How chemical displacement reactions differ from a chemical decomposition reaction? "

Chemical displacement reaction : one group or element in a compound is replaced by another. There is a substitution taking place.

A - X + B - Y - > A - Y + B - X with respective electric charges on them

methane + chlorine - > HCl + methyl chloride

Chemical decomposition is opposite of chemical synthesis. A compoind is broken into its groups or elements.

A - B - > A + B with respective electric charges on them

2 H2O liquid - > 2 H2 gas + O2 gas