slide 1 do now write a balanced nuclear equation for the alpha decay that produces uranium-238

Do Now• Write a balanced nuclear equation for the

alpha decay that produces uranium-238.

Pu ª U + He

Do Now

Solve for x1)x = ()32)x = 3) = 0.1254) = 0.0625x = 8x = 3x = 0.667

x =

Announcement• Test #4 - Wed, Dec 10 (tentative)• Nuclear Chemistry

– The concept of radioactivity– Chemical vs nuclear reactions– Major types of radioactivity– Factors that determine nuclear stability– Band of stability– Predicting nuclear decay pathways– Writing and balancing nuclear equations– Nuclear binding energy & mass defect– Fission and fusion– Half-life and kinetics of decay

Nuclear Reactions

SWBAT explain the concepts of nuclear fission and fusion, predict which will happen based upon nuclear binding energy, and write balanced equations for each.

2-day lesson

Write this in your notes

ReviewNuclear reactions• A reaction that involves a change to an

atom's nucleus. • Can produce enormous amounts of energy

Nuclear fusion - two nuclei joining together into one

Two Types of Nuclear ReactionsNuclear fission - a nucleus splitting into two or more parts


• A nucleus will undergo whichever process makes it more stable

• So how do we know the relative stability of different nuclei?

What determines if a nucleus will undergo fission or fusion?

ΔErequired

componentparts

++++++

nucleus

++++

How do we measure stability of a nucleus?

The amount of energy required is directly

related to the stability of the nucleus


Nuclear Binding Energy• Nuclear binding energy is the energy required

to split the nucleus of an atom into its component parts (e.g protons and neutrons)

• Explaining nuclear binding energy requires a complex discussion of Einstein's theory of mass-energy equivalence.

• You will only be responsible for explaining the concept of mass defect and its use in deriving nuclear binding energy

How do we measure this energy?

Separating a nucleus into its component parts is not easy to do.

It requires huge amounts of energy.

It is hard to measure.

But how to measure this energy?

But there is another way We can measure this

energy indirectly by measuring the mass difference

And using E = mc2

componentparts

++++++

nucleus

++++

Measure the mass differenceMass difference? What mass difference?

Shouldn't the mass of the nucleus should be the sum of the component parts?

componentparts

++++++

nucleus

++++

Measure the mass difference

12.09564 u12.00000 u

Difference is 0.09564 u

Mass-Energy Equivalence• Mass and energy have a direct relationship as

establish by Einstein's famous equation

• E = energy, m = mass, c = speed of light (670,000,000 mph)

• Normally we don't notice this relationship because even large change in energy results in a very small change in mass

• • For example:

• 90,000,000 J of energy changes mass by 0.000001 g

Life is different in the nucleus• Strong nuclear force is the most powerful force

in the universe• Because of strong nuclear force, a HUGE

amount of energy is required to split the nucleus into its component parts

• With amounts of energy this large, the mass-energy equivalence is detectable.

But how to measure this energy?

When this huge amount of energy is added to the system, the mass goes up.

How much?

12.09564 u

++++++

12.00000 u

++++

Measure the mass difference

componentparts

nucleus

Difference is 0.09564 u

This much!!!

The difference between these two is called the mass defect



Mass Defect• Mass defect is the difference between the mass

of a nucleus and the mass of its component parts

• By measuring the mass defect, the nuclear binding energy can be determined– An example of this calculation is shown on pg 878 in your book

• Nuclear binding energy provides an estimate of relative stability of different nuclei• High nuclear binding energy - high stability• Low nuclear binding energy - low stability

Last Class

Sketch this graph in your

notes

Iron is the most stable element, with the highest

binding energy.

This is why iron is such an abundant element

Fission reactions

Fission reactions

FissionProcess where a nuclei splits apart

The daughters are more stable than the parents so

energy is released

Fission reactions

FusionProcess where two nuclei join together


energy is released

Fusion reactions

Fusion Reactions

• A nucleus will undergo whichever process makes it more stable

• Nuclear binding energy measures the relative stability of different nuclei

• The optimum mass is around A=60− Mass number >60, fission is favored− Mass number <60, fusion is favored

Write this in your notes!!!

What determines if a nucleus will undergo fission or fusion?

Fission ReactionsNuclear fission - a nuclei splitting into two or more parts

Do Now• Write a balanced nuclear equation for the

alpha decay that produces uranium-238.

