Topic 24: Nuclear Chemistry Flashcards
Characteristics of chemical reactions (6)
a) Atoms never change their identity
b) e- in orbitals are involved
c) Nuclear particles do not take part
d) Relatively small changes in energy
e) No measurable changes in mass
f) Rate influenced by several external factors
Characteristics of nuclear reactions (4)
a) Atoms converted into atoms of another element
b) e- in orbitals are less involved
c) Nuclear particles are involved
d) Relatively large changes in energy
e) Measurable changes in mass
f) Rate depends on number of nuclei and rarely in the compound in which an element occurs
Nucleons
Protons + Neutrons
Nuclide definition
Nucleus with specific numbers of nucleons
Notation for nuclide
(Z/A)X
(1/1)p
(-1/0)e
(0/1)n
Naming of nuclide
Element name followed by the mass number
Radioactivity definition
Spontaneous disintegration of a nucleus by emitting radiation
Role of Becquerel in nuclear chemistry
Discovery of radiation in a photographic plate exposed to U
Radiation creates an electric discharge in the air
Role of Curie in nuclear chemistry
a) Intensity directly proportional to the concentration of the element in the mineral, not to the formula of the mineral or compound
b) Unaffected by physical/chemical conditions
c) Discovery of Po/Ra
Types of radioactive emission
a) Alpha particles - α | (2/4)He
b) Beta particles - β
c) Gamma rays - γ
Principles of radioactive decay
When a nuclide decays, it becomes a nuclide of lower energy
Excess energy is carried off by the emitted radiation and the recoiling nucle
Definition of parent/daughter nuclide
Decaying nuclide = Parent
Product nuclide = Daughter
Conservation principle of nuclear equations
(TotalZ/TotalA) Reactants = (TotalZ/TotalA) Products
α decay description
a) A = -4
b) Z = -2
c) N = -2
a) Emission of α particles
b) Most common means for heavy, unstable nucleus (Z=83) to become more stable
β- decay description
a) A = 0
b) Z = +1
c) N = -1
Emission of β- particles
Positron emission
a) A = 0
b) Z= -1
c) N = +1
Emission of β+ (antiparticle of e-)
e- capture
a) A = 0
b) Z= -1
c) N = +1
a) Nucleus draws in an e- from a low atomic energy level
b) Usually accompany by the release of x-ray or neutrinos
γ decay
a) γ emission accompanies other (β^-) modes of decay
b) γ rays emitted when a particle and an antiparticle annihilate each other
=> β+ + e- → 2γ
c) γ rays have no mass or charge
General factors that affect nuclear stability
a) Total mass of the nuclide
b) Ratio of (N/Z)
Band of stability (N/Z Graph)
a) Lighter nuclides are stable when N = Z
b) As Z increases, the N/Z for stable nuclei gradually increases
c) All nuclides with Z > 83 are unstable
d) Pairing of spins of like nucleons lead to greater stability
Exceptions to instable light nuclides
(1/1)H and (2/3)He
Magic numbers
2, 8, 20, 28, 50, 82, 126
Effect of strong force
a) Electrostatic repulsive forces between protons would break the nucleus if not for strong force
b) Strong force operates over short distances within the nucleus
Type of emission in
a) Neutron-rich nuclides (Higher than atomic mass)
b) Proton-rich nuclides (Lower than atomic mass)
c) Heavy nuclides (Beyond 83)
a) B- decay
b) B+ emission / e- capture
c) Alpha decay
Decay series definition
Series of decay steps before a stable daughter nuclide form
Principle for detecting radioactivity
Observing the effects of radioactive emission on the surrounding atoms
Methods used to detect and measure radioactivity
a) Ionization counter (Geiger-Muller)
b) Scintillation
Mechanism of ionization counter
a) Detect radioactive emissions as these cause gas (Ar) ionization
b) Ionized gaseous cations (Ar+) and free e- are attracted to electrodes from a recording device
c) Current created is amplified and appears as a meter reading
Structure of Geiger-Muller counter
Tube filled with Ar-CH4 mixture
a) Cathode (-) - Tube shell
b) Anode (+) - Central wire
Scintillation counter mechanism
a) Detect radioactivity emissions by their ability to excite atoms and make them emit light
b) Putting a radioactive sample into a liquid (phosphor)
c) Incoming radioactive particle strike the phosphor
d) Each photon strikes the cathode, releasing an e-
e) # of pulses proportional to the concentration of the radioactive substance
Does the rate of radioactive decay depend on the chemical substance?
No
Decay rate / activity definition
Change in # of nuclei divided by the change in time
Calculation of decay rate
A = - ∆N/∆t
Units of decay rate
1Bq = 1d/s (SI)
1Ci = 3.7x10^10 d/s
What order kinetics does the decay rate follow?
