Hydrogen and Helium nuclei in the synthesis of elements in stars.
Hydrogen and helium are the most plentiful elements. They make up approximately 89% and 11% of atoms in the universe. This means they play a pretty major role.
The production of Hydrogen and Helium nuclei makes it possible for the star to synthesise the majority of the elements in the first three periods of the periodic table.
Heavier chemical elements are made from these through a variety of processes known as nucleogenesis. This is what occurs in stars. The different types of stars depend on the different reactions that take place.
In the sun, the closest star, hydrogen gets converted into helium in a nuclear fusion reaction.
41H → 4He + subatomic particles
This reaction releases a lot of energy, (approximately 2.5 x 109 kJmol-1) and this energy reaches earth as heat and light energy.
As stars evolve, most of the hydrogen gets used up and new fusion reactions begin.
Now, the helium nuclei react to form from Beryllium → Carbon → Oxygen → Neon → Magnesium. First of all reacting 24He, which forms Beryllium; and then reacting 4He with each consecutive product do this. (Ending with 20Ne reacting with a helium nuclei to form 24Mg)
Energy is given out at every stage of these processes. When the helium gets used up, the star begins new processes using carbon nuclei.
When lithium forms in stars it can take one of two routes:
-
4He + 3H → 7Li
2. 4He + 3He → 7Be… … …7Be + electron → 7Li
In the 2nd route shows that the collision between the atom and an electron causes a change to the nucleus of the atom. The proton number and the structure of the nucleus is changed. Lithium could then be produced by the action of cosmic rays on carbon, oxygen and nitrogen. Lithium can be synthesised in exploding stars and even heavier elements can be created.
Main characteristics of fission and fusion
Fission
Fission uses the equation
E=mc2
(E = energy, m = mass and c = velocity of light)
Nuclear fission is likened to the oscillation of a drop of liquid. If there are violent vibrations, the droplet will split in two. If an atom is given enough energy, through the absorption of a neutron, the atom becomes excited and begins to oscillate.
The nucleus then splits into two nuclei of medium mass whilst emitting three neutrons.
These neutrons may start a chain reaction by causing further fission in other nuclei.
A lot of energy is produced and can be used to provide anything from electricity to atomic bombs!
A possible fission reaction of 235U is:
Alternatively, it could have gone something like this:
Nuclear fission is sometimes used to generate electricity. The reactions can be controlled in a nuclear reactor. 238U and 235U are used.
As 238U does not undergo fission, the neutrons collide and then are absorbed. This stops the chain reaction.
These reactions produce energy and are also controlled using a graphite moderator and the control rods.
These are made from boron-coated steel. The moderator slows down the rapidly-moving neutrons that are produced when the nucleus is split. This allows the fission reactions to occur when they collide with the 235U nuclei.
The control rods then absorb the neutrons. Moving them partly in and out allows the operators to control the rate that the reactions occur. All the reactions that take place in the reactor generate a lot of energy. This heat is used to boil water into steam, which in turn turns turbines to produce electricity.
Fusion
Fusion reactions involve the joining together (fusing) of two light nuclei. This requires sustained high temperatures. The reaction chosen for experiments on earth is the fusion of deuterium (found in water) and tritium. These are both two heavy isotopes of hydrogen.
Under the correct conditions, the nuclei fuse to produce both helium and a neutron and excess energy, which is also produced.
For the reaction to occur, particles have to take shape of an ionised gas- also known as a plasma-. This is extremely hot, (approximately 150 million oC) and has an extremely high density. When the temperature reaches this limit, the electrons escape from the nuclei. This causes the plasma to have positively charged ions amongst the repositioned electrons.
The main difficulty is creating these conditions on earth. But the sun’s gases exist as a plasma.
There are advantages and disadvantages of using fission and fusion for generating electricity. The fuels needed for fusion are isotopes of hydrogen. They are in abundant supply. Which can offer longevity.
This is a definite advantage as burning fossil fuels generates 80% of the world’s energy, which brings on climate change and produces pollution.
In addition to this, the radioactivity of the structure is shorter compared to a nuclear fission power station. Fission can produce actinides (highly radioactive waste) which can take thousands of years to decay.
It is a lot safer as well. The fusion reaction cannot get out of control as only small amounts of fuel is used.
Challenges Facing Scientists
If fusion reactions could be controlled on earth, the possibilities would be endless. The energy has the likelihood of being used to generate electricity. But scientists have the problem of safely producing reactions. Fusion reactions require constant high temperatures. Scientists have to combat the fact that at normal temperatures on Earth, the repellent force between the two nuclei would be too huge for fusion to occur.
Higher temperatures are needed to overcome this force, as the nuclei are moving a lot faster and with more energy for the collisions that the force barrier is overcome.
References
Chemical Ideas Salters Advanced Chemistry Heineman 2nd edition 2000
ISBN 0-435-63120-9
Chemical Storylines Salters Advanced Chemistry Heineman 2nd edition 2000
ISBN 0-435-63119-5
“Lisa Meitner Radiochemist, physicist and co-discoverer of nuclear fission”
Gordon Woods Chemistry Review Volume 16 Number 1 September 2006
“Fusion, powering the future?” Chris Warwick Chemistry Review Volume 16 Number 1 September 2006
“Lithium” Chris Ennis, Volume 15 Number 31 February 2006
“From Gaslight to Nuclear Power-Chemistry of the actinides” Nigel Lowe Chemistry Review Volume 17 Number 1 September 2007
Salters Horners Advanced Physics Published 2000 Heinemann ISBN 0 435 628909
“50 Physics ideas you really need to know” Joanne Baker, 2007 Querscus Publishing London
“Revise AS + A2 Physics” Letts Educational, Letts Educational Publishers, London 2004
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