Pu ª U + He

Previous Classes

Transmutation & Fissionable• Fission always involves transmutation• Transmutation is the conversion of one atom of

an element to an atom of another element• Sometimes this occurs spontaneously

– e.g. radioactive decay

• Sometimes this is forced to occur– e.g. caused by bombardment with particles– Called "induced transmutation"

• Fissionable - material capable of undergoing fission upon bombardment


When uranium-235 is bombarded with neutrons, it can breaks apart (fissions) according to the equation:


1 235 99 135 10 92 42 50 02n U Mo Sn n energy

Fission products

Fission of Uranium-235

UNSTABLE

U-236

When uranium-235 is bombarded with neutrons, it can breaks apart (fissions) according to the equation:

1 235 99 135 10 92 42 50 02n U Mo Sn n energy

Fission products

Fission of Uranium-235

Fission reactions

Fission reactions

Fission Converts U-235into More Stable Nuclei

Notice that the nuclei of Mo and Sn have greater binding energy and therefore are more stable than U

Writing Balanced Nuclear Equations

n + U ª Mo + Sn + 2 n

Balanced the Mass Numbers

n + U ª Mo + Sn + 2 n

Mass Number1 + 235 = 236

Mass Number99 + 135 + (2 x 1) = 236

Balanced the Atomic Numbers

n + U ª Mo + Sn + 2 n


Mass Number99 + 135 + (2 x 1) = 236

Atomic Number0 + 92 = 92

Atomic Number42 + 50 + (2 x 0) = 92

Independent Practice 1• The fission of uranium-235 by a neutron can

produce many fission products. For the case where it produces krypton-91 (Kr) and barium-142 (Ba), write the nuclear equation and determine how many neutrons are produced.


produce many fission products. For the case where it produces krypton-91 (Kr) and barium-142 (Ba), write the nuclear equation and determine how many neutrons are produced.

n + U ª Kr + Ba + 3 n


produce many fission products. For the case where it produces yttrium-97 (Y), five neutrons and another fission product, write the nuclear equation and determine how what other fission product is made.


produce many fission products. For the case where it produces yttrium-97 (Y), five neutrons and another fission product, write the nuclear equation and determine how what other fission product is made.

n + U ª + I + 5 n

Independent Practice 3• The fission of plutonium-240 (Pu) by a neutron can

produce many fission products. For the case where it produces lanthanum-137 (La) and rubidium-94 (Rb), write the nuclear equation and determine how many neutrons are produced.

Independent Practice 3• The fission of plutonium-240 (Pu) by a neutron can

produce many fission products. For the case where it produces lanthanum-137 (La) and rubidium-94 (Rb), write the nuclear equation and determine how many neutrons are produced.

n + Pu ª La + Rb + 10 n

SERIOUSLY?!?All this arguing over

a simple isotope, uranium-235?

What is all the arguing about???

Benjamin Netanyahu Mahmoud Ahmadinejad

What makes U-235 so special?• U-235 is a naturally-occurring isotope• Typical uranium ore contains:

>99% U-238 & <1% U-235

• Uranium ore can become "enriched" in U-235 through a complex and difficult process

• 5-20% enriched uranium is needed for nuclear power

• >85% enriched uranium is needed for nuclear weapons

• U-235 is a naturally-occurring isotope capable of starting and propagating a nuclear chain reaction.

Key PointsWrite this

in your notes

What is a Nuclear Chain Reaction?• A nuclear chain reaction is a self-sustaining

sequence of nuclear fission reactions.• One atom undergoes fission• This triggers another atom to undergo fission• Which triggers another atom to undergo fission . . .• The sequence, once started, continues with no external

triggers


• A nuclear chain reaction is a self-sustaining sequence of nuclear fission reactions.


in your notes

Fission products

1 neutron, 1 fission

1 235 99 135 10 92 42 50 02n U Mo Sn n energy

How does U-235 start & propagatea nuclear chain reaction?


1 235 99 135 10 92 42 50 02n U Mo Sn n energy

Can these 2 neutrons cause 2 other fissions?

Can these 2 neutrons cause 2 other fissions? Yes!


2 neutrons, 2 fissions

Fission products




Each neutron then causes one more fission reaction




Each neutron then causes one more fission reaction


How does U-235 start & propagatea nuclear chain reaction?

Because its fission createsmore neutrons than it consumes!

