nuclear chemistry

NUCLEAR CHEMISTRY

Henri Becquerel 1896 - Discovers

natural radioactivity

FYI: Historical Perspective

FYI: Historical Perspective

Marie Sklodowska, Polish chemist marries Pierre Curie, French physicist

Marie died from leukemia caused by her exposure to radiation

Pierre was killed while crossing the street when he was hit by a vegetable wagon.

FYI: Historical Perspective

Earnest Rutherford 1899 – Discovers alpha,

beta and gamma radiation

Nuclear Reactions

Involve changes in the composition of nuclei Accompanied by the release of tremendous

amounts of energy

Nuclear Fission

The splitting of a heavy nucleus into lighter nuclei

Nuclear Fusion

The combination of light nuclei to produce a heavier nucleus

Nuclides

Different atomic forms of all elements Most small nuclides have equal # of protons

and neutrons Some nuclides have “magic #’s” of protons

and neutrons and are especially stable

The neutron-to-proton ratio determines the stability of the nucleus

For low atomic #’s:– Equal #’s of protons and neutrons

Above atomic #20:– More neutrons than protons

Nuclei whose neutron-to-proton ratio is unstable undergo radioactive decay by emitting 1 or more particles and/or electromagnetic rays:

Nuclei whose neutron-to-proton ratio is unstable undergo radioactive decay by emitting 1 or more particles and/or electromagnetic rays:

Type/

symbol

Identity Mass (amu)

Charge Penetration

Alpha

Beta

Gamma

Proton

Neutron

Heor 42

eor 01-

00

Hor p 11

11

n10

helium nucleus

electron

high energy radiationproton, H nucleus

neutron

4.0026 2+

0.00055 1-

0 0

1.0073 1+

1.0087 0

low

low-med

high

low-med

very high

Comparing penetrating ability…

Alpha Particle Decay

Example 1: Radium-226 transmutates by alpha decay. Write the nuclear equation that represents this process.

RnRa 22286

22688

or

HeRnRa 42

22286

22688

Beta Particle Decay

Example 2: Write the nuclear equation for the beta-decay of boron-12.

+ +

CB 126

125

or

eCB 01

126

125

Gamma Radiation

Example 3: Write the nuclear equation representing gamma radiation given off by the unstable radionuclide cobalt-60.

CoCo 6027

6027

Nuclear Fission & Fusion

FISSION: a heavy nucleus splits into 2 lighter nuclei

some elements undergo fission spontaneously

some elements can be induced to undergo fission when bombarded with other particles (e.g. neutrons)

FUSION: 2 nuclei combine to form a heavier nucleus

the sun is a tremendous fusion reaction; the major fusion reaction in the sun is thought to be:

both fission & fusion release large amounts of energy (fusion more than fission)

energy!nHeHH 10

42

31

21

The Atomic Bomb (FISSION)

when the nucleus of U-235 splits, 2 isotopes are formed, plus neutrons are emitted

these neutrons collide with other U-235 atoms, causing them to undergo fission; they release neutrons, and so on…

The result - CHAIN REACTION!!

FISSION

CHAIN REACTION!!!

The Atomic Bomb (FISSION)

The Atomic Bomb (FISSION)

CRITICAL MASS

The minimum mass of fissionable material that must be used to sustain a chain reaction

One type of bomb…

Little Boy: U-235 (Hiroshima)

Fat Man: Pu-239 (Nagasaki)

subcritical mass of U-235

subcritical mass of U-235

TNT(dynamite)

Nuclear Reactors (controlled FISSION)

Nuclear Reactors (FISSION)

use subcritical masses of fissionable material CORE: contains fuel pins made of U-235;

interspersed among the pins are control rods control rods: absorb neutrons

– pull rods out of core: fission increases– push rods back into the core: fission decreases

Safety feature: if power is lost, rods will automatically fall into the core and shut the reaction down.

Nuclear Reactors (FISSION)

“The energy produced by breaking down the atom is a very poor kind of thing. Anyone who expects a source of power from the transformation of these atoms is talking moonshine.”

Ernest Rutherford

Nuclear Reactors (FISSION)

Nuclear Power Plants

Nuclear Power Plants

Nuclear Power Plants

TO GENERATE ELECTRICITY:

1) Fission heats up water in vessel and heat is carried away.

2) This heat is used to heat up water in a second system, which turns into steam.

3) Steam turns turbine of a generator.

4) Generator makes electricity.

