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 $[\mathrm{Cr}(\mathrm{en}{)}_2{\mathrm{Cl}}_2]\mathrm{Cl},$ 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 AgCl per mole of Cr present. (Write two chemical equations.) (c) When an NaOH solution is added to a solution of $\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2,$ 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

The balanced chemical equations for the given observations are: (a) ${\mathrm{AgCl}}_{(\mathrm{s})}+2{\mathrm{NH}}_{3(\mathrm{aq})}\to [\mathrm{Ag}({\mathrm{NH}}_3{)}_2{]}_{(\mathrm{aq})}^{+}+{\mathrm{Cl}}_{(\mathrm{aq})}^{-}$ (b.1) $[\mathrm{Cr}(\mathrm{en}{)}_2{\mathrm{Cl}}_2]{\mathrm{Cl}}_{(\mathrm{aq})}+{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}\to [\mathrm{Cr}(\mathrm{en}{)}_2\mathrm{Cl}(\mathrm{OH})]{\mathrm{Cl}}_{(\mathrm{aq})}+{\mathrm{HCl}}_{(\mathrm{aq})}$ (b.2) $3{\mathrm{AgNO}}_{3(\mathrm{aq})}+[\mathrm{Cr}(\mathrm{en}{)}_2\mathrm{Cl}(\mathrm{OH})]{\mathrm{Cl}}_{(\mathrm{aq})}\to 3{\mathrm{AgCl}}_{(\mathrm{s})}+[\mathrm{Cr}(\mathrm{en}{)}_2\mathrm{Cl}(\mathrm{OH})]({\mathrm{NO}}_3{)}_{3(\mathrm{aq})}$ (c.1) $\mathrm{Zn}({\mathrm{NO}}_3{)}_{2(\mathrm{aq})}+2{\mathrm{NaOH}}_{(\mathrm{aq})}\to \mathrm{Zn}(\mathrm{OH}{)}_{2(\mathrm{s})}+2{\mathrm{NaNO}}_{3(\mathrm{aq})}$ (c.2) $\mathrm{Zn}(\mathrm{OH}{)}_{2(\mathrm{s})}+2{\mathrm{NaOH}}_{(\mathrm{aq})}\to {\mathrm{Na}}_2[\mathrm{Zn}(\mathrm{OH}{)}_4{]}_{(\mathrm{aq})}$ (d) $\text{Double subscripts: use braces to clarify}$

Step-by-step solution

(a) Dissolution of Silver Chloride in Aqueous Ammonia

The solid silver chloride (AgCl) reacts with aqueous ammonia (NH₃) to form a complex, which is soluble in water. The balanced chemical equation for this reaction is: $${\mathrm{AgCl}}_{(\mathrm{s})}+2{\mathrm{NH}}_{3(\mathrm{aq})}\to [\mathrm{Ag}({\mathrm{NH}}_3{)}_2{]}_{(\mathrm{aq})}^{+}+{\mathrm{Cl}}_{(\mathrm{aq})}^{-}$$

(b.1) Conversion of Green Complex to Brown-Orange Complex

The green complex, $[\mathrm{Cr}(\mathrm{en}{)}_2{\mathrm{Cl}}_2]\mathrm{Cl}$, undergoes hydrolysis when treated with water, resulting in the formation of a brown-orange complex. The balanced chemical equation for this reaction is: $$[\mathrm{Cr}(\mathrm{en}{)}_2{\mathrm{Cl}}_2]{\mathrm{Cl}}_{(\mathrm{aq})}+{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}\to [\mathrm{Cr}(\mathrm{en}{)}_2\mathrm{Cl}(\mathrm{OH})]{\mathrm{Cl}}_{(\mathrm{aq})}+{\mathrm{HCl}}_{(\mathrm{aq})}$$

(b.2) Reaction with AgNO₃ forming Precipitate of AgCl

A brown-orange complex, which is the reaction product of part (b.1), reacts with ${\mathrm{AgNO}}_3$ to precipitate 3 moles of AgCl per mole of Cr present. The balanced chemical equation for this reaction is: $$3{\mathrm{AgNO}}_{3(\mathrm{aq})}+[\mathrm{Cr}(\mathrm{en}{)}_2\mathrm{Cl}(\mathrm{OH})]{\mathrm{Cl}}_{(\mathrm{aq})}\to 3{\mathrm{AgCl}}_{(\mathrm{s})}+[\mathrm{Cr}(\mathrm{en}{)}_2\mathrm{Cl}(\mathrm{OH})]({\mathrm{NO}}_3{)}_{3(\mathrm{aq})}$$

(c.1) Formation of Precipitate with NaOH Solution

When a NaOH solution is added to a solution of ${\mathrm{Zn}(\mathrm{NO}}_3{)}_2$, a precipitate forms due to the reaction between the two compounds. The balanced chemical equation for this reaction is: $$\mathrm{Zn}({\mathrm{NO}}_3{)}_{2(\mathrm{aq})}+2{\mathrm{NaOH}}_{(\mathrm{aq})}\to \mathrm{Zn}(\mathrm{OH}{)}_{2(\mathrm{s})}+2{\mathrm{NaNO}}_{3(\mathrm{aq})}$$

