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An induction period in 
Similarly, for an autocatalytic reaction, where one of the reaction products catalyzes the reaction itself, the rate of reaction is low initially until sufficient products have formed to catalyze the reaction.
Reactions generally accelerate when heat is applied. Where a reaction is
An induction period in chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is different from chemical thermodynamics, which deals with the direction in which a ...
is an initial slow stage of a chemical reaction
A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...
; after the induction period, the reaction accelerates. Ignoring induction periods can lead to runaway reactions.
In some catalytic reactions, a pre-catalyst
Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
needs to undergo a transformation to form the active catalyst, before the catalyst can take effect. Time is required for this transformation, hence the induction period. For example, with Wilkinson's catalyst, one triphenylphosphine ligand must dissociate to give the coordinatively unsaturated 14-electron species which can participate in the catalytic cycle:
:
Similarly, for an autocatalytic reaction, where one of the reaction products catalyzes the reaction itself, the rate of reaction is low initially until sufficient products have formed to catalyze the reaction.
Reactions generally accelerate when heat is applied. Where a reaction is exothermic
In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...
, the rate of the reaction may initially be low. As the reaction proceeds, heat is generated, and the rate of reaction increases. This type of reaction often exhibits an induction period as well.
The reactions to form Grignard reagents are notorious for having induction periods. This is usually due to two reasons: Firstly, the thin film of oxide on the magnesium reagent must be removed before the bulk magnesium can react. Secondly, Grignard reactions, while exothermic, are typically conducted at low temperature for better selectivity. For these two reasons, Grignard reactions often can have a long induction period, followed by a thermal runaway
Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing Thermal energy, energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the cond ...
, even causing the reaction solvent to boil-off.
References
{{reflist Catalysis Chemical kinetics