Question

Write balanced chemical equations to represent the following observations. (In some instances the complex involved has been discussed previously in the text.) (a) Solid silver chloride dissolves in an excess of aqueous ammonia. (b) The green complex \(\left[\mathrm{Cr}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right] \mathrm{Cl}\), on treatment with water over a long time, converts to a brown-orange complex. Reaction of \(\mathrm{AgNO}_{3}\) with a solution of the product precipitates \(3 \mathrm{~mol}\) of \(\mathrm{AgCl}\) per mole of Cr present. (Write two chemical equations.) (c) When an \(\mathrm{NaOH}\) solution is added to a solution of \(\mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}, \mathrm{a}\) precipitate forms. Addition of excess \(\mathrm{NaOH}\) solution causes the precipitate to dissolve. (Write two chemical equations.) (d) A pink solution of \(\mathrm{Co}\left(\mathrm{NO}_{3}\right)_{2}\) turns deep blue on addition of concentrated hydrochloric acid.

Short answer

a) AgCl(s) + 2 NH₃(aq) → [Ag(NH₃)₂]⁺(aq) + Cl⁻(aq) b) 1. [Cr(en)₂Cl₂]Cl(s) + H₂O(l) → [Cr(en)₂Cl(OH)]Cl (Orange-brown complex) 2. [Cr(en)₂Cl(OH)]Cl + 3 AgNO₃ → Cr(en)₂(OH)Cl + 3 AgCl + 3 NO₃⁻ c) 1. Zn(NO₃)₂(aq) + 2 NaOH(aq) → Zn(OH)₂(s) + 2 NaNO₃(aq) 2. Zn(OH)₂(s) + 2 NaOH(aq) → Na₂[Zn(OH)₄](aq) d) Co(NO₃)₂(aq) + 6 HCl(aq) → [CoCl₆]⁴⁻(aq) + 2 NO₃⁻(aq) + 6 H⁺(aq)

Step-by-step solution

a) Silver chloride dissolving in aqueous ammonia

A solid silver chloride (AgCl) is dissolving in an excess of aqueous ammonia (NH₃). The silver chloride ionizes to form a complex ion with ammonia, known as diamminesilver(I) complex ion, [Ag(NH₃)₂]⁺. Ammonia acts as a ligand, forming a complex ion with the silver ion. The balanced chemical equation is as follows: AgCl(s) + 2 NH₃(aq) → [Ag(NH₃)₂]⁺(aq) + Cl⁻(aq)

b) Conversion of green complex to a brown-orange complex

The green complex [Cr(en)₂Cl₂]Cl is converted to a brown-orange complex by reacting with water over time. When treated with AgNO₃, the product precipitates 3 moles of AgCl per mole of Cr present. The first and second chemical equations are given below: 1. [Cr(en)₂Cl₂]Cl(s) + H₂O(l) → [Cr(en)₂Cl(OH)]Cl (Orange-brown complex) 2. [Cr(en)₂Cl(OH)]Cl + 3 AgNO₃ → Cr(en)₂(OH)Cl + 3 AgCl + 3 NO₃⁻

c) Precipitation of a product and then dissolution with excess NaOH

A sodium hydroxide (NaOH) solution is added to a solution of zinc nitrate, Zn(NO₃)₂. This reaction produces a precipitate, which dissolves when an excess of NaOH solution is added. The two chemical equations are written below: 1. Zn(NO₃)₂(aq) + 2 NaOH(aq) → Zn(OH)₂(s) + 2 NaNO₃(aq) (Formation of precipitate) 2. Zn(OH)₂(s) + 2 NaOH(aq) → Na₂[Zn(OH)₄](aq) (Dissolution of precipitate)

d) Color change of Co(NO₃)₂ solution on addition of concentrated HCl

A pink solution of cobalt nitrate, Co(NO₃)₂, turns deep blue upon the addition of concentrated hydrochloric acid (HCl). This color change is due to the formation of the hexachlorocobaltate complex ion, [CoCl₆]⁴⁻. The balanced chemical equation for this reaction is as follows: Co(NO₃)₂(aq) + 6 HCl(aq) → [CoCl₆]⁴⁻(aq) + 2 NO₃⁻(aq) + 6 H⁺(aq)

Key concepts

Chemical Complexation

Chemical complexation involves the combination of metal ions with ligands to form complex ions. Ligands are atoms, ions, or molecules that can donate a pair of electrons to form a coordinate bond with a central metal ion. These complex ions often exhibit unique properties such as distinct colors, solubility behaviors, and reactivities that differ from those of their individual components.

In the original exercise, the concept of chemical complexation is observed when solid silver chloride (AgCl) reacts with an aqueous ammonia solution. The ammonia molecules act as ligands, coordinating with the silver ions to form a soluble [Ag(NH₃)₂]⁺ complex. This is a classic example of how ligands can stabilize metal ions in solution, leading to increased solubility of an otherwise insoluble substance.

Precipitation Reactions

A precipitation reaction occurs when two soluble substances react to form an insoluble product, known as a precipitate. Such reactions are essential for understanding solubility rules and predicting which combinations of ionic compounds can lead to the formation of a solid.

In the exercise, we see a precipitation reaction when solutions of zinc nitrate, Zn(NO₃)₂, and sodium hydroxide, NaOH, are mixed, resulting in the formation of zinc hydroxide, Zn(OH)₂. This insoluble substance falls out of solution as a precipitate. Conversely, excess NaOH can result in the re-dissolution of the precipitate, indicating the formation of a soluble complex ion or a change in the precipitate's overall charge. These steps demonstrate the reversibility of precipitation reactions under certain conditions and how a change in the chemical environment can affect solubility.

Ligand Exchange Reactions

Ligand exchange reactions involve the replacement of one or more ligands in a complex ion with other ligands. These reactions can significantly impact the properties of the complex, including its color, shape, and reactivity. This can be seen when additional ligands interact with the central metal ion, leading to a rearrangement of its coordination sphere.

This is exemplified in the given exercise when concentrated hydrochloric acid is added to a pink cobalt nitrate solution, Co(NO₃)₂, resulting in a deep blue complex. The chloride ions from the HCl displace the water molecules from the coordination sphere of cobalt, forming a [CoCl₆]⁴⁻ complex. Ligand exchange reactions like this are often employed in both analytical chemistry to test for the presence of certain metal ions and in various industrial applications.