Answer:
18. Double replacement
19. Single replacement
20. Synthesis
21. Decomposition
Step-by-step explanation:
18. Double replacement:
The double replacement reaction is typical of two ionic compounds reacting.
In a double replacement reaction ions are switched. This is represented by the general form AB + CD → AD + CB.
In 2LiBr + Pb(NO₃) → PbBr₂ + 2LiNO₃ the ion NO₃⁻ from Pb(NO₃) recplaces the Br⁻ ion from LiBr to form LiNO₃, and the Br⁻ ion replaces the NO₃⁻ ion in Pb(NO₃) to form PbBr₂, so they switch places
19. Single replacement
In a single replacement reaction a more active substance replaces a less reacting substance in a compound.
The general representation of single replacement reactions is AB + C → AC + B.
In Fe + 2HCl, Fe (iron) is a more active element than H (hydrogen), so Fe replaces H in HCl. At the end the hydrogen atoms stand as a gas.
20. Synthesis
A synthesis reaction is the combination of two or more substances (elementes or compounds) to form a new compound.
The general representation of a synthesis reaction is A + B → AB: two substances (A and B) produce a new one (AB).
In CaO + H₂O → Ca(OH)₂, calcium oxide and water react together to obtain calcium hydroxide.
21. Decomposition
In a decomposition reaction a single substance decomposes into two or more different substances.
The typical representation of a decomposition reaction is AB → A + B.
So, in NiCl₂ → Ni + Cl₂, the starting compound (reactant) decomposes into two products, so you say that NiCl₂ decomposed.