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TRANSCRIPT
1 of 39 © Boardworks Ltd 2007
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What does rate of reaction mean?
The speed of different chemical reactions varies hugely.
Some reactions are very fast and others are very slow.
What is the rate of these reactions?
The speed of a reaction is called the rate of the reaction.
rusting baking explosion
slow fast very fast
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Changing the rate of reactions
increased temperature
increased concentration of
dissolved reactants, and increased
pressure of gaseous reactants
increased surface area of solid
reactants
use of a catalyst.
Anything that increases the number of successful collisions
between reactant particles will speed up a reaction.
What factors affect the rate of reactions?
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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.
Why does increased temperature
increase the rate of reaction?
At a higher temperature, particles
have more energy. This means
they move faster and are more
likely to collide with other particles.
When the particles collide, they
do so with more energy, and so
the number of successful
collisions increases.
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Effect of concentration on rate of reaction
The higher the concentration of a dissolved reactant, the
faster the rate of a reaction.
Why does increased concentration increase the rate of
reaction?
At a higher concentration, there are more particles in the
same amount of space. This means that the particles are
more likely to collide and therefore more likely to react.
higher concentrationlower concentration
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Effect of pressure on rate of reaction
The gas particles become closer together, increasing the
frequency of collisions. This means that the particles are more
likely to react.
Why does increasing the pressure of gaseous reactants
increase the rate of reaction?
As the pressure increases, the space in which the gas
particles are moving becomes smaller.
lower pressure higher pressure
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Effect of surface area on rate of reaction
Any reaction involving a solid can only take place at the
surface of the solid.
If the solid is split into several pieces, the surface area
increases. What effect will this have on rate of reaction?
The smaller the pieces, the larger the surface area. This
means more collisions and a greater chance of reaction.
This means that there is an increased area for the reactant
particles to collide with.
low surface area high surface area
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reaction (time)
en
erg
y (
kJ)
What are catalysts?
Catalysts are substances that change the rate of a reaction
without being used up in the reaction.
Catalysts never produce more product – they just
produce the same amount more quickly.
Different catalysts work in
different ways, but most
lower the reaction’s
activation energy (Ea).
Ea with
catalyst
Ea without
catalyst
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Unit 16: Nuclear Chemistry
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Radioactivity
• Emission of subatomic particles or high-
energy electromagnetic radiation by nuclei
• Such atoms/isotopes said to be
radioactive
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Its discovery
• Discovered in 1896 by Becquerel
• Called strange, new emission uranic rays
• Cuz emitted from uranium
• Marie Curie & hubby discovered two new
elements, both of which emitted uranic
rays
– Polonium & Radium
• Uranic rays became radioactivity
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Types of radioactivity
• Rutherford and Curie found that emissions
produced by nuclei
• Different types:
– Alpha decay
– Beta decay
– Gamma ray emission
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The properties of the
different types of radiation
The differences between
the three types of radiation
can be seen by passing
them through an electric
field.
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Characteristics of an alpha
particle
The alpha particle
(a) is deflected to
some extent
toward the
negative plate.
This indicated that it is
positively (+) charged
and has a fairly large
mass.
Today we know that an a particle is the same
as the nucleus of a He atom.
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Characteristics of a beta particle
The beta particle (b)
is deflected toward
the positive plate (+).
It is also deflected
more than the a
particle
This indicates that it is
negatively (-) charged and
has a much smaller mass
than the a particle.
Today we know that a b particle is the same as
an electron.
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Characteristics of gamma
radiation
Gamma radiation (g)
is not deflected
toward either the
positive (+) or
negative (-) plate.
This indicates that it has
no charge.
Today we know that gamma rays are a type of
electromagnetic radiation made up of photons
(packets of energy).
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Radiation and Table O
The Regents Reference Tables provides us with a
summary of the different types of radiation:
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Penetrating power of radiation
The ability of radioactive particles to pass through air and other materials is inversely related to their mass.
• Alpha particles – the least penetrating, they travel only a few centimeters through air. They can be stopped by a single sheet of paper.
• Beta particles – more penetrating, they travel several meters through air. They can be stopped by a sheet of Al or plastic.
• Gamma Rays – most penetrating, thick sheets of lead or concrete are required to stop gamma rays.
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22
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• Beta decay
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Diagram showing
penetrating ability
www.epa.gov
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Where does the radiation
come from?
Rutherford suggested that the radiation resulted
from the breakdown of the nucleus of an atom,
resulting in radiation being given off, and the
nucleus of the atom changing into a new element.
For instance, the fact that U-238 undergoes
alpha decay (emits an a particle) can be shown
by this reaction:
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Why does the atom break up?
Remember that the
nucleus of the atom is
held together by the
strong nuclear force.
This force is normally
strong enough to hold
the protons and
neutrons together.
However, sometimes the force of repulsion due
to the protons having the same charge
overcomes the strong nuclear force and the
atom breaks apart.
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How does beta decay occur?
