Chain reactions usually consist of many repeating elementary steps, each of which has a chain carrier. Once started chain reactions continue until the reactants are exhausted. Fire and explosion are some of the phenomena associated with chain reactions. The chain carriers are some intermediates appear in the repeating elementary steps. These are usually free radicals.
Once initiated, repeating elementary steps continue until the reactants are exhausted. When repeating steps generate more chain carriers, they are called chain branching reactions, which leads to explosions. If the repeating elementary steps do not lead to the formation of new product, they are called chain inhibition reactions. Addition of other materials in the reaction mixture can lead to the inhibition reaction to prevent the chain propagation reaction. When chain carriers react with one another forming stable product, the elementary steps are called chain termination reactions.
Explosions, polymerizations, and food spoilage often involve chain reactions. The chain reaction mechanism is involved in nuclear reactors, in this case the chain carriers are neutrons.
The mechanisms describing chain reactions are useful models for describing chemical reactions.
Usually, a free radical is marked by a dot beside the symbol. A dot represents an odd electron on the species. The odd electron on the species makes it very reactive. We represent the odd electron with *, because a period is too confusing. For example, the oxygen, chlorine and ethyl radicals are represented by O*, Cl*, and C2H5* respectively.
The Cl* radicals can be formed by the photo dissociation reaction,
If we mix chlorine, Cl2, and ethane, CH3CH3, together at room temperature, there is no detectable reaction. However, when the mixture is exposed to light, the reaction suddenly initiates, and explodes. To explain this, the following mechanism is proposed.
The light initiates the reaction by providing free radicals. The light causes a photodissociation reaction, and the initiation step, can be written as:
Elementary steps in which the number of free radicals consumed is equal to the number of free radicals generated are called chain propagation steps. Once initiated, the following chain propagation steps repeat indefinitely or until the reactants are exhausted:
For example, in the reaction between hydrogen and oxygen, the following reaction may take place:
where *O* is a di-radical, because the O atom has an electronic configuration 2s2 2px2 2py1 2pz1. In this elementary step, three radicals are generated, whereas only one is consumed.
The di-radical may react with a H2 molecule to form two radicals.
Thus, together chain branching reactions increase the number of chain carriers. Branching reactions contribute to the rapid explosion of hydrogen-oxygen mixtures, especially if the mixtures have proper portion.
Furthermore, sometimes another reactive substance *A may be added to the system to reduce the chain carriers to inhibit the chain reactions.
The species A* is often called radical scavengers. In food industry, radical scavengers are added to prevent spoilage due to oxidation.
The mechanism in chain reactions is very complicated. When intermediates are detected, a reasonable mechanism can be proposed. Adding radical scavenger to prevent food spoilage is an important application in food chemistry. This application came from the application of the chain reaction model to natural phenomena.
In chain reactions, many products are produced.
Argon exists as a mono-atomic gas. All noble gases have mono-atomic molecules.
Identify steps for the names in the multiple choices.
Predicting the intermediate from the nature of the reactants.
The reactant HCl in the step is a product in overall reaction. When HCl reacts with Cl*, the reaction is retarted. Cl* attacked one of the product molecule HCl causing a reversal of the reaction.