Question

Without first writing a full molecular or ionic equation, write the net ionic equations for any precipitation reactions that occur when aqueous solutions of the following compounds are mixed. If no reaction occurs, so indicate. a. iron(III) nitrate and sodium carbonate b. mercurous nitrate and sodium chloride c. sodium nitrate and ruthenium nitrate d. copper(II) sulfate and sodium sulfide e. lithium chloride and lead(II) nitrate f. calcium nitrate and lithium carbonate g. gold(III) chloride and sodium hydroxide

Short answer

a. \[Fe^{3+}(aq) + 3CO_3^{2-}(aq) \rightarrow Fe_2(CO_3)_3(s)\] b. \[Hg_2^{2+}(aq) + 2Cl^-(aq) \rightarrow Hg_2Cl_2(s)\] c. No reaction occurs. d. \[Cu^{2+}(aq) + S^{2-}(aq) \rightarrow CuS(s)\] e. \[Pb^{2+}(aq) + 2Cl^-(aq) \rightarrow PbCl_2(s)\] f. \[Ca^{2+}(aq) + CO_3^{2-}(aq) \rightarrow CaCO_3(s)\] g. \[Au^{3+}(aq) + 3OH^-(aq) \rightarrow Au(OH)_3(s)\]

Step-by-step solution

Form the possible product:

Combine the cations and anions in a double displacement reaction: Iron(III) cation: Fe^{3+} Nitrate anion: NO_3^- Sodium cation: Na^+ Carbonate anion: CO_3^{2-} Possible products: Fe_2(CO_3)_3(s) and NaNO_3(aq)

Write the net ionic equation:

\[Fe^{3+}(aq) + 3CO_3^{2-}(aq) \rightarrow Fe_2(CO_3)_3(s)\] b. Mercurous nitrate and sodium chloride

Form the possible product:

Combine the cations and anions in a double displacement reaction: Mercurous cation: Hg_2^{2+} Nitrate anion: NO_3^- Sodium cation: Na^+ Chloride anion: Cl^- Possible products: Hg_2Cl_2(s) and NaNO_3(aq)

Write the net ionic equation:

\[Hg_2^{2+}(aq) + 2Cl^-(aq) \rightarrow Hg_2Cl_2(s)\] c. Sodium nitrate and ruthenium nitrate #Analysis#: Both sodium nitrate and ruthenium nitrate are soluble salts, so mixing them will not result in a precipitation reaction. The answer is: No reaction occurs. d. Copper(II) sulfate and sodium sulfide

Form the possible product:

Combine the cations and anions in a double displacement reaction: Copper(II) cation: Cu^{2+} Sulfate anion: SO_4^{2-} Sodium cation: Na^+ Sulfide anion: S^{2-} Possible products: CuS(s) and Na_2SO_4(aq)

Write the net ionic equation:

\[Cu^{2+}(aq) + S^{2-}(aq) \rightarrow CuS(s)\] e. Lithium chloride and lead(II) nitrate

Form the possible product:

Combine the cations and anions in a double displacement reaction: Lithium cation: Li^+ Chloride anion: Cl^- Lead(II) cation: Pb^{2+} Nitrate anion: NO_3^- Possible products: LiNO_3(aq) and PbCl_2(s)

Write the net ionic equation:

\[Pb^{2+}(aq) + 2Cl^-(aq) \rightarrow PbCl_2(s)\] f. Calcium nitrate and lithium carbonate

Form the possible product:

Combine the cations and anions in a double displacement reaction: Calcium cation: Ca^{2+} Nitrate anion: NO_3^- Lithium cation: Li^+ Carbonate anion: CO_3^{2-} Possible products: CaCO_3(s) and LiNO_3(aq)

Write the net ionic equation:

\[Ca^{2+}(aq) + CO_3^{2-}(aq) \rightarrow CaCO_3(s)\] g. Gold(III) chloride and sodium hydroxide

Form the possible product:

Combine the cations and anions in a double displacement reaction: Gold(III) cation: Au^{3+} Chloride anion: Cl^- Sodium cation: Na^+ Hydroxide anion: OH^- Possible products: Au(OH)_3(s) and NaCl(aq)

Write the net ionic equation:

\[Au^{3+}(aq) + 3OH^-(aq) \rightarrow Au(OH)_3(s)\]

Key concepts

Precipitation Reactions

Precipitation reactions are a subset of chemical reactions that result in the formation of an insoluble solid, called a precipitate, from the mixing of two soluble compounds. In our context, when aqueous solutions are mixed, the ions in the individual solutions can come together to form new compounds. If the product of this exchange is not soluble in water according to solubility rules, it will precipitate out of the solution.

