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 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

#tag_title#b) Reaction of the green complex with water and AgNO₃# #tag_content#The balanced chemical equations for the reaction of the green complex with water and AgNO₃ are: 1. $\left[\mathrm{Cr}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right] \mathrm{Cl} + 4 \mathrm{H}_{2} \mathrm{O} \rightarrow \left[\mathrm{Cr}(\mathrm{en})_{2} (\mathrm{OH})_{4}\right] + 2 \mathrm{Cl}^{-}$ 2. $\left[\mathrm{Cr}(\mathrm{en})_{2} (\mathrm{OH})_{4}\right] + 3 \mathrm{AgNO}_{3} \rightarrow \left[\mathrm{Cr}(\mathrm{en})_{2} (\mathrm{OH})_{4}\right] \mathrm{[AgCl]_{3}} + 3 \mathrm{NO}_{3}^{-}$ #tag_title#c) Reaction of NaOH with Zn(NO₃)₂ and excess NaOH# #tag_content#The balanced chemical equations for the reaction of NaOH with Zn(NO₃)₂ and the addition of excess NaOH are: 1. Zn(NO₃)₂(aq) + 2NaOH(aq) -> Zn(OH)₂(s) + 2NaNO₃(aq) 2. Zn(OH)₂(s) + 2NaOH(aq) -> Na₂Zn(OH)₄(aq) #tag_title#d) Reaction of Co(NO₃)₂ with concentrated hydrochloric acid# #tag_content#The balanced chemical equation for the reaction of Co(NO₃)₂ with concentrated hydrochloric acid is: Co(NO₃)₂(aq) + 4HCl(aq) -> CoCl₄²⁻(aq) + 2NO₃⁻(aq) + 4H⁺(aq)

Step-by-step solution

a) Dissolution of silver chloride in aqueous ammonia#

The balanced chemical equation for the dissolution of silver chloride in aqueous ammonia is: AgCl(s) + 2NH₃(aq) -> [Ag(NH₃)₂]⁺(aq) + Cl⁻(aq)

Key concepts

Chemical Reaction

Understanding the basics of a chemical reaction is essential for grasping the complexity of chemistry. A chemical reaction is a process where substances, known as reactants, are transformed into different substances, called products. This transformation happens through the breaking of old bonds and the formation of new ones. A reaction can be indicated by changes such as the release or absorption of energy, color change, the formation of a gas, or the appearance of a solid in a liquid, referred to as a precipitate.

For a reaction to be considered balanced, the same number of each type of atom must be present on both the reactant and product sides. Balancing chemical equations is a fundamental skill in chemistry that allows us to predict the quantities of substances consumed and produced in a reaction, which is crucial for experiments and industrial processes.

Precipitation Reaction

A precipitation reaction is a type of chemical reaction that occurs when two soluble salts react in solution to form one or more insoluble products, known as precipitates. This type of reaction is instrumental in determining the presence of various ions in a solution. For example, when a silver nitrate solution is added to a solution containing chloride ions, a white precipitate of silver chloride, AgCl, forms. The overall equation for the reaction is often written as:
\[ \text{Ag}^{+}(aq) + \text{Cl}^{-}(aq) \rightarrow \text{AgCl}(s) \]
Through precipitation reactions, it is possible to isolate and remove ions from a solution, which is a critical process in water treatment and other chemical applications.

Complex Ion

Formation of Complex Ions

A complex ion is a species formed from a metal ion attached to one or more ligands by coordinate bonds. Ligands are molecules or ions that are able to donate a pair of electrons to the metal ion. A classic example is the silver-ammonia complex, \([Ag(NH_3)_2]^+\), formed when silver ions in solution react with ammonia. The ammonia molecules, acting as ligands, donate their lone pair of electrons to the silver ion.

Characteristics of Complex Ions

These ions exhibit unique properties such as specific colors, which makes them useful in identifying metal ions in solution. Furthermore, the formation of complex ions can alter the solubility of certain substances. In our exercise, the silver complex ion formed increases the solubility of silver chloride in water, a phenomenon exploited in the field of analytical chemistry.

Stoichiometry

Stoichiometry is the area of chemistry that pertains to the quantitative relationships between the amounts of reactants and products involved in a chemical reaction. It is based on the conservation of mass and the concept of atoms and molecules reacting in fixed ratios outlined by the balanced chemical equations. Understanding stoichiometry is essential for calculating how much of each substance is needed or produced in a reaction.

For instance, when using stoichiometry to analyze the dissolution of silver chloride in ammonia, one can calculate precisely how much ammonia is needed to dissolve a given amount of silver chloride, which directly relates to the coefficients in the balanced chemical equation. Mastery of stoichiometry is pivotal for anyone aspiring to conduct experiments, work in a lab, or understand the workings of reactions in the real world.