Classify the reaction type.
18. 2libr + pb(no3) pbbr2 + 2lino3
19. fe + 2hcl fecl2 + h2
20. cao + h2o ca(oh)2
21. nicl2 ni + cl2

18. Double replacement

19. Single replacement

20. Synthesis

21. Decomposition

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.

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