First order kinetics (A = kN)
ln(No/Nt) = kt
Half-life equation
Time taken for half the nuclei in a sample to decay
a) t(1/2) = ln(2)/k
Radioisotope dating purpose
Determine the ages of certain objects
Radiocarbon dating
Measure relative amounts of C-14 and C-12 in materials of biological origin (36000yrs)
a) Ratio remains the same for all living organisms
b) Once the organism dies, the amount of C-14 starts to decrease as it decays
c) Measuring its amount present indicates the time that has passed
Formula for radiocarbon dating
(1/k) ln(A0/At) = t
A0 = Activity in living organism
At = Activity in object whose age is unknown
Nuclear transmutation definition
a) Induced conversion of the nucleus of one element into the nucleus of another
b) Achieved by high-energy bombardment of nuclei in a particle accelerator
Notation for nuclear transmutation
Reactant nucleus (Particle in, Particle(s) out) Prdouct nucleus
Characters that contributed to nuclear transmutation
a) Rutherford bombarded the N with a particles to form O
b) Chadwick discovered the neutron by bombarding Li with a particles
c) Irene and Frederic created the first artificial radioisotope.
Particle accelerator principle
Imparting high KE to particles by placing them in an electric field with a magnetic field
Linear accelerator
a) Series of separated tubes increasing in length with alternating voltage
b) Particle is accelerated from one tube to the next by repulsion/attraction
Cyclotron (Lawrence, 1930)
a) Use of electromagnets to give the particle a spiral path to save space
b) Same principle as linear accelerator
Synchrotron
Synchronously increasing magnetic field to make particle’s path circular
Large Hadron Collider
World’s most powerful accelerator
First successful collision of protons
Application of particle accelerators
a) Production of radioisotopes used in medical field
b) Synthesis of transuranium elements (higher atomic number than U)
What do nuclear changes cause?
a) Chemical changes in surrounding matter
b) Ionization in surrounding matter (Cation + e- pair)
What is the relationship between incoming energy and the number of cation-electron pairs?
Directly proportional
General principle of the penetrating power of a particle
If a particle interacts strongly with matter, it penetrates only slightly.
Ionizing ability and penetrating power of a particle
a) Interact with matter most strongly
b) Piece of paper can stop radiation from external source
Ionizing ability and penetrating power of B particle
a) Interact less strongly with matter
b) Thick piece of metal is required to stop these particles
Ionizing ability and penetrating power of γ particle
a) Interact least with matter and penetrate most
b) Block of Pb several inches thick is needed to stop them
Factors that affect the danger from a radioactive nuclide
a) Type of radiation
b) Half-life of the radioactive nuclide
c) Biological behavior of the radioactive nuclide
Explanation of the biological behavior of the radioactive nuclide
a) Sr-90 (Similar to Ca and easily absorbed by bones)
b) Water is the most likely molecule to absorb γ radiation
Reaction of water after absorbing y radiation
H2O + γ → H2O∙ + e-
Products collide with other water molecules to form more free radicals
Free radicals are unstable and extremely reactive
Attacking the bonding and structure of surrounding molecules
Units of energy absorption
SI: 1 Gy = 1 J/kg
Common: 1 rad = 0.01 Gy
Definition of Rem
Unit of radiation dosage equivalent to a given amount of tissue damage
A) no. of rems = rads x RBE
SI Units of tissue damage
1 Sv = 100 rem
Sources of ionizing radiation
a) Cosmic radiation
b) Th and U minerals present in soil
c) Medical diagnostic techniques
d) Radioactive waste disposal
e) Radioactive K-14 in daily intake of food/tap water
Explanation of radiation in Rn
Production of Rn from Th and U minerals present in soil
Cause of lung cancer
What factors influence the risk from ionizing radiation?