Fissile Material for Chain Reactions• A material capable of sustaining a nuclear fission chain

reaction with neutrons of any energy• Fissile rule

90 ≤ Z ≤ 100 2Z - N = 43 ± 2

• Different from a fissionable materials. All fissile materials are fissionable materials but not the other way around. Materials that require high energy neutrons are not considered fissile



• U-235 can do this because its fission creates more neutrons than it consumes.


in your notes

Critical Mass• Critical mass is another key factor to consider• For a chain reaction to occur, there has to be

enough fissionable atoms around to collide with released neutrons and propagate the chain.

• Not enough - subcritical mass

• Just enough - critical mass

• More than enough - supercritical mass

• Critical mass is the minimum amount of fissionable material necessary to sustain a nuclear chain reaction



• U-235 can do this because its fission creates more neutrons than it consumes.

• Enough U-235 is needed to achieve critical mass, the minimum amount of fissionable material necessary to sustain a nuclear chain reaction


in your notes

So where do we stand?

Hassan Rouhani Ali HosseiniKhamenei

Fusion ReactionsNuclear fusion - two nuclei joining together into one

Fission reactionsFusion reactions

FusionProcess where two nuclei join together


energy is released

The products are more stable than the starting material so: • the reaction proceeds forward • energy is released

Fusion reactions

Fusion• Fusion is less familiar to most people• Fusion is more difficult to initiate and sustain

than fission• The stars use fusion to generate energy

Fusion Energy is Very Attractive• Fuels are cheap and abundant

– Reaction can hydrogen gas or other abundant elements

• Little radioactive waste• Reactors can't get out of control

– Fusion would just stop

• Large projects are trying to achieve fusion– Seeking to produce more energy than required to start

the reaction

Fusion of Carbon & Hydrogen

+ +

Balance the Mass Number

+ +



Balance the Atomic Number

+ +





Equation is Balanced

+ +





Solve This ProblemTwo helium-3 atoms fuse together to form one helium-4 atom and

some hydrogen-1. Write a balanced equations and determine how

many atoms of hydrogen-1 are produced.

Write the ReactantsTwo helium-3 atoms fuse together to form one helium-4 atom and



helium-3 =

helium-4 =

hydrogen-1 =

Write the Equation

2 + ?

Two helium-3 atoms fuse together to form one helium-4 atom and



Balance the Mass Number

2 + 2

Mass Number(2 x 3) = 6

Mass Number4 + (2 x 1) = 6

Two helium-3 atoms fuse together to form one helium-4 atom and some hydrogen-1. Write a balanced equations and determine how many atoms of hydrogen-1 are produced.

Balance the Atomic Number

2 + 2

Mass Number(2 x 3) = 6

Mass Number4 + (2 x 1) = 6

Atomic Number(2 x 2) = 4

Atomic Number2 + (2 x 1) = 4

Two helium-3 atoms fuse together to form one helium-4 atom and some hydrogen-1. Write a balanced equations and determine how many atoms of hydrogen-1 are produced.

Balanced Equation

2 + 2

Two atoms of hydrogen-1 are produced.

Two helium-3 atoms fuse together to form one helium-4 atom and



Independent Practice 4• The fusion of uranium-238 and nitrogen-14

produces a fusion product and four neutrons. Write a balanced nuclear equation that describes this fusion reaction.

U + N ª Es + 4 n

Independent Practice 5• An unknown nuclide fuses with helium-4 to produce

oxygen-16 and a neutron. Write a balanced nuclear equation that describes this fusion reaction.

C + He ª O + n

Independent Practice 6• Lawrencium-257 and six neutrons were produced

through the fusion of californium-252 and another nuclide. Write a balanced nuclear equation that describes this fusion reaction.

Cf + B ª Lr + 6 n

Worksheet• Start in class• Finish for homework• Be sure to ask for help if you need it

Good video on mass defecthttps://www.youtube.com/watch?v=4HgvPBAOea8

Decay versus Fission• By definition, most decay qualifies as a form of fission• In common usage, however, the meanings are different• Radioactive decay is a spontaneous process where the nucleus

tries to get into the band of stability. The fragment lost is usually no bigger than a-particles

• In common usage, fission is an induced transmutation. It leads to the transient production of an unstable nucleus which undergoes fission. The fragments are usually larger than a-particles

slide 1 do now write a balanced nuclear equation for the alpha decay that produces uranium-238

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