What chemical change occur when a more reactive substance replaces a less reactive substance in a compound?

A single replacement reaction (single displacement reaction). The generic form is: A +BC-->B+AC.

For example:

2Al+3CuSO4-->3Cu+Al2(SO4)3

The more reactive aluminum replaced the copper in the copper sulfate compound. An activity series indicates which elements will replace other elements. You can find activity series online.