Problem 80 - Chemistry

Question

The solubility of copper(II) hydroxide in water can be increased by adding either the base \(\mathrm{NH}_{3}\) or the acid \(\mathrm{HNO}_{3}\) . Explain. Would added \(\mathrm{NH}_{3}\) or \(\mathrm{HNO}_{3}\) have the same effect on the solubility of silver acetate or silver chloride? Explain.

Step-by-Step Solution

Answer

Adding \(\mathrm{NH}_{3}\) increases the solubility of copper(II) hydroxide, silver acetate, and silver chloride by forming soluble complexes with their respective metal ions, while adding \(\mathrm{HNO}_{3}\) only increases the solubility of copper(II) hydroxide by reacting with \(\mathrm{OH^{-}}\) ions. The solubility of silver acetate and silver chloride remains unaffected by the addition of an acid like \(\mathrm{HNO}_{3}\) since their anions are not basic.

Step 1: 1. Solubility of Copper(II) Hydroxide and Effect of NH₃ and HNO₃

Copper(II) hydroxide (\(\mathrm{Cu(OH)_{2}}\)) is only slightly soluble in water and the equilibrium can be represented as: \[\mathrm{Cu(OH)_{2}(s) \rightleftharpoons Cu^{2+}(aq) + 2\,OH^{-}(aq)}\] When we add \(\mathrm{NH}_{3}\) (ammonia, a base) to the solution, it reacts with \(\mathrm{Cu^{2+}}\) ions to form the soluble copper-ammonia complex ion, \(\mathrm{[Cu(NH_{3})_{4}]^{2+}}\). The reaction is: \[\mathrm{Cu^{2+}(aq) + 4\,NH_{3}(aq) \rightleftharpoons [Cu(NH_{3})_{4}]^{2+}(aq)}\] This reaction reduces the concentration of \(\mathrm{Cu^{2+}}\) ions in the solution, causing the equilibrium of \(\mathrm{Cu(OH)_{2}}\) dissolution to shift to the right, and making it more soluble. Now, when we add \(\mathrm{HNO}_{3}\) (nitric acid) to the solution, it reacts with the \(\mathrm{OH^{-}}\) ions as follows: \[\mathrm{H^{+}(aq) + OH^{-}(aq) \rightleftharpoons H_{2}O(l)}\] In this case, the addition of acid neutralizes the \(\mathrm{OH^{-}}\) ions and decreases their concentration, causing the \(\mathrm{Cu(OH)_{2}}\) solubility equilibrium to shift to the right.

Step 2: 2. Comparing the Solubility of Silver Acetate and Silver Chloride with Copper(II) Hydroxide

Now, let's consider the solubility of silver acetate (\(\mathrm{AgC_{2}H_{3}O_{2}}\)) and silver chloride (\(\mathrm{AgCl}\)) in water, represented by the following equilibria: \[\mathrm{AgC_{2}H_{3}O_{2}(s) \rightleftharpoons Ag^{+}(aq) + C_{2}H_{3}O_{2}^{-}(aq)}\] \[\mathrm{AgCl(s) \rightleftharpoons Ag^{+}(aq) + Cl^{-}(aq)}\] When we add ammonia to these equilibria, \(\mathrm{Ag^{+}}\) ions form soluble complexes – \(\mathrm{[Ag(NH_{3})_{2}]^{+}}\) – similar to the copper case, increasing the solubility of both silver acetate and silver chloride. \[\mathrm{Ag^{+}(aq) + 2\,NH_{3}(aq) \rightleftharpoons [Ag(NH_{3})_{2}]^{+}(aq)}\] However, if we add an acid like \(\mathrm{HNO}_{3}\) to these solutions, there will be no significant reaction with the anions of silver acetate (\(\mathrm{C_{2}H_{3}O_{2}^{-}}\)) and silver chloride (\(\mathrm{Cl^{-}}\)), as they are not basic anions. Hence, the solubility of silver acetate and silver chloride will not change significantly when an acid is added. In conclusion, adding ammonia increases the solubility of all three compounds, while adding nitric acid only has a significant effect on the solubility of copper(II) hydroxide in water and not on the solubility of silver acetate or silver chloride.

