No Slide Titleteachermarten.com/APChem/APLectureNotes_files/ChNuclearZum3r… · 5 8 4 β Be 0 +1....
Transcript of No Slide Titleteachermarten.com/APChem/APLectureNotes_files/ChNuclearZum3r… · 5 8 4 β Be 0 +1....
Nuclear Chemistry
Atoms are composed of
PROTONS
NEUTRONS
ELECTRONS
positively charged mass = 1.6726 x 10–27 kg+
neutral mass = 1.6750 x 10–27 kg
negatively charged–• mass = 9.1096 x 10–31 kg
the Nucleus
+
+
made up of protons and neutrons
Strong Nuclear Force
+
+
the neutrons within the nucleus act as sort of glue countering the electrostatic
repulsion between the protons
Why/how do the protons stay so close to each other?
when an unstable nuclei increases its stability by altering its number of
neutrons and protons
Radioactivity
Types of Nuclear Decay
Alpha emission ( α)42
0 -1Beta emission ( β )
Electron capture ( e) 0 -1
Positron emission ( β ) 0 +1
Gamma emission ( γ)00
Alpha emission ( α)42
the nucleus emits a helium nuclei ( 2 protons and 2 neutrons
α42 = 4
2 He
Po Pb +210 20684 82
42 He
Beta emission ( β )the nucleus changes a neutron into a proton by
emitting an electron
0 -1
β 0 -1
10
n 1 1p+
C +146
147 Nβ 0
-1
Positron emission ( β )the nucleus changes a proton into a neutron by
emitting a positron
0 +1
β 0 +1
11
p 1 0n+
B +85
84 Beβ 0
+1
Electron capture ( e )the nucleus captures an electron and changes a
proton into a neutron
0
β 0 -1
11
p 1 0n+
-1
C116
115 Bβ 0
-1 +
electromagnetic radiation emitted during nuclear decay
Gamma emission ( γ )00
Nuclear Stability
the principle factor in determining whether a nucleus is stable is the neutron-to-proton ratio
as the mass number increases, the neutron-to-proton ratios become greater than one
for elements of low atomic number the value is close to one
nuclei that contain 2, 8, 20, 50, 82, and 126 protons are generally more stable (magic
numbers)
nuclei with even numbers of both protons and neutrons are generally more stable than odd
numbers
Nuclear Stability
If an isotope’s mass number is greater than its atomic weight, beta emission is expected
If an isotope’s mass number is less than its atomic weight, positron emission or electron
capture is expected
All elements having an atomic number greater than 83 are radioactive. Alpha particles are
emitted by most of these isotopes.
Nuclear Stability
Po Pb +210 20684 82 X
Cs Ba +137 13755 56
Na Ba +20 2011 10
X
X
balance the following nuclear equations
Po Pb +210 20684 82
42 He
Cs Ba +137 13755 56
Na Ba +20 2011 10
X
X
balance the following nuclear equations
Po Pb +210 20684 82
42 He
Cs Ba +137 13755 56
Na Ne +20 2011 10 X
0-1 β
balance the following nuclear equations
Po Pb +210 20684 82
42 He
Cs Ba +137 13755 56
Na Ne +20 2011 10
0-1 β
0+1β
balance the following nuclear equations
a decay series
when a radioactive nucleus disintegrates, the products formed may also be unstable and under go further disintegration’s until a stable product
is formed
U Pb238 20692 82
involves 14 steps
U238
92 Pb206
82
+U23892 Th234
9042 He
+Th23490 Pa234
910
-1β
+Pa23491 U234
920
-1β+U
23492 Th230
9042 He
+Th23090 Ra226
8842 He
+Ra22688 Rn222
8642 He
+Rn22286 Po218
8442 He
+Rn22286 Po218
8442 He
+Po21884 Pb214
8242 He
+Pb21482 Bi214
830
-1β
+Bi21483 Po214
840
-1β
+Po21484 Pb210
8242 He
+Pb21082 Bi210
830
-1β
+Bi21083 Po210
840
-1β
+Po21084 Pb206
8242 He
all radioactive decays obey first-order kinetics
Kinetics of Radioactive Decay
Rate of decay at time t = Nk
k = rate constant
the number of radioactive nuclei present at time tN =
Time ( s )The plot shows the decay of uranium-238 to thorium-234
First-order rate plot
U23892 U Th +
238 23492 90
42 He
First-Order rate law Integrated
the integrated form of the rate law is:
t1/2 = k
0.693= kt
N0
Nt
ln
Integrated rate law
is an equation for a straight line
Plot ln Nt versus t
ln Nt = -kt + ln N0
y = mx + b
Slope = -k
y intercept is ln N0
Half-life
the time for the concentration of a reactant to decrease to one-half of its
initial concentration
Time ( s )0
U23892
t 1/2 = 4.51 x 107yr
U Th +238 23492 90
42 He
Half-life
Radiocarbon Dating
Willard Libby (Nobel Prize, 1960)
Carbon-14
Natural abundance: 1 part in 1012
β − emitter
Half-life = 5730 yearsused to date archeological artifacts younger
than 30,000 years
- +
14 C614 N7
0 e-1 +
Example
The C-14 decay rate of wood obtained from a live tree is 0.260 disintegration per second per gram of sample A sample of wood from an archaeological site has C-14 decay rate
of 0.186 disintegration per second per gram. How old is the sample?
