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A single replacement reaction occurs when an element reacts with a compound to produce a new element and a new compound. It can only occur when the element that is doing the replacing is more reactive than the element that is being replaced. Therefore, it is useful to have a list of elements in order of their relative reactivities. The activity series is a list of elements in decreasing the order of reactivity. Since metals replace other metals, while nonmetals replace other nonmetals, they each have a separate activity series.

There are two types of single replacement reactions:

A metal replaces another metal that is in solution:  

A + BC → B + AC

Example:  Zn + CuC → Cu + ZnC

A halogen replaces another halogen that is in solution:

A + BC → C + BA

Example: Br + 2KI → I

an active site is a region that is present on an enzyme where the reactants or the substrate molecules we can say bind and experience a chemical reaction. the substrate is basically a reactant whose concentration is changing and that is converting into a product after binding at the active site of an enzyme