Key Concepts

Complex Ion Formation

In the world of chemistry, complex ion formation plays a significant role in increasing the solubility of certain compounds. When certain ions, like copper ions (Cu^{2+}), interact with ligands like ammonia (NH_{3}), they can form complex ions, such as [Cu(NH_{3})_{4}]^{2+}. This formation process effectively decreases the concentration of free copper ions in the solution.
By binding with Cu^{2+} ions, complex ion formation promotes the dissolution of compounds like copper(II) hydroxide, (Cu(OH)_{2}).
The reaction between Cu^{2+} ions and ammonia can be written as:\[Cu^{2+}(aq) + 4\,NH_{3}(aq) \rightleftharpoons [Cu(NH_{3})_{4}]^{2+}(aq)\]

• This complexing process causes a shift in the solubility equilibrium, enhancing solubility.
• Similar complex ions can form with silver ions, (Ag^{+}), when ammonia is added, making compounds like silver acetate and silver chloride more soluble.

Equilibrium Shift

In chemistry, the concept of equilibrium shift refers to the change in the position of equilibrium in a chemical reaction when external conditions are modified. The addition of ligands or acids can cause this shift.
For example, the solubility equilibrium of copper(II) hydroxide in water can be represented by:\[Cu(OH)_{2}(s) \rightleftharpoons Cu^{2+}(aq) + 2\,OH^{-}(aq)\]When ammonia is added, it reduces the concentration of Cu^{2+} ions via complex ion formation, shifting the equilibrium to the right, enhancing the dissolution of Cu(OH)_{2}.
Conversely, the addition of an acid like HNO_{3} reduces OH^{-} ion concentration by forming water:\[H^{+}(aq) + OH^{-}(aq) \rightleftharpoons H_{2}O(l)\]This reduction causes the equilibrium to shift to the right, further dissolving Cu(OH)_{2}.
These equilibrium shifts help explain why changes in solubility occur under different conditions.

Acid-Base Reaction

Acid-base reactions are fundamental to understanding solubility changes within different compounds. An acid-base reaction involves the transfer of a proton (H^{+}) from an acid to a base.
When nitric acid (HNO_{3}) is added to a Cu(OH)_{2} solution, the acid reacts with the OH^{-} (hydroxide) ions:\[H^{+}(aq) + OH^{-}(aq) \rightleftharpoons H_{2}O(l)\]This neutralization reaction decreases the concentration of OH^{-} ions, prompting the solubility equilibrium of Cu(OH)_{2} to shift towards dissolving more solid into ions.
However, for silver acetate and silver chloride, the presence of HNO_{3} does not significantly impact solubility.

• The reason is that their anions, (C_{2}H_{3}O_{2}^{-}) and Cl^{-}, do not engage substantially in acid-base reactions with H^{+} because they are not basic.
• This highlights how specific interactions in an acid-base reaction differ based on the substances involved.

Solubility Rules

Solubility rules are a set of guidelines that predict the solubility of ionic compounds in water. These rules help determine whether a specific ion combination forms a soluble or insoluble compound.
For example, compounds containing alkali metal ions and ammonium (NH_{4}^{+} ) are usually soluble.
Understanding these rules, paired with the knowledge of complex ion formation and equilibrium shifts, can explain the solubility behaviors observed in chemical reactions like those discussed here.

• Ammonia can complex with metal ions, thus increasing solubility.
• Acids can increase solubility by neutralizing bases, as seen with copper(II) hydroxide.

In practice, the solubility rules guide predicting whether a salt will dissolve in water. By applying them alongside equilibrium and complex ion concepts, one achieves a comprehensive understanding of solubility phenomena.