How do I predict products given only the formulas of the reactants?

Balance this equation, please.
NiCl2(aq) + (NH4)2S(aq) = ?

chemical change*
double displacement reaction*
monatomic ions
polyatomic ions*
strong electrolyte*
weak electrolyte*
This reaction involves two soluble ionic compounds and so is a potential double-displacement reaction. Follow this procedure to write balanced molecular equations for double displacement reactions, when you know only the formulas of the reactants:

  1. Write down the cations and anions present in each of the reactants. You'll have to have memorized a list of common ions to do this! In this case, the NiCl2 contains Ni2+ and Cl-, and the (NH4)2S contains NH4+ and S2-. Tables of common monatomic ions and polyatomic ions can be found elsewhere on this site.
  2. Swap the ions to build the products. The Ni2+ will pair up with the S2- to produce NiS. The NH4+ and the Cl- will produce NH4Cl. Make sure that you combine the cations and anions so that the charges balance.
  3. Label the products as solid, liquid, or aqueous. In general, you'll have to have memorized the solubility guidelines, or looked up the solubilities of the products in a handbook to do this. In this case, because sulfides of metals that are not alkali metals are insoluble, write the NiS as a solid, and the NH4Cl as an aqueous (dissolved) salt.
  4. Does a reaction occur? If one of the products is not a weak electrolyte, a nonelectrolyte, or a solid, no reaction occurs. In this case, NiS is a solid, so a reaction does occur.
  5. Balance the equation. Change the coefficients in front of each compound in the equation so that you have the same number of each type of atom on each side of the equation. See the section on chemical change for more on this; also see 10 tips for balancing simple equations.

Author: Fred Senese

General Chemistry Online! How do I predict products given only the formulas of the reactants?

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