Elementary Reactions

Elementary Reactions

• Elementary reactions are the simplest form of chemical reactions and occur in a single step.
• They involve a change from one set of reactants to another set, with no intermediate stages.
• Elementary reactions are classified based on the number of molecules involved in the reaction.

Unimolecular Reactions

• Unimolecular reactions involve only a single molecule.
• They are characterised by a molecule dissociating into two or more products. For example, radioactive decay of unstable isotopes is a unimolecular process.

Bimolecular Reactions

• In a bimolecular reaction, two reactant particles collide and react to form products.
• The reaction rate of bimolecular reactions is directly proportional to the product of the concentrations of the two reactant species.
• An example of a bimolecular reaction is the combination of hydrogen gas and iodine gas to form hydrogen iodide.

Termolecular Reactions

• Termolecular reactions involve three particles that collide simultaneously and react.
• Termolecular reactions are very rare due to the low likelihood of three particles colliding at the same time and in the correct orientation.
• An example of a termolecular reaction is the combination of three hydrogen atoms to form a molecule of hydrogen gas.

Rate Law Expressions and Elementary Reactions

• The order of reactions can be derived directly from the reactants in the rate law expressions of elementary reactions.
• For example, in the unimolecular reaction 2NO2 → 2NO + O2, the rate of reaction can be expressed as: Rate = k[NO2], where k is the rate constant and [NO2] is the concentration of NO2.