chapter 21 lecture- nuclear chemistry
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Chapter 21 lecture for AP Chemistry on Nuclear ChemistryTRANSCRIPT
Chapter 21Nuclear Chemistry
Nuclear Chemistry
The nucleus of an atomcontains
protons (+1charge)and
neutrons (no charge)
The nucleus is held togetherby the strong nuclear force
The strong nuclear force is the strongestforce known
Protons and neutrons are very closetogether
They exchange a teeny bit of mass backand forth.
When disrupted, the mass is convertedto energy according to E=mc2
The mass is tiny. The energy isimmense.
Protons and neutrons experiencethe strong nuclear force if close
enough
Because protons repel each otherthe nucleus needs a certain proton
to neutron ratio for stability
An unstable atom decays byemitting radiation
These unstable atoms are radioactive.Carbon-12 is stable. Carbon-14 is
radioactive.Carbon-14 is a radioisotope.There are many naturally occurring
radioisotopes and some that are human-made.
Radioisotopes decay intostable isotopes of a
DIFFERENT element
In nuclear reactions, the makeup of thenucleus changes.
Sometimes the number of protons willchange.
If the number of protons changes, theelement has changed.
The three most commonforms of radioactive decay are
alpha(α), beta(β) andgamma(γ)
Alpha particles are the leastpenetrating.
Gamma rays are the mostpenetrating.
25.1
To balance nuclear equationsyou must know the symbol for
the emitted particle
You must balance the atomic number (number onbottom) and the atomic mass (number on top)
Here 222 + 4 = 226 and 86 + 2 = 88 its balanced
The stuff that radiatesAn alpha particle is a helium nucleus it
has a +2 charge and a mass of 4amu.
A beta particle is an electronwhich is formed when a
neutron becomes a proton
A gamma ray is a high energyelectromagnetic wave.
A gamma ray has no mass orcharge so it is not in a nuclear
equation.
particle Alpha α Beta β Gamma γMass 4amu 0 0Charge +2 -1 0Effect Radioisotope
loses twoprotons and twoneutronsThe ELEMENTchanges
Radioisotopeconverts a neutronto a proton &ejects an electronThe ELEMENTchanges
Radioisotopeloses energyThe elementdoes notchange
What it is Heliumnucleus
electron High energyelectromagneticradiation
Stop it Paper/skin 1cm/metal foil Lead/concretedamage High ionization Medium
ionizationLowestionization
Damage from nuclearradiation is due to ionization of
living tissue.Nuclear radiation is called ionizing
radiation because it produces ions fromneutral molecules.
Alpha radiation has a low penetration, butit is the most damaging to living tissuebecause it deposits all its energy alonga short path
Nuclear Fission
Fission separates heavy elements into two lighterelements.
Uranium-235 Barium-141 + Krypton-92
+3neutrons
A huge amount of energy is produced.Used by humans as an energy sourceThe fission bomb was used in WWII
Nuclear fusion
Fusion combines two light nuclei into oneheavier element.
Produces even more energy than fission.Occurs in the sun.Requires extremely high temperatures
and pressures.
FUSION = to put togetherFISSION = to break apart
The Nucleus
• Remember that the nucleus is comprised ofthe two nucleons, protons and neutrons.
• The number of protons is the atomic number.• The number of protons and neutrons together
is effectively the mass of the atom.
Isotopes
• Not all atoms of the same element havethe same mass due to differentnumbers of neutrons in those atoms.
• There are three naturally occurringisotopes of uranium:Uranium-234Uranium-235Uranium-238
Radioactivity
• It is not uncommon for some nuclides ofan element to be unstable, orradioactive.
• We refer to these as radionuclides.• There are several ways radionuclides
can decay into a different nuclide.
Types ofRadioactive Decay
Alpha Decay:
Loss of an α-particle (a helium nucleus)
He42
U23892
→ U23490 He4
2+
Beta Decay:
Loss of a β-particle (a high energy electron)
β0−1 e0
−1or
I13153 Xe131
54→ + e0
−1
Positron Emission:
Loss of a positron (a particle that has thesame mass as but opposite charge thanan electron)
e01
C116
→ B115 + e0
1
Gamma Emission:
Loss of a γ-ray (high-energy radiationthat almost always accompanies the lossof a nuclear particle)
γ00
Electron Capture (K-Capture)
Addition of an electron to a proton in thenucleusAs a result, a proton is transformed into a
neutron.p1
1 + e0−1
→ n10
Neutron-Proton Ratios
• Any element with morethan one proton (i.e.,anything but hydrogen)will have repulsionsbetween the protons inthe nucleus.
• A strong nuclear forcehelps keep the nucleusfrom flying apart.
Neutron-Proton Ratios
• Neutrons play a key rolestabilizing the nucleus.
• Therefore, the ratio ofneutrons to protons is animportant factor.
Neutron-Proton Ratios
For smaller nuclei(Z ≤ 20) stablenuclei have aneutron-to-protonratio close to 1:1.
