Question

For each of the following slightly soluble salts, write the net ionic equation, if any, for reaction with acid: (a) MnS, (b) \(\mathrm{PbF}_{2}\), (c) \(\mathrm{AuCl}_{3}\) (d) \(\mathrm{Hg}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\) (e) CuBr.

Short answer

(a) MnS + 2HCl: \[S^{2-}(aq) + 2H^+(aq) \rightarrow H_2S(g)\] (b) \(\mathrm{PbF}_{2}\) + 2HCl: \[2F^-(aq) + 2H^+(aq) \rightarrow 2HF(g)\] (c) \(\mathrm{AuCl}_{3}\) + 3HCl: No net ionic equation, as there is no reaction with acid. (d) \(\mathrm{Hg}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\) + 4HCl: \[Hg_{2}^{2+}(aq) + 2C_{2}O_{4}^{2-}(aq) + 4H^+(aq) \rightarrow Hg_{2}Cl_{2}(s) + 4HCO2H\] (e) CuBr + HCl: \[Br^-(aq) + H^+(aq) \rightarrow HBr(g)\]

Step-by-step solution

Write the dissociation equations for the salts

For each salt, we need to write the dissociation equation, which shows how the salt breaks down into its component ions when mixed with water. (a) MnS: \[MnS(s) \rightleftharpoons Mn^{2+}(aq) + S^{2-}(aq)\] (b) \(\mathrm{PbF}_{2}\): \[\mathrm{PbF}_{2}(s) \rightleftharpoons Pb^{2+}(aq) + 2F^-(aq)\] (c) \(\mathrm{AuCl}_{3}\): \[\mathrm{AuCl}_{3}(s) \rightleftharpoons Au^{3+}(aq) + 3Cl^-(aq)\] (d) \(\mathrm{Hg}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\): \[\mathrm{Hg}_{2} \mathrm{C}_{2} \mathrm{O}_{4}(s) \rightleftharpoons Hg_{2}^{2+}(aq) + 2C_{2}O_{4}^{2-}(aq)\] (e) CuBr: \[CuBr(s) \rightleftharpoons Cu^{+}(aq) + Br^-(aq)\]

Determine the reaction between the dissociated ions and the acid

In this step, we will write the reaction between the dissociated ions and the acid, which is usually a strong acid like HCl or H2SO4. This will lead to the formation of a new compound or compounds. (a) MnS + 2HCl: \[Mn^{2+}(aq) + S^{2-}(aq) + 2H^+(aq) + 2Cl^-(aq) \rightarrow Mn^{2+}(aq) + 2Cl^-(aq) + H_2S(g)\] (b) \(\mathrm{PbF}_{2}\) + 2HCl: \[Pb^{2+}(aq) + 2F^-(aq) + 2H^+(aq) + 2Cl^-(aq) \rightarrow Pb^{2+}(aq) + 2Cl^-(aq) + 2HF(g)\] (c) \(\mathrm{AuCl}_{3}\) + 3HCl: \[Au^{3+}(aq) + 3Cl^-(aq) + 3H^+(aq) + 3Cl^-(aq) \rightarrow AuCl_{3}(s) + 3H^+(aq)\] (d) \(\mathrm{Hg}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\) + 4HCl: \[Hg_{2}^{2+}(aq) + 2C_{2}O_{4}^{2-}(aq) + 4H^+(aq) + 4Cl^-(aq) \rightarrow Hg_{2}Cl_{2}(s) + 4HCO2H\] (e) CuBr + HCl: \[Cu^{+}(aq) + Br^-(aq) + H^+(aq) + Cl^-(aq) \rightarrow Cu^{+}(aq) + Cl^-(aq) + HBr(g)\]

