Problem 40 - Chemistry: Principles and Reactions

Question

Write a net ionic equation for the reaction with ammonia by which (a) \(\mathrm{Cu}(\mathrm{OH})_{2}\) dissolves. (b) \(\mathrm{Cd}^{2+}\) forms a complex ion. (c) \(\mathrm{Pb}^{2+}\) forms a precipitate.

Step-by-Step Solution

Answer

Question: Write the net ionic equations for the following reactions involving ammonia: a) Dissolution of copper(II) hydroxide in ammonia. b) Formation of a complex ion with cadmium and ammonia. c) Precipitation of lead(II) ions in the presence of ammonia. Answer: a) The net ionic equation for the dissolution of copper(II) hydroxide in ammonia is: $$\mathrm{Cu(OH)_2 + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+} + 2OH^-}$$ b) The net ionic equation for the formation of a complex ion with cadmium and ammonia is: $$\mathrm{Cd^{2+} + 4NH_3 \rightarrow [Cd(NH_3)_4]^{2+}}$$ c) The net ionic equation for the precipitation of lead(II) ions in the presence of ammonia is: $$\mathrm{Pb^{2+} + 2OH^- \rightarrow Pb(OH)_2}$$

Step 1: Write the chemical equation for the dissolution of copper(II) hydroxide

Copper(II) hydroxide dissolves in ammonia to form a complex ion called tetraamminecopper(II) and water. The chemical equation for this reaction can be written as: $$\mathrm{Cu(OH)_2 + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+} + 2OH^-}$$

Step 2: Write the net ionic equation for the dissolution of copper(II) hydroxide

Since there are no spectator ions in this reaction, the net ionic equation is the same as the chemical equation: $$\mathrm{Cu(OH)_2 + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+} + 2OH^-}$$ #b)

Step 1: Write the chemical equation for the formation of the complex ion with cadmium

Cadmium ions form a complex ion with ammonia to create the tetraamminecadmium(II) complex. The chemical equation for this reaction can be written as: $$\mathrm{Cd^{2+} + 4NH_3 \rightarrow [Cd(NH_3)_4]^{2+}}$$

Step 2: Write the net ionic equation for the formation of the complex ion with cadmium

Since there are no spectator ions in this reaction, the net ionic equation is the same as the chemical equation: $$\mathrm{Cd^{2+} + 4NH_3 \rightarrow [Cd(NH_3)_4]^{2+}}$$ #c)

Step 1: Write the chemical equation for the formation of the precipitate with lead(II) ions

When lead(II) ions react with ammonia, they form a precipitate of lead(II) hydroxide and ammonium ions. The chemical equation for this reaction can be written as: $$\mathrm{Pb^{2+} + 2NH_3 + 2H_2O \rightarrow Pb(OH)_2 + 2NH_4^+}$$

Step 2: Write the net ionic equation for the formation of the precipitate with lead(II) ions

To write the net ionic equation, we will remove the ammonium ions (NH₄⁺) and water (H₂O) as spectator ions: $$\mathrm{Pb^{2+} + 2OH^- \rightarrow Pb(OH)_2}$$

Key Concepts

Chemical Reactions in Chemistry

Chemical reactions are essential transformations within chemistry where substances, known as reactants, change into new substances called products. The hallmark of a chemical reaction is the formation of new chemical bonds and the breaking of old ones. This can be caused by various types of changes, such as temperature, pressure, presence of catalysts, or interaction with other chemical substances.

Demonstrated through the exercise, when copper(II) hydroxide reacts with ammonia, the reactants (copper(II) hydroxide and ammonia) transform into products (tetraamminecopper(II) complex ion and hydroxide ions). Chemical equations represent these reactions, balancing the number of atoms on each side to obey the Law of Conservation of Mass. The net ionic equation further simplifies this by focusing only on the ions that undergo a change during the reaction, which are directly involved in the process of forming the products.

Complex Ion Formation

Complex ions consist of a metal ion at the center surrounded by molecules or anions called ligands. These ligands are bonded to the central ion by coordinate covalent bonds. In the exercise, both copper(II) and cadmium(II) ions react with ammonia to form complex ions, namely tetraamminecopper(II) and tetraamminecadmium(II), respectively.

In these reactions, the lone pairs of electrons on the ammonia molecules donate to the empty d-orbitals of the metal ions, creating a bond and resulting in a charged entity known as a complex ion. Such processes are critical in areas like biochemistry and material science, as they directly impact the properties and behavior of substances.

Precipitation Reactions

Precipitation reactions occur when two soluble reactants produce an insoluble product, known as a precipitate. This transformation is signified by the formation of a solid from a solution and is used extensively in various analytical and synthetic chemistry techniques. In the provided exercise, lead(II) ions react with hydroxide ions to form an insoluble substance, lead(II) hydroxide, which appears as a precipitate.

The net ionic equation for a precipitation reaction includes only the species that participate in forming the precipitate, leaving out the spectator ions—ions that do not change their physical state or oxidation number. The net ionic equation thus vividly illustrates the fundamental chemistry taking place, making it a valuable tool for understanding chemical processes.

Ammonia Reactions

Ammonia, a compound with the formula NH3, is a common reactant in chemistry that can act as a base or a ligand. It readily donates a pair of electrons, which enables it to neutralize acids or bind to metal ions to form complex ions. For instance, in the context of this exercise, we see ammonia reacting in two different ways: neutralizing the hydroxide ions from copper(II) hydroxide and forming a complex ion with cadmium and lead ions.

Ammonia’s reactivity stems from the nitrogen atom's ability to share its lone pair of electrons, which explains its role in these reactions. Understanding ammonia reactions is fundamental in understanding a wide range of chemical systems, from industrial synthesis to biochemical pathways.