PROS OF NUCLEAR ENERGY:

no air pollution enormous amt. of energy released alternative to using our rapidly decreasing

fossil fuels

CONS OF NUCLEAR ENERGY

containers for waste products may erode or break

thermal pollution (heated water returned to rivers, etc.)

potential theft of fuel (Pu-239) for use in weapons

Controlled Nuclear FUSION

PROS:– A very abundant supply of energy world wide.– Environmentally clean– No creation of weapon materials– No chance of runaway reactions leading to

accidents

CONS:– It doesn’t work; at least not yet…

Nuclear Fusion

"Every time you look up at the sky, every one of those points of light is a reminder that fusion power is extractable from hydrogen and other light elements, and it is an everyday reality throughout the Milky Way Galaxy."

Carl Sagan, Spitzer Lecture, October 1991

Nuclear Fusion

Obstacles…– HOT – plasma at least 100 million C– High density plasma– Containment of plasma– Confinement time

Rates of Decay & Half Life

Radionuclides have different stabilities and decay at different rates.

Integrated Rate Equation

where… A = the amt. of decaying sample remaining at

some time, t Ao= the amt. of sample present at the beginning k = rate constant; different for each radionuclide t = time

303.2kt

AA

log

Integrated Rate Equation

OR…

where… N = # of disintegrations per unit of time; relative activity No= original activity

303.2kt

NN

log

HALF-LIFE= the amount of time required for half of the original sample to decay

Half Life

0

20

40

60

80

100

120

0 5 10 15 20

Time

Am

ou

nt

HALF-LIFE= the amount of time required for half of the original sample to decay

0

50

100

150

200

250

0 5 10 15 20

Amount

Tim

e

Parent

Daughter

HALF-LIFE

Half-life = the amount of time required for half of the original sample to decay

k693.0

k2 ln

t2

1

21t

0.693k

Example: Cobalt-60 decays with the emission of beta particles and gamma rays, with a half-life of 5.27 years. How much of a 3.42 g of cobalt-60 remains after 30.0 years?

5.27 years0.693

k -1yr1315.0

303.2kt

AA

log

303.2

)(30.0yrs)(0.1315yr

A

g42.3log

-1

713.1A

g42.3log

How do you solve for A???

Take the ANTILOG (10x) of both sides.

Example: Cobalt-60 decays with the emission of beta particles and gamma rays, with a half-life of 5.27 years. How much of a 3.42 g of cobalt-60 remains after 30.0 years?

713.110A

g42.3

6.51

g42.3A

A6.51g42.3

g0.0662A

713.1Ag42.3

log1010 OR 713.1antilog

A

g42.3logantilog

Uses of Radionuclides

Radiocarbon dating: the ages of specimens of organic origin can be estimated by measuring the amount of cabon-14 in a sample.

Example: A piece of wood taken from a cave dwelling in New Mexico is found to have a carbon-14 activity (per gram of carbon) only 0.636 times that of wood today. Estimate the age of the wood. (The half-life of carbon-14 is 5730 years.)

.636NN

years57300.693

k -1yr00012094.0

303.2kt

NN

log

303.2

)(t)4yr(0.0001209

.636log

-1

NN

)(t)yr(5.25x10.636

1log 1-5-

Example: A piece of wood taken from a cave dwelling in New Mexico is found to have a carbon-14 activity (per gram of carbon) only 0.636 times that of wood today. Estimate the age of the wood. (The half-life of carbon-14 is 5730 years.)

)(t)yr(5.25x10.636

1log 1-5-

)(t)yr(5.25x1019654.0 -1-5

t=3744 yrs = 3740 yrs

Uses of Radionuclides

***NOTE: Objects older than 50,000 years have too little activity to be dated accurately using carbon dating; instead the following methods are used:

– Potassium-40 decays to argon-40: half-life = 1.3 x 109 years

– Uranium-238 decays to lead-206: half-life = 4.51 x 109 years

Example: A sample of uranium ore is found to contain 4.64 mg of uranium-238 and 1.22 mg of lead-206. Estimate the age of the ore.

Pb g/mol 206

U g/mol 238 x Pb mg 22.1 decayed have 238-U mg 41.1

U mg 6.05 mg 4.64 mg 14.1A

yrs10 x 4.51

0.693k

9 -1-10 yr10 x 1.54

Example: A sample of uranium ore is found to contain 4.64 mg of uranium-238 and 1.22 mg of lead-206. Estimate the age of the ore.

303.2kt

AA

log

yearsbillion 73.1t

303.2

)(t)yr10(1.54

mg64.4

mg05.6log

-1-10