For example, when iron(III) nitrate \(Fe(NO_3)_3\) is mixed with sodium carbonate \(Na_2CO_3\), iron(III) carbonate \(Fe_2(CO_3)_3\) forms as a precipitate because it is not soluble in water. The net ionic equation for this reaction is: \[Fe^{3+}(aq) + 3CO_3^{2-}(aq) \rightarrow Fe_2(CO_3)_3(s)\].

The formation of the solid precipitate is indicated by the appearance of a substance that does not dissolve, often changing the clarity or color of the solution. Precipitation reactions are useful in various applications, including water treatment, qualitative analysis, and the manufacturing of products like cement and plaster.

Double Displacement Reactions

Double displacement reactions, also known as metathesis, involve the exchange of ions between two compounds to form two new compounds. In the context of precipitation reactions, such an exchange results in at least one insoluble product.

When two ionic compounds in solution are combined, the cations of one compound swap places with the anions of the other. This is well illustrated by the combination of copper(II) sulfate \(CuSO_4\) and sodium sulfide \(Na_2S\). The copper(II) ions \(Cu^{2+}\) pair with the sulfide ions \(S^{2-}\) to form copper(II) sulfide \(CuS\), a solid precipitate, while the sodium \(Na^+\) and sulfate \(SO_4^{2-}\) ions remain in solution as sodium sulfate \(Na_2SO_4\). The net ionic equation is: \[Cu^{2+}(aq) + S^{2-}(aq) \rightarrow CuS(s)\].

Understanding double displacement reactions is essential for predicting the products of a chemical reaction and determines whether the mixing of two compounds will result in a visible change, such as the formation of a precipitate.

Solubility Rules in Chemistry

Solubility rules are guidelines that predict the solubility of various compounds in water. These rules are based on empirical observations and are crucial in determining whether a substance will dissolve to form an aqueous solution or precipitate out as a solid.

Key solubility rules include:

• Most nitrate \(NO_3^-\) salts are soluble.
• Salts containing alkali metal ions (like \(Li^+\), \(Na^+\), \(K^+\)) and \(NH_4^+\) are generally soluble.
• Chloride \(Cl^-\), bromide \(Br^-\), and iodide \(I^-\) salts are soluble, except when paired with lead \(Pb^{2+}\), silver \(Ag^+\), or mercury \(Hg_2^{2+}\).
• Most sulfate \(SO_4^{2-}\) salts are soluble, with the exception of salts formed with calcium \(Ca^{2+}\), strontium \(Sr^{2+}\), barium \(Ba^{2+}\), and lead \(Pb^{2+}\).
• Carbonates \(CO_3^{2-}\), phosphates \(PO_4^{3-}\), chromates \(CrO_4^{2-}\), sulfides \(S^{2-}\), and hydroxides \(OH^-\) are generally insoluble, with exceptions for compounds involving alkali metals and \(NH_4^+\).

Applying these rules to our examples, we find that calcium carbonate \(CaCO_3\) is insoluble and therefore precipitates when calcium nitrate and lithium carbonate are mixed, as does lead(II) chloride \(PbCl_2\) when lithium chloride and lead(II) nitrate are mixed. The net ionic equations for these reactions are: \[Ca^{2+}(aq) + CO_3^{2-}(aq) \rightarrow CaCO_3(s)\] and \[Pb^{2+}(aq) + 2Cl^-(aq) \rightarrow PbCl_2(s)\], respectively.

Solubility rules are not absolute, but they provide a valuable first step in predicting the products of a chemical reaction and determining whether a precipitate will form.