Question

Another step in the qualitative analysis of cations (see problem 28 ) involves precipitating some of the metal ions as the insoluble sulfides (followed by subsequent treatment of the mixed sulfide precipitate to separate the individual ions). Write balanced net ionic equations for the reactions of \(\mathrm{Co}(\mathrm{II}), \mathrm{Co}(\mathrm{III})\) \(\mathrm{Fe}(\mathrm{II}),\) and \(\mathrm{Fe}(\mathrm{III})\) ions with sulfide ion, \(\mathrm{S}^{2-}\).

Short answer

The balanced net ionic equations for the reactions between cobalt(II), cobalt(III), iron(II), and iron(III) ions with sulfide ions are as follows: 1. \(\mathrm{Co^{2+}(aq) + S^{2-}(aq) \rightarrow CoS(s)}\) 2. \(\mathrm{2Co^{3+}(aq) + 3S^{2-}(aq) \rightarrow Co_2S_3(s)}\) 3. \(\mathrm{Fe^{2+}(aq) + S^{2-}(aq) \rightarrow FeS(s)}\) 4. \(\mathrm{2Fe^{3+}(aq) + 3S^{2-}(aq) \rightarrow Fe_2S_3(s)}\)

Step-by-step solution

Reaction of Cobalt(II) Ion with Sulfide Ion

To write the balanced net ionic equation, we start by combining the cobalt(II) ion (\(\mathrm{Co^{2+}}\)) with the sulfide ion (\(\mathrm{S^{2-}}\)) to form the precipitated cobalt(II) sulfide (\(\mathrm{CoS}\)). The balanced net ionic equation for this reaction is: \[ \mathrm{Co^{2+}(aq) + S^{2-}(aq) \rightarrow CoS(s)} \]

Reaction of Cobalt(III) Ion with Sulfide Ion

Next, we need to find the balanced net ionic equation for the reaction between the cobalt(III) ion (\(\mathrm{Co^{3+}}\)) and the sulfide ion (\(\mathrm{S^{2-}}\)). This will result in the formation of cobalt(III) sulfide (\(\mathrm{Co_2S_3}\)). The balanced net ionic equation for this reaction is: \[ \mathrm{2Co^{3+}(aq) + 3S^{2-}(aq) \rightarrow Co_2S_3(s)} \]

Reaction of Iron(II) Ion with Sulfide Ion

The third reaction we need to balance is the reaction between the iron(II) ion (\(\mathrm{Fe^{2+}}\)) and the sulfide ion (\(\mathrm{S^{2-}}\)) which results in the formation of iron(II) sulfide (\(\mathrm{FeS}\)). The balanced net ionic equation for this reaction is: \[ \mathrm{Fe^{2+}(aq) + S^{2-}(aq) \rightarrow FeS(s)} \]

Reaction of Iron(III) Ion with Sulfide Ion

Finally, we need to find the balanced net ionic equation for the reaction between the iron(III) ion (\(\mathrm{Fe^{3+}}\)) and the sulfide ion (\(\mathrm{S^{2-}}\)). The product of this reaction is iron(III) sulfide (\(\mathrm{Fe_2S_3}\)). The balanced net ionic equation for this reaction is: \[ \mathrm{2Fe^{3+}(aq) + 3S^{2-}(aq) \rightarrow Fe_2S_3(s)} \] In summary, we have used the general form of precipitation reactions to write balanced net ionic equations for the reactions between cobalt(II) and cobalt(III) ions as well as iron(II) and iron(III) ions with sulfide ions.

Key concepts

Qualitative Analysis of Cations

Qualitative analysis of cations is a laboratory process used to identify the presence of metal ions in a sample. This typically involves a series of tests that precipitate various cations under specific conditions. These procedures often result in the formation of distinctive insoluble salts, allowing chemists to deduce the identity of the cations present.

A common qualitative test for cations involves the use of reagents that will form precipitates with specific metal ions. For example, when sulfide ions are introduced to a solution containing metal cations, such as cobalt or iron, the sulfide will combine with the metal to form an insoluble compound. This is advantageous because it can separate cations that might otherwise be difficult to identify amidst other ions in a complex mixture.

Precipitation Reactions

Precipitation reactions are a type of chemical reaction where two soluble substances react to form an insoluble product, known as a precipitate. This process can be represented by a net ionic equation, which shows only the ions that directly participate in the reaction.

The net ionic equation excludes spectator ions that do not undergo a change and remain in solution. Precipitation is important in qualitative analysis as it helps isolate and identify different ions. When a metal cation reacts with a sulfide anion in solution, a precipitate of the corresponding metal sulfide is formed, which is a key step in the analysis of metal ions.

Chemical Equation Balancing

Balancing chemical equations is a fundamental skill in chemistry that ensures the law of conservation of mass is obeyed. Each element must have the same number of atoms on both sides of the equation. For ionic equations, this also includes balancing the charge.

Balanced reactions are crucial for calculations in stoichiometry and can predict the amounts of reactants and products. In the step-by-step solutions provided, balancing was done by matching the number of cobalt and iron atoms, as well as the charges of the ions, to generate balanced net ionic equations for the precipitation of each metal sulfide.

Sulfide Precipitation

Sulfide precipitation is a chemical process used in qualitative analysis to form solid sulfides from metal cations and sulfide anions. This is often utilized for the separation and identification of heavy metals. The insolubility of most metal sulfides in aqueous solution is leveraged to achieve this.

Ions like \( \mathrm{Co^{2+}} \), \( \mathrm{Co^{3+}} \) (forming \( \mathrm{CoS} \) and \( \mathrm{Co_2S_3} \), respectively), and iron ions like \( \mathrm{Fe^{2+}} \), \( \mathrm{Fe^{3+}} \) (forming \( \mathrm{FeS} \) and \( \mathrm{Fe_2S_3} \) respectively) will typically form sulfide precipitates. The balancing of the net ionic equations in this context involves combining the appropriate ratios of metal cations to sulfide anions to produce the solid metal sulfides.