Sometimes an atom will emit a b
particle when it breaks up.
In beta decay a
neutron apparently
“spits out” an electron
(the b particle) and
becomes a proton.
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Balanced nuclear equations
Nuclear reactions can be represented by equations.
These reactions are governed by two “laws”:
The law of conservation of mass number – the sum of
the mass numbers on the reactant sides must be
equal to the sum of the mass numbers on the product
side.
This law applies to all nuclear equations!
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Balanced nuclear equations
The second law is:
The law of conservation of charge (atomic #) – the
sum of the atomic numbers on the reactant sides
must be equal to the sum of the atomic numbers on
the product side.
This law applies to all nuclear equations!
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Predicting products in alpha
decay
The Law of Conservation of Mass Number and the
Law of Conservation of Charge allows us to predict
products in a nuclear reaction.
For instance, suppose we wanted to predict the
atom produced when Radon-222 undergoes
alpha decay.
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Predicting products
Based on the two laws we can predict:
The mass number of particle X must be
218.
The charge (atomic #) of particle X must be 84.
The symbol of the element can then be
determined from the Periodic Table.
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How about beta decay?
It works the same way. Let’s look at the beta decay
of Strontium-90.
Remember the sum of the mass numbers and
atomic number on both sides MUST be the same.
So atom X must be:
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Using balanced nuclear
equations to identify the type
of radioactivity.
Suppose we know that a particular atom undergoes
radioactive decay and we are able to identify the atom
that is produced.
Using the Laws of Conservation of Mass # and Charge, we
can identify the type of radiation given off.
For instance, Iodine-131 is known to form Xenon-131
when it decays. What radioactive particle must it emit?
Particle X must be a b particle:
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Another type of radioactive
decay
Some atoms undergo a decay process that
produces a positron. A positron has the same
mass as an electron, but is positively charged.
Symbols for the positron include:
Positrons are a form of anti-matter. Antimatter is
made up of particles with the same properties as
normal matter, but are opposite in charge.
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Positrons are also listed in
Table O
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Positron Emission
We can use our ability to balance nuclear equations
to predict what will be given off when Potassium-37
undergoes positron emission.
There’s only one atom that will work, and
that’s Argon-37.
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Your turn!
Using the Laws of Conservation of Mass # and
Charge, write balanced nuclear equations for the
following nuclear reactions:
1. Beta decay of Phosphorus-32
2. Alpha decay of U-238
3. Positron decay of Iron-53
4. Decay of Oxygen-17 into Nitrogen-17
5. Decay of Potassium-42 into Calcium-42
6. Decay of Plutonium-239 into Uranium-235
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Half Life
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Half-lifes
The rate at which a particular radioisotope decays is
described by its half-life.
The half-life is defined as the time that it takes for
one half of a sample of a radioactive element to
decay into another element.
The half-life of a radioisotope is dependent only on
what the radioisotope is.
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Table N provides us
with a list of various
nuclides, their decay
modes, and their half-
lifes.
Using Table N, what is
the decay mode and
half-life for Radium-
226?
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Using Table N
Table N indicates that Radium-226 undergoes alpha
decay.
Based on this we can write a balanced nuclear equation to
represent this reaction:
This tells us that for every atom of Radium that
decays an atom of Radon is produced.
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Using Half-life
Table N also tells us that Radium-226 has a half-life of
1600 years.
Starting with a 100g
sample, after 1 half-
life (or 1600 years),
50g remain.
After another 1600
years, half of the
50g will remain
(25g).
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Carbon-14 Dating
The age of objects that were once alive can be
determined by using the C-14 dating test. In this test,
scientists determine how much C-14 is left in a sample
and from this determine the age of the object.
From Table N we can determine that C-14 undergoes
b decay:
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Where does the Carbon-14
come from?
C-14 is created in the
atmosphere by
cosmic rays.
It becomes part of living
things through
photosynthesis and the
food chain.
When the plant or
animal dies, the C-14
begins to decay.
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Using C-14 to Age Objects
By comparing the amount of C-14 left in a sample to the amount that
was present when it was alive, and using the half-life of 5700 years
(Table N), one can determine the age of a sample.
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Sample Half-life Problem 1
A 10 gram of sample of Iodine-131undergoes b decay, what
will be the mass of iodine remaining after 24 days?
From Table N, the ½ life of iodine is determined to be
approximately 8 days.
That means that 24 days is equivalent to 3 half-lifes.
The decay of 10 grams of I-131 would produce:
1.25 grams of I-131 would remain after 24
days.
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Sample Half-life Problem 2
A sample of a piece of wood is analyzed by C-14 dating. The
percent of C-14 is found to be 25% of what the original C-14
concentration was. What is the age of the sample?
First, let’s analyze how many half-lives have taken place.
Two half-lives have gone by while the sample decayed from
the original C-14 concentration to 25% of that concentration.
Based on Table N, the half-life of C-14 is 5730 years,
so…