the fall started
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Watch the sugar potassium chlorate flame thrower

• We had to disrupt the electron clouds of the
  reactants with acid to start the reaction. Then
  the exothermic nature of the reaction provided
  enough heat to keep things going

8.90 grams Potassium Chlorate, KClO3 18.20
grams Sucrose (Sugar) 1 drop concentrated
Sulfuric Acid, H2SO4
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It takes a great deal of energy to get some
charcoal started

• The activation energy for charcoal is great
• So what do they do to make it easier to light

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2H2O2 ? 2H2O O2

• It takes very little energy to decompose peroxide
  this why the peroxide should be stored in what
  kind of bottle at what temperature?

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Another common medicine that is very sensitive to
heat is aspirin or salicylic acid don't store
your medicines in a hot moist place.

• like a bathroom.

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We have seen how the concentration, surface
areas, temperature and the nature of the
reactants effect the reaction rate.
One additional factor is the presence of a
catalyst
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And we know that increased temperature will
increase the number of collisions
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Is there another way we can increase the reaction
rate without messing with the concentrations or
temperature or number of collisions?
Catalysts
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A catalyst is a chemical that increases the rate
of the reaction without being involved in the
reaction.

• It is kind of like a table or funnel in which the
  reaction takes place. It is not one of the
  reactants or products but it enables the reaction
  to take place

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Getting all three atoms together is both
difficult on both a energy and physical basis
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Enter the Catalyst
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By adding a mechanism that will make the
collisions more efficient and funnel the
reaction, the rate will increase without having
to increase concentration, temperature, surface
or anything
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In the manufacture of peroxide H2O2 you need to
get two hydrogen's and two oxygen's together at
the exact time. If you get the H2O together first
you just get water and O2. A catalyst allows
the creation of H2O2
MnO2
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Can anyone name another catalyst in nature

• Chlorophyll

The human body has millions of reactions of each
of the 5 classes taking place at the same time
under the same temperature.
How does the body regulate these reactions so
there is no excess products or reactants?
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The body contains millions of catalysts but in a
living biological system they are not called
catalysts

• Enzymes

hemoglobin
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It recognizes, confines and orients the substrate
in a particular direction.
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Special enzymes often force stable atoms apart
until they are unstable.

• In the body, enzymes crack complex proteins or
  dissolve bone

Plants crack stable CO2 into C O2
Taking these stable proteins and making them
unstable causes the creation of unstable radical
molecules.
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In Fire flies an enzyme makes an unstable
chemical

• These unstable molecules that result vibrate
  widely and like our beach ball with a few
  misplaced weights applied, are unstable. This
  unstable state results in a shifting of the
  electrons into a more stable electron orbit.

Eventually the electrons fall back into a more
stable orbit and in doing so give off light.
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What is it called when light is given off by a
living organism

• Bioluminescence

Enzymes that take a stable molecule, make it
unstable and then allow it to become more stable
and give off light as it does so
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Enzymes that take a stable molecule, make it
unstable and then allow it to drop back to a
stable state and give off light as it does sois
Bioluminescence