Neutron-Proton Ratios
As nuclei getlarger, it takes agreater number ofneutrons tostabilize thenucleus.
Stable Nuclei
The shaded region inthe figure showswhat nuclides wouldbe stable, the so-called belt of stability.
Stable Nuclei
• Nuclei above thisbelt have too manyneutrons.
• They tend to decayby emitting betaparticles.
Stable Nuclei
• Nuclei below the belthave too manyprotons.
• They tend tobecome more stableby positron emissionor electron capture.
Stable Nuclei
• There are no stable nuclei with anatomic number greater than 83.
• These nuclei tend to decay by alphaemission.
Radioactive Series
• Large radioactivenuclei cannot stabilizeby undergoing onlyone nucleartransformation.
• They undergo a seriesof decays until theyform a stable nuclide(often a nuclide oflead).
Some Trends
Nuclei with 2, 8, 20,28, 50, or 82 protonsor 2, 8, 20, 28, 50,82, or 126 neutronstend to be morestable than nuclideswith a differentnumber of nucleons.
Some Trends
Nuclei with an evennumber of protonsand neutrons tend tobe more stable thannuclides that haveodd numbers ofthese nucleons.
Nuclear Transformations
Nucleartransformationscan be inducedby acceleratinga particle andcolliding it withthe nuclide.
Particle AcceleratorsThese particle accelerators are enormous,having circular tracks with radii that aremiles long.
Kinetics of Radioactive Decay
• Nuclear transmutation is a first-orderprocess.
• The kinetics of such a process, you willrecall, obey this equation:
= kt NtN0
ln
ln[A]t – ln[A]0 = -kt for 1st order kinetics
Kinetics of Radioactive Decay
• The half-life of such a process is:
= t1/2 0.693
k
• Comparing the amount of a radioactivenuclide present at a given point in timewith the amount normally present, onecan find the age of an object.
Not given, but canbe derived fromprevious equationwith [A]t equal tohalf of [A]0.
First order processes (including nuclear decay)always show a constant half-life
Measuring Radioactivity
• One can use a device like this Geiger counter tomeasure the amount of activity present in aradioactive sample.
• The ionizing radiation creates ions, which conducta current that is detected by the instrument.
Kinetics of Radioactive Decay
A wooden object from an archeological siteis subjected to radiocarbon dating. Theactivity of the sample that is due to 14C ismeasured to be 11.6 disintegrations persecond. The activity of a carbon sample ofequal mass from fresh wood is 15.2disintegrations per second. The half-life of14C is 5715 yr. What is the age of thearcheological sample?
Kinetics of Radioactive Decay
First we need to determine the rateconstant, k, for the process.
= t1/2 0.693
k
= 5715 yr 0.693k
= k 0.6935715 yr
= k 1.21 × 10−4 yr−1
Kinetics of Radioactive Decay
Now we can determine t:
= kt NtN0
ln
= (1.21 × 10−4 yr−1) t 11.615.2ln
= (1.21 × 10−4 yr−1) t ln 0.763= t 6310 yr
Energy in Nuclear Reactions
• There is a tremendous amount ofenergy stored in nuclei.
• Einstein’s famous equation, E = mc2,relates directly to the calculation of thisenergy.
Energy in Nuclear Reactions
• In the types of chemical reactions wehave encountered previously, theamount of mass converted to energyhas been minimal.
• However, these energies are manythousands of times greater in nuclearreactions.
Energy in Nuclear Reactions
For example, the mass change for the decayof 1 mol of uranium-238 is −0.0046 g.The change in energy, ΔE, is then
ΔE = (Δm) c2
ΔE = (−4.6 × 10−6 kg)(3.00 × 108 m/s)2
ΔE = −4.1 × 1011 J
Nuclear Fission
• How does one tap all that energy?• Nuclear fission is the type of reaction carried
out in nuclear reactors.
Nuclear Fission
• Bombardment of the radioactive nuclide witha neutron starts the process.
• Neutrons released in the transmutation strikeother nuclei, causing their decay and theproduction of more neutrons.
Nuclear Fission
This process continues in what we call anuclear chain reaction.
Nuclear Fission
If there are not enough radioactive nuclides in thepath of the ejected neutrons, the chain reactionwill die out.
Nuclear Fission
Therefore, there must be a certain minimumamount of fissionable material present for thechain reaction to be sustained: Critical Mass.
Fission reactor
Nuclear ReactorsIn nuclear reactors the heat generated by thereaction is used to produce steam that turns aturbine connected to a generator.
Nuclear Reactors• The reaction is kept in
check by the use ofcontrol rods.
• These block the paths ofsome neutrons, keepingthe system from reachinga dangerous supercriticalmass.
Nuclear Fusion
• Fusion would be a superiormethod of generatingpower.The good news is that the
products of the reaction arenot radioactive.
The bad news is that inorder to achieve fusion, thematerial must be in theplasma state at severalmillion kelvins.
Nuclear Fusion
• Tokamak apparati like theone shown at the rightshow promise for carryingout these reactions.
• They use magnetic fieldsto heat the material.