Problem 14 - Chemistry: Principles and Reactions

Question

Name the reagent, if any, that you would add to a solution of cobalt(III) chloride to precipitate (a) cobalt(III) phosphate. (b) cobalt(III) carbonate. (c) cobalt(III) hydroxide.

Step-by-Step Solution

Answer

Answer: The reagents that should be added are (a) ammonium phosphate, (b) sodium carbonate, and (c) sodium hydroxide.

Step 1: (Step 1: Write down the chemical formulas for the given compounds)

First, let's write down the chemical formulas for cobalt(III) chloride, and the three cobalt(III) salts we want to precipitate: 1. Cobalt(III) chloride: CoCl3 2. Cobalt(III) phosphate: CoPO4 3. Cobalt(III) carbonate: Co2(CO3)3 4. Cobalt(III) hydroxide: Co(OH)3 Now, we need to identify the appropriate reagents to add to the cobalt(III) chloride solution to form the precipitates.

Step 2: (Step 2: Find the reagent for cobalt(III) phosphate)

To form cobalt(III) phosphate, we need to add a reagent that will provide the phosphate anions (PO4^3-) to the solution. A common source of phosphate ions is ammonium phosphate [(NH4)3PO4]. When ammonium phosphate is added to the solution, the following reaction occurs: CoCl3 + (NH4)3PO4 -> CoPO4 + 3NH4Cl Cobalt(III) phosphate will precipitate out of the solution because it is insoluble in water.

Step 3: (Step 3: Find the reagent for cobalt(III) carbonate)

To form cobalt(III) carbonate, we need to add a reagent that will provide the carbonate anions (CO3^2-) to the solution. A common source of carbonate ions is sodium carbonate (Na2CO3). When sodium carbonate is added to the solution, the following reaction occurs: 2CoCl3 + 3Na2CO3 -> Co2(CO3)3 + 6NaCl Cobalt(III) carbonate will precipitate out of the solution because it is insoluble in water.

Step 4: (Step 4: Find the reagent for cobalt(III) hydroxide)

To form cobalt(III) hydroxide, we need to add a reagent that will provide the hydroxide anions (OH^-) to the solution. A common source of hydroxide ions is sodium hydroxide (NaOH). When sodium hydroxide is added to the solution, the following reaction occurs: CoCl3 + 3NaOH -> Co(OH)3 + 3NaCl Cobalt(III) hydroxide will precipitate out of the solution because it is insoluble in water. So, the reagents that need to be added for each cobalt(III) salt are: (a) ammonium phosphate (b) sodium carbonate (c) sodium hydroxide

Key Concepts

Cobalt(III) Compounds

Cobalt(III) compounds, often referred to as trivalent cobalt compounds, are chemical species where cobalt, the transition metal, exhibits a +3 oxidation state. These compounds have distinct chemical and physical properties, which are pivotal in inorganic chemistry.

For instance, cobalt(III) chloride (CoCl₃), cobalt(III) phosphate (CoPO₄), cobalt(III) carbonate (Co₂(CO₃)₃), and cobalt(III) hydroxide (Co(OH)₃) are cobalt(III) salts with various anions. They have wide applications, including catalysis, battery manufacturing, and pigment production. Here's how they look at the atomic level:

• Cobalt(III) chloride: Consists of one cobalt ion and three chloride ions.
• Cobalt(III) phosphate: Formed between cobalt and the phosphate group.
• Cobalt(III) carbonate: Includes two cobalt ions and three carbonate groups forming a complex unit.
• Cobalt(III) hydroxide: Comprises cobalt ions surrounded by hydroxide groups.

The choice of reagents to form these compounds is critical. Typically, a reagent that provides the necessary anion to react with CoCl₃ is used, resulting in a precipitation reaction where the cobalt(III) compound of interest becomes a solid and drops out of solution.

Solubility Rules

Understanding solubility rules is essential for predicting the outcome of chemical reactions, especially when dealing with precipitation reactions such as those involving cobalt(III) compounds. Solubility rules are guidelines that help predict whether a salt will dissolve in water. These rules are not absolute, but they provide a good basis for making educated guesses about a substance's solubility.

For example, most of the nitrates and many sulfates are soluble, while hydroxides, carbonates, and phosphates are generally insoluble, with some exceptions. Knowing if a compound is insoluble is the key to identifying whether it'll form a precipitate in a reaction. This is demonstrated in our exercise where cobalt(III) phosphate, cobalt(III) carbonate, and cobalt(III) hydroxide are all predicted to be insoluble and, therefore, precipitate out when mixed with their respective ions in solution.

Ionic Equations

Ionic equations provide a more detailed representation of what happens in a chemical reaction, particularly precipitation reactions. Unlike molecular equations, ionic equations show only the species that change during the reaction and omit the spectator ions that do not participate.

Let's consider the formation of cobalt(III) hydroxide as an example. The full ionic equation would look like this:
\[ \text{Co}^{3+}(aq) + 3\text{Cl}^{-}(aq) + 3\text{Na}^{+}(aq) + 3\text{OH}^{-}(aq) \rightarrow \text{Co(OH)}_{3}(s) + 3\text{Na}^{+}(aq) + 3\text{Cl}^{-}(aq) \]
When we remove the spectator ions, we are left with the net ionic equation which shows the direct formation of the precipitate:
\[ \text{Co}^{3+}(aq) + 3\text{OH}^{-}(aq) \rightarrow \text{Co(OH)}_{3}(s) \]
These equations are critical for understanding the core of the reaction. It's the hydroxide ions that combine directly with the cobalt(III) ions to form the insoluble cobalt(III) hydroxide, separating out of the solution as a solid. The key takeaway here is recognizing the action and interaction of ions in a solution, which fuels the process of precipitation.