Problem 15 - 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) \(\operatorname{Fe}(\text { OH })_{3}(\text { s) from } \operatorname{Fe} S( \text { s) }\) (b) \(\mathrm{BaCrO}_{4}(\mathrm{s})\) from \(\mathrm{BaCO}_{3}(\mathrm{s})\) and \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}(\mathrm{aq})\)

Step-by-Step Solution

Answer

Iron(III) hydroxide can be synthesized from iron sulfide through a two-step process involving an intermediary step of forming iron(II) chloride. Similarly, barium chromate can be synthesized from barium carbonate and potassium dichromate in the presence of sulfuric acid.

Step 1: Synthesis of \(Fe(OH)_3\) from \(FeS\)

Firstly, our target is to convert \(FeS\) to \(Fe(OH)_3\). This synthesis can be achieved in two steps: \n1. \(FeS + 2HCl \rightarrow FeCl_2 + H_2S\) \nThis reaction can occur under normal conditions, producing iron(II) chloride and hydrogen sulfide. \n2. \(FeCl_2 + 3NaOH \rightarrow Fe(OH)_3 + 2NaCl\) \nIron(II) chloride reacts with sodium hydroxide to give iron(III) hydroxide and sodium chloride. This reaction occurs under mild conditions.

Step 2: Synthesis of \(BaCrO_4\) from \(BaCO_3\) and \(K_2Cr_2O_7\)

The synthesis of \(BaCrO_4\) from \(BaCO_3\) and \(K_2Cr_2O_7\) can be achieved in one step: \n1. \(BaCO_3 + K_2Cr_2O_7 + H_2SO_4 \rightarrow BaCrO_4 + K_2SO_4 + H_2O + CO_2\) \nThe strong acid, sulfuric acid is required here to allow the reaction to proceed, producing our target barium chromate along with potassium sulfate, water and carbon dioxide as by-product. This reaction needs to be carried out in acidic medium.

Key Concepts

Fe(OH)3 synthesis

Fe(OH)₃, also known as iron (III) hydroxide, can be synthesized from iron sulfide (FeS) through a straightforward chemical process. This synthesis involves two main reactions. To start with, you need to convert iron sulfide into iron(II) chloride. This is achieved by reacting it with hydrochloric acid (HCl):

• Reaction: \( FeS + 2HCl \rightarrow FeCl_2 + H_2S \)
• Products: Iron(II) chloride (FeCl₂) and hydrogen sulfide (H₂S)

In this reaction, the iron component of FeS reacts with the hydrochloric acid, forming FeCl₂ and releasing H₂S gas.

Next, we convert iron (II) chloride into iron (III) hydroxide using a simple reaction with sodium hydroxide (NaOH):

• Reaction: \( FeCl_2 + 3NaOH \rightarrow Fe(OH)_3 + 2NaCl \)
• Products: Iron(III) hydroxide (Fe(OH)₃) and sodium chloride (NaCl)

In this step, the FeCl₂ reacts with NaOH, resulting in our target compound, Fe(OH)₃, and salt as a by-product. This reaction is typically conducted under mild conditions, making it feasible in a standard laboratory setup. Understanding these reactions is crucial for mastering inorganic synthesis, especially concerning transitions metals like iron.

BaCrO4 preparation

The synthesis of barium chromate (BaCrO₄) from barium carbonate (BaCO₃) and potassium dichromate (K₂Cr₂O₇) is an exemplary reaction demonstrating the role of acids in chemical synthesis. This reaction stands out because it is carried out in a single step using sulfuric acid (H₂SO₄) as a catalyst.

• Reaction: \( BaCO_3 + K_2Cr_2O_7 + H_2SO_4 \rightarrow BaCrO_4 + K_2SO_4 + H_2O + CO_2 \)
• Products: Barium chromate (BaCrO₄), potassium sulfate (K₂SO₄), water (H₂O), and carbon dioxide (CO₂)

In this reaction, the interaction between BaCO₃ and K₂Cr₂O₇, in the presence of H₂SO₄, leads to the formation of insoluble BaCrO₄, which precipitates from the solution. The CO₂ gas and water formed are released during the reaction, while K₂SO₄ remains dissolved in the solution.

The acidic medium, provided by H₂SO₄, is essential for the reaction to proceed. It ensures that the reactants are in the right state to form the desired products. Such reactions highlight the intriguing balance between reactants and conditions, crucial for achieving successful inorganic preparations in the lab.

Iron and barium reactions

Reactions involving iron and barium compounds illustrate fundamental principles in inorganic chemistry. Both Fe(OH)₃ and BaCrO₄ syntheses showcase how diverse reactions can lead to different types of compounds using elements like iron and barium.

• Iron reactions typically involve oxidation states and possible transformations from ferrous to ferric forms (e.g., from FeS to Fe(OH)₃).
• Similarly, barium's reactivity can be seen when forming complex salts like barium chromate from simple barium carbonate.

In Fe(OH)₃ synthesis, iron underwent an oxidation change through hydrochloric acid and finished as an iron(III) compound. Meanwhile, barium's transformation requires an acidic medium to yield BaCrO₄ as a precipitate.

These processes underscore the significance of initial reactants and reaction conditions (e.g., pH, presence of catalysts) in determining the course and success of inorganic syntheses. Understanding these principles is crucial for students delving into the vast field of inorganic chemistry, as they often reflect broader chemical laws and characteristics of transition metals and alkaline earth metals.