(c.2) Dissolving the Precipitate with Excess NaOH

The addition of excess NaOH solution causes the precipitate formed in part (c.1) to dissolve. The balanced chemical equation for this reaction is: $$\mathrm{Zn}(\mathrm{OH}{)}_{2(\mathrm{s})}+2{\mathrm{NaOH}}_{(\mathrm{aq})}\to {\mathrm{Na}}_2[\mathrm{Zn}(\mathrm{OH}{)}_4{]}_{(\mathrm{aq})}$$

(d) Color Change with Addition of Concentrated Hydrochloric Acid

When concentrated hydrochloric acid is added to a pink solution of ${\mathrm{Co}(\mathrm{NO}}_3{)}_2$, the color changes to deep blue. This indicates a formation of complex ions with the ligand exchange. The balanced chemical equation for this reaction is: $$\text{Double subscripts: use braces to clarify}$$

Key concepts

Understanding Complex Ion Reactions

In chemistry, complex ion reactions involve the formation of coordination compounds where transition metal ions are surrounded by molecules or ions, known as ligands. These ligands donate electron pairs to the metal ion, resulting in a stable arrangement known as a coordination complex. In step 1, when solid silver chloride dissolves in aqueous ammonia, a complex ion is formed where ammonia acts as the ligand bonding to the silver ion.

A classic example is when silver chloride, which is poorly soluble in water, becomes soluble when ammonia is added because it forms the complex $$[\mathrm{Ag}({\mathrm{NH}}_3{)}_2{]}_{(\mathrm{aq})}^{+}+{\mathrm{Cl}}_{(\mathrm{aq})}^{-}$$. Complex ion reactions are pivotal in areas such as analytical chemistry where they are used to identify the presence of certain ions in a solution.

Ligand Exchange Reactions

A ligand exchange reaction is where one ligand in a complex ion is replaced by another ligand. Such reactions can be observed through color changes, as the new ligand may alter the electronic transitions that are possible for the metal ion at the center of the complex. This can be seen in step 6, where a change in color of a cobalt solution upon the addition of hydrochloric acid signifies the exchange of nitrate ions for chloride ions, forming a new complex. The reaction $$\text{Double subscripts: use braces to clarify}$$ illustrates this concept. The deep blue color is indicative of the $$[{\mathrm{CoCl}}_6{]}^{4-}$$ complex being formed. This type of reaction is essential in biochemical processes such as oxygen transport in blood where the ligands bound to the iron in hemoglobin are exchanged with oxygen molecules.

Precipitation Reactions

Precipitation reactions occur when two soluble salts react in solution to form one or more insoluble products, known as precipitates. When a solution of sodium hydroxide is mixed with a zinc nitrate solution, as seen in step 4, a precipitate of zinc hydroxide is created according to the equation $$\mathrm{Zn}({\mathrm{NO}}_3{)}_{2(\mathrm{aq})}+2{\mathrm{NaOH}}_{(\mathrm{aq})}\to \mathrm{Zn}(\mathrm{OH}{)}_{2(\mathrm{s})}+2{\mathrm{NaNO}}_{3(\mathrm{aq})}$$. Furthermore, in step 3, the reaction product of a green complex with silver nitrate leads to the formation of a precipitate of silver chloride. In the equation $$3{\mathrm{AgNO}}_{3(\mathrm{aq})}+[\mathrm{Cr}(\mathrm{en}{)}_2\mathrm{Cl}(\mathrm{OH})]{\mathrm{Cl}}_{(\mathrm{aq})}\to 3{\mathrm{AgCl}}_{(\mathrm{s})}+[\mathrm{Cr}(\mathrm{en}{)}_2\mathrm{Cl}(\mathrm{OH})]({\mathrm{NO}}_3{)}_{3(\mathrm{aq})}$$, for every mole of the chromium complex, three moles of silver chloride precipitate out of solution. Precipitation reactions are vital for understanding processes such as water purification and mineral formation.

Hydrolysis Reactions

The term hydrolysis reactions refers to chemical reactions that involve the breaking of a bond in a molecule using water. This type of reaction plays a crucial role in the transformation of chemicals in different environments. As depicted in step 2, the conversion of a green chromium complex to a brown-orange one involves the hydrolysis of a chloride ligand, producing $$[\mathrm{Cr}(\mathrm{en}{)}_2\mathrm{Cl}(\mathrm{OH})]{\mathrm{Cl}}_{(\mathrm{aq})}$$ and hydrochloric acid. The equation is $$[\mathrm{Cr}(\mathrm{en}{)}_2{\mathrm{Cl}}_2]{\mathrm{Cl}}_{(\mathrm{aq})}+{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}\to [\mathrm{Cr}(\mathrm{en}{)}_2\mathrm{Cl}(\mathrm{OH})]{\mathrm{Cl}}_{(\mathrm{aq})}+{\mathrm{HCl}}_{(\mathrm{aq})}$$. Hydrolysis reactions are particularly important because they contribute to the stability and reactivity of many complexes and play a role in shaping the bioavailability of nutrients and metals in natural waters.