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
