Chemistry Unit 8 Lesson 6

Nuclear reactions can yield tremendous energy.

-Albert Einstein showed that mass and energy are equivalent (E = mc2). -Chemical reactions have insignificant changes in mass, while nuclear reactions have significant ones and yield much more energy. -In fission, heavy atoms absorb neutrons, split into lighter ones, and yield particles and energy. A critical mass is essential for a chain reaction. -In fusion, lighter elements combine into heavier ones and yield particles and energy. High temperatures and pressure are needed. -Nuclear power plants harness the heat energy from fission (and will do so from fusion, too, at some point) to drive steam turbine electrical generators. RTGs use radioactive decay to make small amounts of electrical power for spacecraft.

Two ways to initiate nuclear reactors were developed.

-Gun-type - Explosives fire a piece of 92235U into subcritical 92235U to create supercritical mass. -Implosion - Explosives create shock waves that are directed inside a sphere to compress the subcritical 94244Pu core into supercritical mass.

Scientists can use radioactivity to make electricity.

-In deep space, sunlight provides little energy for powering a spacecraft. Instead, NASA uses heat from radioisotopes to make electrical power. The device for doing this is called a radioisotope thermal generator (RTG). -RTGs do not produce as much power as fission or fusion, but they are small, have no moving parts, and can operate for long periods of time, making them great for spacecraft systems.

In stars like the sun, hydrogen fusion is not a simple process, but rather requires many steps.

-Protons fuse to make deuterium. -Deuterium fuses with protons to make helium-3. -Helium-3 fuses together to make helium-4 and protons Each step releases high-energy particles and/or radiation.

Fusion

-The process whereby smaller nuclei fuse to form larger ones, with mass converted into energy that is released -In nuclear fusion, lighter elements combine to make both heavy elements and energy.

Radioactive decay has changes in mass and energy.

Assume that you have 1 mole of the reactants and products. Remember that the molecular mass is the molecular weight, expressed in grams. Therefore, the amu will translate to g/mol. It's possible to determine how much energy is released.

When atoms split, they release energy.

At first, they were puzzled by the production of such light atoms as Ba and Kr, but later they realized that the neutron had split the uranium atom. They had discovered fission.

When atoms fuse, they release energy.

Australian physicist Mark Oliphant discovered the hydrogen isotope tritium (13H) and the helium isotope (23He). He realized that the hydrogen isotopes deuterium (12H) and tritium (13H) could combine to form 24He and vast amounts of energy in a process called nuclear fusion.

Physicist Enrico Fermi helped develop the first nuclear fission reactor in 1942.

Critical mass is the amount of 235U (or other fissile material) necessary to sustain a continuous fission chain reaction. If the mass is below the critical mass, then the outward pressure from the heat, energy, and motion of the fragments will push the target atoms too far apart to absorb the emitted neutrons and continue the fission reaction. The critical mass provides enough attractive force to position the target atoms close enough together to initiate the chain reaction and to hold the fissile material together against this outward pressure. Thus, it is enough mass to sustain the chain reaction.

Fission provides energy for electricity.

Fermi's nuclear reactions could be controlled in a contained nuclear reactor and could produce heat to make electricity. In a nuclear power plant, the following happens: -Fission occurs in the reactor core, and boron rods control the reaction by absorbing excess neutrons. -The heat from fission transfers to pressurized water circulated through the core (the primary coolant loop). -The hot pressurized water transfers heat to water in another loop, called the secondary loop, creating steam. -Steam drives a turbine attached to an electrical generator to make electricity. -After passing the turbine, the steam gets condensed to water, which is then recirculated. -The reactor's concrete structure prevents radiation leaks from the reactor to the environment.

Einstein's idea led to nuclear power.

In his theory of special relativity, he realized that mass has energy even when it is not moving. He derived an equation that related mass and energy: E = mc2 where E is energy, m is mass in kilograms, and c is the speed of light (3 × 108 m/s). In this equation, mass has equivalent energy and energy has equivalent mass. In any reaction where energy is changed, there is a slight change in mass. Conversely, if there is a change in mass, then there is a change in energy. So for any reaction, there is conservation of mass and energy.

The first nuclear bombs used fission.

In the early days of World War II, physicist Robert Oppenheimer and General Leslie Groves led the U.S. effort to develop an atomic bomb in what was called the Manhattan Project. The problems involved enriching 92235U, storing subcritical 92235U mass in a bomb casing, and finding a way to initiate a chain reaction of 92235U fission.

Stars shine by nuclear fusion.

It takes tremendous heat (millions of kelvins) and pressure to bring the nuclei of 12H and 13H together and overcome the repulsive forces so that fusion can occur. One place where these temperatures and pressures can be found is inside stars.

Nuclear reactions convert matter to energy.

Radioactive decay naturally changes one element into another in the process called transmutation. But in this process, the mass of the products is sometimes less than that of the reactants. Physicists discovered that the additional mass is converted to energy—a lot of energy. Later, scientists developed technology to make controllable nuclear reactions (nuclear fission and fusion) to generate electric power.

Fusion may be harnessed to produce electricity.

Scientists attempt to create fusion in the laboratory by focusing laser energy on a small amount of matter.

In fission, the unstable target nucleus absorbs a neutron and immediately splits into two lighter element fragments (products) and one or more neutrons.

The mass difference between products and reactants is converted to energy (E = mc2). The emitted neutrons can split other atoms in a chain reaction, as shown in the animation.

Two bombs were detonated in Japan, over Hiroshima (August 6, 1945) and over Nagasaki (August 8, 1945). The immense devastation forced the Japanese to surrender on August 10, 1945, and ended World War II.

True

You have learned that the protons and neutrons of the nucleus are made of even smaller particles, called quarks, and that strong forces hold the nucleus together. Often, the balance of forces in the nucleus is unstable, and the nucleus decays by emitting particles and energy in a process known as radioactive decay. Radioactive decay can change one element into another. Furthermore, new elements can be produced artificially in particle accelerators. Finally, you have learned that nuclear reactions yield vast amounts of energy compared to their chemical counterparts, and people can harness this energy.

True

Nuclear Reactor

a contained vessel in which a nuclear reaction takes place

Fission

the process whereby a heavy nucleus splits into two intermediate-sized nuclei, with the emission of neutrons and conversion of mass into energy that is released