Ch. 26 Nuclear Chemistry

Ordinary Chemical Rxns

No new elements can be produced

Only the e- participate

Relatively small amounts of energy are released or absorbed

Rate of rxn depends on factors such as concentration, temperature, catalysts, and pressure.

Elements may be converted from one element to another.

Particles within the nucleus are involved.

Tremendous amounts of energy are released or absorbed

Rate of rxn is not influenced by external factors

vs. Nuclear Rxns

Nuclear particles

α = 4α or 4He n = 1n

p = 1p e = -1e β- = -1β β+ = +1β

δ = gamma (energy)

Balancing Nuclear Rxns

For the general reaction

The two conservation principles demandM1 = M2 + M3

andZ1 = Z2 + Z3

◦M's are mass numbers ◦Z's are atomic numbers

YRQ 3

3

2

2

1

1

MZ

MZ

MZ

Beta Emission

A beta particle is an electron ejected from the nucleus when a neutron is converted to a proton.

β emission = electron 1n = 1p + -1β

228Ra -1β + 228Ac

14C 14N + -1β

Positron emission or electron capture (K capture)

+1β = positron

1p 1n + +1β

38K 38Ar + +1β

15O 15N + +1β

K capture106Ag + -1e 106Pd 37Ar + -1e 37Cl

Alpha emission

Alpha emission 4α or 4He204Pb 200 Hg + 4α

All nuclides with atomic # greater than 83 are radioactive. Most decay by alpha emission

*only stable nuclide with atomic # 83 is 209Bi

Types of Nuclear Rxns

83+ protons alpha decay

neutron rich β emission

neutron poor K capture or positron emission

History

1896 - Henri Becqurel –discovers radioactivity in U salts

1898 - Marie and Pierre Curie –discover two new radioactive elements, Po and Ra

1898 - Ernest Rutherford –discovers that radioactivity has two forms: α and β radiation

Belt of stability

~ neutrons have a stabilizing effect on proton – proton repulsion

~ neutrons and protons swap particles called gluons which keeps the atom together

~ as # of protons increases, atoms need even more neutrons.

Mass deficiency Δm

– Δm – for a nucleus is the difference between the sum of the masses of e-, p+ and no in the atom and the actual measured mass of the atom.

Table 26-1 Δm = (sum of all e-, p+ and no) – (actual

mass of the atom)

1 amu = 1.661 x 10-24 grams

Example Problem

Ex. 1) Calculate the mass deficiency for 39K in amu/atom and in g/mol. The actual mass of 39K is 39.32197 amu per atom

Nuclear binding energy

(BE) provides the powerful short-range force that holds the nuclear particles together in a small volume.

Rewrite Einsteins E = mc2 BE = (Δm)c2

Ex. 2) Use the value for Δm 39 K to calculate the nuclear binding energy in J/mol of K atoms. 1J = 1kg m2/s2.

Fission and Fusion

Both processes generate large amounts of energy

Nuclear fission

◦splitting of a heavy nucleus into two lighter nuclei

Nuclear fusion

◦combining two light nuclei into one heavier nucleus

Fissionoccurs when large nuclei break down into

smaller ones. Ex. U, Th, Pa, Pu,

Some fission rxns are spontaneous while others require activation by neutron bombardment

Very unstable – chain reaction - mass goes down and energy is produced.

Controlled at Nuclear Power Plants pg 1027. ~ know the different parts◦Reactors, Fuel, Moderator, Control Rods, Cooling

Systems, and Shielding

Pressurizes water reactor Boling water reactorPWR ~ 2000 psi BWR ~ 1000 psi

Cerenkov Radiation

Nuclear Power Plant accidents

–Three Mile Island, PA (1979) Nuclear reactor malfunctioned – no meltdown, but some radioactive contamination. Affected a 25 mile radius

–Chernobyl , Russia (1986) Nuclear reactor’s cooling system failed – meltdown. Released thirty times the radioactivity of the atomic bombs dropped on Hiroshima and Nagasaki. 31 lives were lost immediately. Radiation in soil & atmosphere still presents significant health risks.

One of the main concerns: Acute radiation to cells causes them to divide and grow without control – this creates a tumor (cancer)

Mythbusters● Myth # 1: An event similar to Chernobyl can

happen in the USA–The Chernobyl design is vastly different than what is operating in the US

–Chernobyl used graphite as a moderator not water

–Graphite has postive reactivity coefficient, water has a negative reactivity coefficient

–Chernobyl did not have containment, American reactors have 3 levels of containment; a Fuel Rod, a Reactor Vessel, and a Containment building

Mythbusters● Myth # 2:

A nuclear power plant can explode like a nuclear bomb

–It is impossible for a reactor to explode like a bomb.

–Bombs require much, much, much higher levels of fuel enrichment and must be configured in a specific geometry. Neither of which are present in a power plant

Mythbusters

● Myth # 3● The smoke you can see from a cooling tower is

radioactive

– The `smoke' is actually water vaper. The water is very clean and has no detectible radiation

Mythbusters● Myth #4

– Americans get most of their yearly radiation dose from nuclear power plants

● Dental X-ray ~ 1 mrem (millirem ~ the amount of ionizing radiation whose effect is equal to that produced by one x-ray)

● Natural Radiation ~ 30 mrem per yr

● 3 hour flight ~ 1.5 mrem

● Living within 50 miles of a Nuclear plant ~0.01 mrem

Benefits of Nuclear Radiation

Radiation & radioactive materials can be used in a number of ways. The following merely touches on the subject:

Agriculture - The increase in the volume and quality of grains & cereals has been vastly improved by growing superior strains labeled with radioactive isotopes.  These improvements are helping to alleviate famine in third world countries.

