Question

If aqueous ammonia is added gradually to a solution of copper sulfate, a pale blue precipitate forms that then dissolves to give a deep blue solution. Describe the chemical reactions that take place during these changes.

Short answer

Ammonia reacts with copper sulfate to form a pale blue precipitate of copper(II) hydroxide, which, upon excess ammonia, dissolves to form a deep blue solution containing the tetraamminecopper(II) complex.

Step-by-step solution

Formation of Pale Blue Precipitate

When aqueous ammonia (NH3) is added gradually to a solution of copper sulfate (CuSO4), the ammonia acts as a base and reacts with the water to produce ammonium ions (NH4+) and hydroxide ions (OH-). The hydroxide ions then react with the copper(II) ions (Cu2+) from the copper sulfate to form a pale blue precipitate of copper(II) hydroxide (Cu(OH)2). The chemical equation for this reaction is: CuSO4(aq) + 2 NH3(aq) + 2 H2O(l) -> Cu(OH)2(s) + (NH4)2SO4(aq).

Dissolution of the Precipitate to Form a Deep Blue Solution

Upon continued addition of ammonia, the copper(II) hydroxide precipitate reacts further to form a complex ion, tetraamminecopper(II), [Cu(NH3)4]2+. The ammonia ligands coordinate with the copper ion, and this complex ion gives the deep blue color to the solution. The sulfate anion remains in solution paired with ammonium ions. The chemical equation for this reaction is: Cu(OH)2(s) + 4 NH3(aq) -> [Cu(NH3)4](OH)2(aq) which further dissociates in water to form [Cu(NH3)4]2+(aq) and 2 OH-(aq).

Key concepts

Chemical Reaction Steps

Understanding chemical reactions often involves breaking them down into steps. The transformation of copper sulfate and ammonia into a deep blue solution is an excellent illustration of this.

Initially, when ammonia is mixed into a copper sulfate solution, there are actually two key steps taking place. In the first step, ammonia reacts with water to produce hydroxide ions, which in turn interact with copper(II) ions to create a pale blue precipitate - copper(II) hydroxide. As more ammonia is added, a second step occurs where the precipitate dissolves, forming a complex ion solution that is characteristically deep blue. This sequence showcases the progression from initial reactants through an intermediate phase to the final solution.

Formation of Copper(II) Hydroxide

Let's delve deeper into the initial phase of our reaction. When aqueous ammonia is added to the copper sulfate solution, it raises the pH by generating hydroxide ions, a consequence of ammonia being a weak base. These hydroxide ions waste no time in combining with the copper(II) ions present, leading to the formation of the compound copper(II) hydroxide (\( Cu(OH)_2 \)).

This substance is responsible for the pale blue precipitate that appears first. It's quite interesting to note that the color and solid nature of copper(II) hydroxide help us to visually track the progress of the reaction.

Complex Ion Formation

After copper(II) hydroxide takes the stage, the addition of even more ammonia prompts the entry of a new player: complex ions. Ammonia acts as a ligand in this scenario, donating its lone pair of electrons to bond with the copper(II) ion. This coordination creates a complex ion, which is a central metal ion surrounded by ligands - in this case, ammonia molecules.

Complex ions are fascinating entities in themselves, with their stability, color, and reactivity being a topic of study in coordination chemistry. The understanding of complex ion formation is crucial as it's pivotal to many processes in both industry and biology.

Tetraamminecopper(II) Complex

Within the scope of complex ions, the tetraamminecopper(II) complex deserves a special mention. The name itself reveals its structure: a copper(II) ion surrounded by four ammonia molecules (\( [Cu(NH_3)_4]^{2+} \)). This is the complex ion that gives the resulting solution its deep blue color.

The formation of this complex indicates a strong bond between copper(II) ions and ammonia, stabilized by the donation of electron pairs from ammonia to the metal ion. This type of interaction is key to many intriguing aspects of chemistry, such as the transport of metal ions in biological systems and the fine-tuning of colors in art materials.

Precipitate Dissolution

The journey from a precipitate to a clear solution is emblematic of precipitate dissolution, which occurs when the solid phase is rendered soluble. In our case, the initial precipitate of copper(II) hydroxide undergoes dissolution upon further addition of ammonia, transitioning into the soluble tetraamminecopper(II) complex.

Precipitate dissolution is not merely mixing; it involves specific chemical changes, often resulting in the formation of new compounds. Such changes are integral to various industrial processes, wastewater treatment, and even geological phenomena like the formation of certain types of mineral deposits.