Problem 16 - General Chemistry: Principles and Modern Applications

Question

Suggest a series of reactions, using common chemicals, by which each of the following syntheses can be performed. (a) \(\mathrm{Cu}(\mathrm{OH})_{2}(\mathrm{s})\) from \(\mathrm{CuO}(\mathrm{s})\) (b) \(\operatorname{Cr} \mathrm{Cl}_{3}\left(\text { aq) } \text { from }\left(\mathrm{NH}_{4}\right)_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}(\mathrm{s})\right.\)

Step-by-Step Solution

Answer

a) Copper(II) Hydroxide can be synthesized from Copper(II) Oxide by combining it with water. \n b) Chromium(III) Chloride can be synthesized from ammonium dichromate through a two-step process involving a redox reaction with zinc and sulfuric acid to form Chromium(III) ions, which can then combine with Chloride ions from hydrochloric acid to form Chromium(III) Chloride.

Step 1: Part (a): Synthesis of Copper(II) Hydroxide

The reaction needed to transform CuO into Cu(OH)2 requires adding a substance that can produce hydroxide ions in solution. A common source of this is water (H2O). As the reaction occurs, CuO reacts with H2O to form Cu(OH)2. This can be represented as: \[ \mathrm{CuO(s)} + \mathrm{H2O(l)} \rightarrow \mathrm{Cu(OH)2(s)} \] This reaction is a type of chemical reaction called a combination reaction.

Step 2: Part (b): Synthesis of Chromium(III) Chloride

We will first transform the dichromate ion into a chromium(III) ion with a redox reaction involving zinc and sulfuric acid. This is done using the reaction: \[ \left(\mathrm{NH4}\right)_{2}\mathrm{Cr2O7} + 14\mathrm{Zn} + 8\mathrm{H2SO4} \rightarrow 2\mathrm{Cr^{3+}} + 7\mathrm{Zn^{2+}} + 16\mathrm{OH^-} + 2\mathrm{N2} + 8\mathrm{SO4^{2-}} \] Then, we turn the Cr3+ ion into CrCl3, this is achieved by introducing a Chlorine ion source such as hydrochloric acid (HCl). The reaction can be expressed as follows: \[ \mathrm{Cr^{3+}} + 3\mathrm{Cl^-} \rightarrow \mathrm{CrCl3} \] These reactions are both redox reactions as there are changes in the oxidation state of elements.

Key Concepts

Copper(II) Hydroxide

Copper(II) Hydroxide, \( \mathrm{Cu(OH)_2} \), is a blue, water-insoluble solid used in various chemical applications.
It forms when copper(II) oxide, CuO, reacts with water. This process is simple but requires careful handling to ensure purity.
- **Reaction Overview**: The process requires a source of hydroxide ions, typically water.
- **Chemical Equation**: \[ \mathrm{CuO(s)} + \mathrm{H2O(l)} \rightarrow \mathrm{Cu(OH)_2(s)} \] - **Properties**: This solid is important in organic synthesis and serves as a precursor for producing other copper compounds. Understanding this process helps in grasping basic inorganic reactions and the transformation of simple compounds.

Chromium(III) Chloride

Chromium(III) Chloride, \( \mathrm{CrCl_3} \), is often synthesized in laboratories for its role in catalysis and material science.
It's commonly prepared by a series of redox reactions starting with ammonium dichromate.- **Initial Transformation**: The dichromate ion is first converted to chromium(III) ions through a redox reaction utilizing zinc and sulfuric acid. This reaction releases nitrogen gas and sulfate ions.
- **Secondary Reaction**: By adding a chlorine ion source like hydrochloric acid, Cr³⁺ ions react to form \( \mathrm{CrCl_3} \). This step illustrates how complex transformation of compounds happens through multiple reaction steps. Precision in redox reactions is key here.

Combination Reaction

A combination reaction is where two or more substances unite to form a single product.
It's one of the simplest forms of chemical reactions that students often encounter first.- **Example**: The formation of Copper(II) Hydroxide from Copper(II) Oxide and water is a classic combination reaction.
- **General Form**: \[ A + B \rightarrow AB \] - **Significance**: These reactions are foundational in chemical synthesis and industrial production processes.
Understanding this concept aids in recognizing how compounds form and interact to produce new materials.

Redox Reaction

Redox reactions, short for reduction-oxidation reactions, involve the transfer of electrons between two substances.
They are integral to energy production and biochemical processes. - **Key Features**: - **Reduction**: Gain of electrons. - **Oxidation**: Loss of electrons. - **Example in Action**: The synthesis of Chromium(III) Chloride begins with a redox reaction converting chromium from hexavalent to trivalent state using zinc and sulfuric acid as reducing agents.
- **Applications**: These reactions are vital in metallurgy, battery chemistry, and photosynthesis. Grasping redox mechanisms is crucial for understanding how electron movement powers chemical change.

Chemical Reactions in Solutions

Chemical reactions in solutions are processes where reactants are in liquid form, usually in water.
This medium allows ions and molecules to move freely, facilitating chemical interactions.
- **Why Solvents Matter**: Solutions provide a medium for reactants to collide and interact effectively.
- **Relevance to Example**: The synthesis of both Copper(II) Hydroxide and Chromium(III) Chloride involves reactions in aqueous solutions, highlighting how water acts not only as a reactant but also as a solvent. Understanding this concept emphasizes the role of solvents in enabling everyday chemical reactions and industrial applications.