The C-14 decay rate of wood obtained from a live tree is 0.260 disintegration per second per gram of sample A sample of wood
from an archaeological site has C-14 decay rate of 0.186 disintegration per second per gram. How old is the sample?
t 1/2 for 14C is known to be 5730 years
t1/2 = k
0.693
Therefore,k = (0.693/5730 yr)= 1.21 x 10-4 yr-1
ln[A]0
[A]= kt ln
260
186= (1.21 x 10-4 yr-1 ) t
t =2770 years
Some representative half-lives
Tc-99 6 hours
C-14 5730 years
Sr-90 28.8 years
Mo-99 67 hours
K-40 1,300,000 years
U-238 45, 000,000 years
By bombarding a sample of nitrogen with α particles an oxygen-17 isotope was produced
with the emission of a proton.
An experiment performed by Rutherford in 1919 produced artificial radioactivity
He +42
178 Op1
1147 N +
Converting one element into another element
Nuclear Transmutation
He +42
178 Op1
1147 N +
balance the following nuclear equation
(d,a )5626Fe 54
25Mn , where d represents the
deuterium nucleus H )21(
+H21+56
26Fe 5425MnHe4
2
Transuranium Elements
Elements with atomic numbers greater than 92
made in particle accelerators
a device used to accelerate nuclear particles near the speed of light
Nuclear Fission
Nuclear Fission
process in which a heavy nucleus (mass number> 200) divides to form smaller nuclei of
intermediate mass and one or more neutrons
Nuclear Fission
Although many heavy nuclei can be made to undergo fission only uranium-235 and
plutonium-239 have any practical importance
Enriched Uranium
Isotope Natural % Abundance
238U 99.2745
235U 0.72
234U 0.0055
http://www.epa.gov/radiation/radionuclides/uranium.htm
http://www.washingtonpost.com/wp-dyn/content/article/2007/04/24/AR2007042401055.html
http://www.cbsnews.com/stories/2007/04/26/eveningnews/main2732837.shtml?source=search_story
Nuclear Fission
n +10 n1
023592U +
14354Xe
9038Sr + 3
For one mole of uranium-235, the energy released is 2.0 x 1013 J
For one ton of coal, the energy released is only 8.0 x 107 J
n +10 n1
023592U +
14354Xe
9038Sr + 3
the fact that more neutrons are produced then captured during uranium-235 fission makes a
possible chain reaction possible
a self-sustaining sequence of nuclear fission reactions
Chain Reaction
••
•
Critical Mass
••
•••
•
••
•
•
••
•
••
•
••
•
••
••
•
••
•
••
•The minimum mass of fissionable material required to
generate a self-sustaining nuclear chain reaction
the first application of nuclear fissionAtomic Bomb
critical mass is formed by using TNT to force the fissionable sections together
Lowell H.S.
yehwoooo
yeh
excellent•
••
•
• ••
•
• •
•
•
••
•••
Nuclear Fusion
Nuclear Fusion
combining small nuclei into larger ones
HHHe
Nuclear Fusion
Because fusion reactions take place at very high temperatures, they are often
called thermonuclear reactions.
H1 1
32
He2 1H+
H1 1
21
H1 1H+ 0
+1β+
He3 2
42
He3 2He+ + H1
12
H1 1
16 O1 0n+8 8
8
Mass of = 2.65535 x 10-2316 O8 g
Mass of H1 1
1 0n+8 8 = 2.67804 x 10-23 g
the difference in mass for the formation of one mole16 O8
= -0.1366 g /molof
Mass and Energy
difference in mass = 2.269 x 10-25
Mass Defect
- equivalence of mass and energy (derived from Einstein’s theory of special
relativity)
E = MC2
energymass
speed of light3.00 x 108 m/s
when a system gains or loses energy, it also gains or loses a quantity of mass.