Write the net ionic equation

Now that we have the complete reactions, we can eliminate the spectator ions and write the net ionic equations for the reactions. (a) MnS + 2HCl: \[Mn^{2+}(aq) + S^{2-}(aq) + 2H^+(aq) \rightarrow Mn^{2+}(aq) + H_2S(g)\] (b) \(\mathrm{PbF}_{2}\) + 2HCl: \[2F^-(aq) + 2H^+(aq) \rightarrow 2HF(g)\] (c) \(\mathrm{AuCl}_{3}\) + 3HCl: \[Au^{3+}(aq) + 3H^+(aq) \rightarrow AuCl_{3}(s) + 3H^+(aq)\] (d) \(\mathrm{Hg}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\) + 4HCl: \[Hg_{2}^{2+}(aq) + 2C_{2}O_{4}^{2-}(aq) + 4H^+(aq) \rightarrow Hg_{2}Cl_{2}(s) + 4HCO2H\] (e) CuBr + HCl: \[Br^-(aq) + H^+(aq) \rightarrow HBr(g)\] These are the net ionic equations for the given slightly soluble salts reacting with acid.

Key concepts

Slightly Soluble Salts

Slighly soluble salts are salts that have a limited solubility in water. They do not dissolve completely, which means that not all of them dissociate into ions in a solution. Therefore, some of the salt remains in solid form. Examples of such salts include MnS and PbF extsubscript{2}.
When these salts are placed in water, only a small fraction dissolves into their respective ions. This creates a saturated solution where an equilibrium is established between the undissolved salt and the dissolved ions. The position of this equilibrium can shift when conditions change, such as temperature or the presence of other chemicals.
Understanding slightly soluble salts is important because their behavior affects many chemical reactions, especially those involving acids. This can be seen in the context of acid reactions, where these salts may further react depending on their solubility and the nature of the acid involved.

Acid Reactions

Acid reactions play a crucial role in transforming slightly soluble salts in solution. When an acid is introduced, it can react with the anions of the dissolved salt to form gases or precipitates. This occurs because acids contribute hydrogen ions, which can combine with the anions present in the solution.
For instance, when HCl is added to MnS, the hydrogen ions react with the sulfide ions to form hydrogen sulfide gas (H extsubscript{2}S). The low solubility of some gases, like H extsubscript{2}S, drives the reaction forward by removing ions from the solution, resulting in more salt dissolving.
These reactions are important for explaining the behavior of certain salts in acidic conditions, providing insights into chemical equilibria and the strength of the acids involved. In a classroom setting, understanding this can aid in predicting the outcomes of acid-base reactions involving slightly soluble salts.

Dissociation Equations

Dissociation equations illustrate how a compound breaks into its ions in a solution. They are essential in understanding the initial steps of any chemical reaction process involving salts. For slightly soluble salts, the dissociation is incomplete, as the equilibrium between the solid form and ions in the solution is established.
The purpose of a dissociation equation is to identify the ions available for reactions like those involving acids. For MnS, the dissociation would be represented as:

• \[ MnS(s) \rightleftharpoons Mn^{2+}(aq) + S^{2-}(aq) \]

The double arrow signifies a dynamic equilibrium rather than a full dissociation, which is typical for slightly soluble compounds.
Dissociation equations help predict the behavior of salts in new conditions by highlighting present ions. They set the stage for understanding subsequent reactions, such as those with acids, by detailing the ions available for interaction.

Spectator Ions Elimination

In chemical equations, spectator ions are ions that do not participate directly in the chemical reaction and remain unchanged on both sides of the equation. Eliminating these ions helps simplify equations to better focus on the actual chemical changes occurring.
Consider the reaction of MnS with HCl; the main reactive ions are sulfide and hydrogen ions that will form hydrogen sulfide gas. The manganese and chloride ions do not change, making them spectator ions.
By eliminating spectator ions from the full ionic equation, we arrive at the net ionic equation. This is crucial for understanding the heart of the reactions. For example, in the reaction involving MnS with HCl, the net ionic equation becomes:

• \[ Mn^{2+}(aq) + S^{2-}(aq) + 2H^{+}(aq) \rightarrow H_2S(g) \]

Identifying and removing spectator ions streamlines problem-solving and enhances comprehension of the fundamental processes during reactions, enabling clearer insight and predictions of the chemical behavior.