Increase in the cancer incidence from
a) High, single exposure
b) Low chronic exposure
Ex. Fruit flies with genetic defects (Linear)
Models for assessing ionizing radiation
a) Linear response - Radiation effects accumulate over time
b) S-shaped response - Threshold above which effects are more significant
Principles of radioactive tracers
Tiny amount of a radioisotope mixed with large amount of stable isotope of the same element
Applications of radioactive tracers
a) Reaction pathways
=> Periodate-iodide reaction
=> Photosynthesis
b) Physiological studies
=> I-125 labeled albumin
=> Cr-51 labeled RBC
c) Material flow
=> Semiconductor chips and metal plating
=> Map of water flow from land to lakes
Neutron activation analysis
a) Converting a small fraction of its atoms to radioisotopes
b) Exhibition of a characteristic decay pattern that reveals the elements present
Medical diagnosis with radioisotopes
a) Production of an image of the thyroid gland with I-131
b) Positron-emission tomography
=> Imaging method for observing brain structure and function
=> Injected substance in bloodstream emits positrons annihilated by e-
Applications of radioisotopes
a) Radiation therapy with Au/Sr (Interference with cell division)
b) Destruction of microbes (Killing organisms that cause rotting)
c) Insect control
d) Power for spacecraft instruments
Types of nuclear processes
a) Fission (Heavy nucleus splits into two)
b) Fusion (Lighter nuclei combine to form one)
c) Decay (Emission of few small particles to become a more stable lighter nucleus)
Conservation principle for nuclear reactions
Total quantity of mass-energy in the universe is constant
E = mc^2
Mass difference in nuclear reactions
There is always a mass decrease when nucleons form a nucleus
a) Decrease in mass is due to mass being converted to energy to hold the nucleus together (Mass effect)
Definition of nuclear binding energy
Energy required to break 1 mol of nuclei of an element into individual nucleons
eV Unit
Energy an electron acquires when it moves through a potential difference of 1 V
Binding energy per nucleon
BE/n = (Binding energy) / (No of nucleons)
Relationship between binding energy per nucleon and nuclear stability
The greater the binding energy per nucleon, the more stable the nuclide is
The role of fission and fusion in binding energy per nucleon
Binding energy per nucleon peaks at element with A = 60
a) Fission - A heavier nucleus that can split into lighter nuclei (closer to A = 60)
b) Fusion - Lighter nuclei can combine to form a heavier nucleus
Historical context of fission
a) Enrico Fermi bombarded U with n
b) Otto Han found an isotope of Ba in sample
c) Meitner proposed fission model to explain this
General process of fission reaction
a) Neutron bombardment results in a highly excited U-236
b) Process is harnessed by chain reaction
Description of chain reaction during fission
a) Few neutrons that are released by the fission of one nucleus collide with other fissionable nuclei and cause them to split
b) This releases more neutrons in a self-sustaining process
c) Each event in a chain reaction releases about 2.5 times as much energy as the preceding one
Definition of critical mass
Mass required to achieve a chain reaction
How does an atomic bomb work?
a) Manhattan Project (1941)
b) Detonation of 2 (1945)
a) Small explosions of TNT bring subcritical masses of fissionable material together to exceed the critical mass and ensuing chain reaction brings about the explosion
Principle of nuclear energy reactor
Generate heat to produce steam, turning a turbine attached to an electric generator
Where does heat generation occur?
Reaction core
Elements of nuclear energy reactor
a) Fuel rods
b) Control rods
c) Reflector
d) Moderator
Fuel rods
a) Composition
b) Function
a) Fuel (U-235) enclosed in tubes of a corrosion-resistant Zr alloy
b) Release of neutrons
How are fuel rods enriched with U-235 3% / 4%?
Gas centrifugation / Graham law
Control rods
a) Composition
b) Function
a) Made up of Cd / B absorb neutrons efficiently
b) Regulate nuclear activity
Reflector
a) Composition
b) Function
a) Made up of Be
b) Absorb very few neutrons and reflect those back to fuel rods
Moderator
a) Composition
b) Function
a) H2O (Light) / D2O (Heavy)
b) Slows the neutrons, making them much better for fission / Acts as a coolant by transferring heat to the steam producing region
Advantages of heavy water reactors
a) Absorbs very few neutrons
b) Use U that has been less enriched
Breeder reactors principles and disadvantages (Not longer used in the US)
Consume one type of nuclear (U-238) fuel as it produces another (Pu-239) by neutron release
Difficult and expensive to build
Products are extremely toxic
Power plant accidents (3)
1979 - Malfunctions of coolant pumps and valves in Pennsylvania
1986 - Cooling system failure at the Chernobyl plant in Ukraine
2011 - Destruction of Fukushima nuclear facility by Tsunami, melting 3 reactors
Use of nuclear reactors in Europe
Important source of electricity due to climate change policies
Germany committed to closing all its nuclear power plants by 2022
Thermal and waste pollution in nuclear reactors
a) Water used to condense the steam is several degrees warmer when returned to its source, which can harm aquatic organisms
b) Many of the fission products formed in nuclear reactors have long half-lives, and no plan for their permanent disposal has yet been devised.
How were all elements heavier than H formed in the stars?
Fusion and decay processes
Example of fusion reaction to produce He
H-2 + H-3 → He-3 + n-1
Benefits and disadvantages of He fusion reaction from tritium
a) Enormous quantity of energy with no radioactive byproducts
b) Tritium is scarce and only produced in cosmic radiation or nuclear accelerators by bombarding Li
Complication of fusion reactions
Require enormous energy (heat) to give the + charged nuclei enough KE to force themselves together
Solutions to fusion complication
a) Atoms are stripped of their e- at high temperatures that results in a gaseous plasma enclosed within a magnetic field (Tokamak)
b) Using many focused lasers to compress and heat the fusion reactants
