Question

On the basis of the general solubility rules given in Table 7.1 , predict the identity of the precipitate that forms when aqueous solutions of the following substances are mixed. If no precipitate is likely, indicate why (which rules apply). a. iron(III) chloride and sodium hydroxide b. nickel(II) nitrate and ammonium sulfide c. silver nitrate and potassium chloride d. sodium carbonate and barium nitrate e. potassium chloride and mercury(I) nitrate f. barium nitrate and sulfuric acid

Short answer

a. The precipitate formed is iron(III) hydroxide (Fe(OH)₃). b. The precipitate formed is nickel(II) sulfide (NiS). c. The precipitate formed is silver chloride (AgCl). d. The precipitate formed is barium carbonate (BaCO₃). e. The precipitate formed is mercury(I) chloride (Hg₂Cl₂). f. The precipitate formed is barium sulfate (BaSO₄).

Step-by-step solution

a. Iron(III) chloride and sodium hydroxide

First, we write the chemical formulas for the reactants: Iron(III) chloride is FeCl₃, and sodium hydroxide is NaOH. When these two substances react, they will undergo a double displacement reaction where the positive ions (cations) will pair up with the negative ions (anions) of the other reactant: FeCl₃(aq) + 3NaOH(aq) → Fe(OH)₃(s) + 3NaCl(aq) The precipitate formed is iron(III) hydroxide (Fe(OH)₃) because most hydroxides are insoluble, except those of group 1 elements (alkali metals) and ammonium (NH₄⁺).

b. Nickel(II) nitrate and ammonium sulfide

The reactants are nickel(II) nitrate, Ni(NO₃)₂, and ammonium sulfide, (NH₄)₂S. The reaction is: Ni(NO₃)₂(aq) + (NH₄)₂S(aq) → NiS(s) + 2NH₄NO₃(aq) The precipitate formed is nickel(II) sulfide (NiS) because most sulfides are insoluble, except those of group 1 and 2 elements (alkali metals and alkaline earth metals) and ammonium (NH₄⁺).

c. Silver nitrate and potassium chloride

The given reactants are silver nitrate, AgNO₃, and potassium chloride, KCl. The reaction is: AgNO₃(aq) + KCl(aq) → AgCl(s) + KNO₃(aq) The precipitate formed is silver chloride (AgCl) because most chlorides, bromides, and iodides are soluble, except those of silver (Ag⁺), lead (Pb²⁺), and mercury(I) (Hg₂²⁺).

d. Sodium carbonate and barium nitrate

The reactants are sodium carbonate, Na₂CO₃, and barium nitrate, Ba(NO₃)₂. The reaction is: Na₂CO₃(aq) + Ba(NO₃)₂(aq) → BaCO₃(s) + 2NaNO₃(aq) The precipitate formed is barium carbonate (BaCO₃) because most carbonates are insoluble, except those of group 1 elements (alkali metals) and ammonium (NH₄⁺).

e. Potassium chloride and mercury(I) nitrate

The given reactants are potassium chloride, KCl, and mercury(I) nitrate, Hg₂(NO₃)₂. The reaction is: 2KCl(aq) + Hg₂(NO₃)₂(aq) → Hg₂Cl₂(s) + 2KNO₃(aq) The precipitate formed is mercury(I) chloride (Hg₂Cl₂) because most chlorides, bromides, and iodides are soluble, except those of silver (Ag⁺), lead (Pb²⁺), and mercury(I) (Hg₂²⁺).

f. Barium nitrate and sulfuric acid

The reactants are barium nitrate, Ba(NO₃)₂, and sulfuric acid, H₂SO₄. The reaction is: Ba(NO₃)₂(aq) + H₂SO₄(aq) → BaSO₄(s) + 2HNO₃(aq) The precipitate formed is barium sulfate (BaSO₄) because most sulfates are soluble, except those of barium (Ba²⁺), lead (Pb²⁺), calcium (Ca²⁺), strontium (Sr²⁺), and mercury(I) (Hg₂²⁺).

Key concepts

Solubility Rules

Understanding the solubility rules is crucial when predicting the formation of a precipitate in chemical reactions. These rules are derived from empirical observations and indicate whether certain ionic compounds are soluble or insoluble in water. For instance, most nitrates \(NO_3^-\) and alkali metal compounds are typically soluble, whereas many carbonates \(CO_3^{2-}\), hydroxides \(OH^-\), and sulfides \(S^{2-}\) are insoluble unless they are paired with specific cations such as those from Group 1 elements or ammonium \(NH_4^+\).

To improve the student's grasp of this concept, let's consider the example of silver nitrate \(AgNO_3\) and potassium chloride \(KCl\). According to the solubility rules, most chlorides are soluble with exceptions including silver. When mixed, these compounds will undergo a reaction that results in the formation of silver chloride \(AgCl\), an insoluble compound that precipitates out of the solution. This practical application helps reinforce the rule that while most chlorides are soluble, there are important exceptions to remember.

Double Displacement Reactions

Double displacement reactions, also known as metathesis reactions, occur when parts of two ionic compounds are exchanged to form two new compounds. This is a type of chemical reaction commonly identified by its general form: \(AB + CD \rightarrow AD + BC\). One of the most common outcomes in an aqueous solution is the formation of an insoluble compound, a precipitate, which can be predicted by solubility rules.

For example, when we mix solutions of barium nitrate \(Ba(NO_3)_2\) and sulfuric acid \(H_2SO_4\), a double displacement reaction will swap the ions to form barium sulfate \(BaSO_4\) and nitric acid \(HNO_3\). Here, \(BaSO_4\) is a precipitate due to its low solubility. It's important for students to practice with exercises that require them to write and balance these equations, so they can identify the products formed and determine which will precipitate based on the solubility rules.

Insoluble Compounds

In chemistry, an insoluble compound is one that does not dissolve significantly in a solvent. The formation of such compounds is often indicated by the appearance of a solid precipitate in a liquid solution. Knowing which compounds are insoluble can be essential for predicting the outcomes of reactions, such as when mixed solutions will form a precipitate.

Taking the reaction between sodium carbonate \(Na_2CO_3\) and barium nitrate \(Ba(NO_3)_2\) as an example, the insoluble compound barium carbonate \(BaCO_3\) is formed. This prediction is based on an established solubility rule that most carbonates are not soluble in water, except for those of the alkali metals and ammonium. The ability to predict whether a compound is soluble or insoluble allows chemists and students alike to anticipate the formation of precipitates in various chemical reactions.