1 of 39 © boardworks ltd 2007

1 of 39 © Boardworks Ltd 2007

© Boardworks Ltd 20072 of 39

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


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?


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.


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


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


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


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

12

Unit 16: Nuclear Chemistry


13

Radioactivity

• Emission of subatomic particles or high-

energy electromagnetic radiation by nuclei

• Such atoms/isotopes said to be

radioactive


14

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


15

Types of radioactivity

• Rutherford and Curie found that emissions

produced by nuclei

• Different types:

– Alpha decay

– Beta decay

– Gamma ray emission


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.


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.


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.


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).


Radiation and Table O

The Regents Reference Tables provides us with a

summary of the different types of radiation:


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.


22


23

• Beta decay


Diagram showing

penetrating ability

www.epa.gov


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:


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.


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.


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!


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!


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.


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.


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:


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:


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.


Positrons are also listed in

Table O


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.


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


Half Life


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.


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?


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.


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).


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:


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.


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.


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.


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…

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