Cancer Treatments - Cancerous cells can be selectively killed by the use of radioactivity, either in the form of directed beams, as for breast cancer, or as radioactive bullets that are designed to migrate directly to the cancerous cells that need killing. 

Chemotherapy, one of the only current alternatives, which involves the use of invasive drugs, but it is very difficult for the patient.

Environmental Measurements - The movement of pollutants through the environment (ex. ground water and rivers)- can be accurately measured by the use of radioactive tracers.

Food - Food, such as beef and chicken, that has been sterilized by irradiation(the process of being exposed to radiation) has a longer shelf life and is free of E. coli ~ a bacterium that can kill as a result of eating poorly cooked food. (children are more susceptible to E. coli than adults)An extension of food irradiation could save the lives of

many children and would be particularly useful in developing countries where refrigeration is not available.

Generation of Electricity - Over 440 nuclear plants around the world contribute some 16% of the world's electrical energy needs.  109 plants in the U.S. contributed 22% of the US's consumption of electricity in 2000.

Medical Diagnostics - The use of radiation in the medical world extends from X-rays, through magnetic resonance imaging (MRI), to the use of radioactive tracers to diagnose such varied conditions as faulty thyroid glands or bone problems.  The use of radioactive tracers often replaces the use of invasive surgical diagnosis.

Polymerization of Plastics - Plastics can be polymerized by radiation instead of damaging heat treatments.  The polymerized plastics are used in such applications as car dashboards, which would, otherwise, crack badly under heat in the summer.

Quality Control of Metal Parts - The integrity of metal parts such as aircraft engine blades can be verified by radiophotography on a conveyor belt instead of having to destroy a sampling of blades to ensure they are intact.

Research in Biology - The use of radioactive tracers allows the non-invasive tracking of elements and drugs through the body for both metabolic studies and medicine.

Space Power - When small amounts of power are needed in space in regions where solar power is inefficient (on the dark side of the moon or when large solar panels are impossible), plutonium batteries are ideal producers of compact energy.

Fusion(fuse – put together) small nuclei into bigger

ones.

Extremely high energies or temperatures are necessary to initiate fusion reactions.Ex. Stellar energy source is fusion (stars)

~ still a mass loss E = mc2

~ fusion typically uses H as a fuel 1H ~ Hydrogen (protium) 2H ~ Heavy H (deuterium) 3H ~ tritium

2H + 2H 3H + 1H

Fusion good – Why?~ no chance of chain rxns; no radioactive

products; Hydrogen is easy to get (75% of universe is Hydrogen); cheap; and fusion produces more energy per amu.

Bad – Why? ~ needs extreme heat and harder to do.

Radioactive Half-lives and Decayradioisotopes turn into other elements

the closer they are to the Belt of stability, the longer it takes

every single nuclide has a different rate of decay, we measure the different rates of decay with half-life.

Half-life: the time it takes for one half of the nuclei to decay into something else.

Half-life equations

t1/2 = half-lifek = decay constanta = 1, a is always 1 for radioactive decayAo = initial activity t = timeA = activity (disintegrations per gram)

t1/2 = 0.693 ak

ln Ao = akt or lnA = -akt A Ao

Example Problems

Ex. 3) What is k for 60Co? How much 60Co remains 15.0 years after it is initially made? 60Co has a half-life of 5.27 years.

Ex. 4) Estimate the age of an object whose 14C activity is only 55% that of living wood. The half-live of carbon-14 is 5730 years.

Detection of Radiation

Detection methods available depend on the fact that particles and radiations emitted by radioactive decay are energetic and some carry charges

Photographic Detection◦Radioactivity affects photographic plates or film as does

ordinary light.Cloud Chambers

◦contain air saturated with a vapor, the particles emitted in radioactive decay ionize air molecules in the chamber and then the vapor subsequently condenses on these ions. Photographing the ion tracks can let you study their nature in detail

Gas Ionization Counters Ex. Geiger-Mueller counter

ions produced by ionizing radiation passing between high voltage electrodes cause a current to flow between the electrodes and then the current is amplified.

Fluorescence Detection Ex. Scintillation counter

Fluorescent substances absorb energy from high energy rays and then emit the energy through visible light.

Radioactive Dating

Radiocarbon dating can be used to estimate the ages of items of organic origin.

14C is produced continuously in the upper atmosphere by the bombardment of 14N by cosmic-ray neutrons:

14C atoms react with O2 to form CO2

◦CO2 then is incorporated into plant life by photosynthesis.

After material dies 14C content decreases from radioactive decay◦ 14C half-life is 5730 years.

p C n N 11

146

10

14

01-147

146 N C

The potassium-argon and uranium-lead methods are used for dating older objects.

y103.1t

Ar K 9

01-

4018

4019

21

e

y105.4t

He Th U9

42

23490

23892

21

Extra credit problems

1) Stars are enormous thermonuclear fusion reactors generating enormous amounts of heat and energy. What keeps stars from blowing themselves apart and how do they remain stable for millions and billions of years?

2) How are thermonuclear reactors designed so that the hot plasma that’s around 10 million degrees does not touch the sides of the reactor and melt it?