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Methane chlorination is a chain reaction. If only the products and reactants are analyzed, the result is: CH 4 + 4 Cl 2 → CCl 4 + 4 HCl. However, this reaction has 3 intermediate reactants which are formed during a sequence of 4 irreversible second order reactions until we arrive at the final product. This is why it is called a chain reaction.
In 1918, Walther Nernst proposed that the photochemical reaction between hydrogen and chlorine is a chain reaction in order to explain what is known as the quantum yield phenomena. This means that one photon of light is responsible for the formation of as many as 10 6 molecules of the product HCl .
A chain reaction is an example of a complex mechanism, in which the propagation steps form a closed cycle. In a chain reaction, the intermediate produced in one step generates an intermediate in another step. Intermediates are called chain carriers. Sometimes, the chain carriers are radicals, they can be ions as well.
Reactions that proceed in the backward direction to approach equilibrium are often called non-spontaneous reactions, that is, is positive, which means that if they occur at constant temperature and pressure, they increase the Gibbs free energy of the reaction. They require input of energy to proceed in the forward direction.
The mere fact that an assembly is supercritical does not guarantee that it contains any free neutrons at all. At least one neutron is required to "strike" a chain reaction, and if the spontaneous fission rate is sufficiently low it may take a long time (in 235 U reactors, as long as many minutes) before a chance neutron encounter starts a chain reaction even if the reactor is supercritical.
Another method of termination is the reaction between a lipid radical and a lipid peroxide, or the combination of two lipid peroxide molecules, resulting in stable nonreactive molecules. [ 4 ] [ 5 ] Reinforced lipids that become part of the membrane if consumed with heavy isotope diet also inhibit peroxidation.
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Since nuclear chain reactions may only require natural materials (such as water and uranium, if the uranium has sufficient amounts of 235 U), it was possible to have these chain reactions occur in the distant past when uranium-235 concentrations were higher than today, and where there was the right combination of materials within the Earth's crust.