Problem 28 - General Chemistry: Principles and Modern Applications

Question

Write plausible equations for the (a) reaction of \(\mathrm{Al}(\mathrm{s})\) with \(\mathrm{Br}_{2}(1)\) (b) production of \(\mathrm{Cr}\) from \(\mathrm{Cr}_{2} \mathrm{O}_{3}(\mathrm{s})\) by the thermite reaction, with Al as the reducing agent; (c) separation of \(\mathrm{Fe}_{2} \mathrm{O}_{3}\) impurity from bauxite ore.

Step-by-Step Solution

Answer

The balanced equations for the reactions are:\n(a) 2Al(s) + 3Br2(l) -> 2AlBr3(s),\n(b) 2Al(s) + Cr2O3(s) -> Al2O3(s) + 2Cr(s),\n(c) Fe2O3(s) + 3NaOH(l) -> 2NaFeO2(s) + H2O(l).

Step 1: Write and Balance the Equation for Reaction (a)

Firstly, consider the reaction between aluminium (Al) and bromine (Br2). When these two react, they will form aluminium bromide (AlBr3). The chemical equation is:\nAl(s) + Br2(l) -> AlBr3(s)\nHowever, this is not balanced, as there are 3 bromine atoms on the right side and 2 on the left. Therefore, the balanced chemical equation is:\n2Al(s) + 3Br2(l) -> 2AlBr3(s)

Step 2: Write and Balance the Equation for Reaction (b)

The thermite reaction involves reduction of chromium (III) oxide (Cr2O3) to chromium using aluminium as the reducing agent. The reaction equation is: \nAl(s) + Cr2O3(s) -> Al2O3(s) + Cr(s)\nAgain, this isn't balanced. The balanced equation is:\n2Al(s) + Cr2O3(s) -> Al2O3(s) + 2Cr(s)

Step 3: Write and Balance the Equation for Reaction (c)

The removal of iron (III) oxide impurity from bauxite (naturally occurring aluminium ore) is a key step in the production of aluminium. Caustic soda (NaOH) is often used to dissolve the impurity, while aluminium oxides remain insoluble. It forms sodium aluminate and water. The reaction is:\nFe2O3(s) + 3NaOH(l) -> 2NaFeO2(s) + H2O(l), which shows how Fe2O3 is separated from the bauxite ore.

Key Concepts

Equation Balancing

Balancing chemical equations is a fundamental skill in chemistry. In a chemical equation, each element must have the same number of atoms on both sides of the equation. This follows the law of conservation of mass, which states that matter cannot be created or destroyed.

When we look at chemical equations, such as the reaction between aluminium (\(\text{Al}\)) and bromine (\(\text{Br}_2\)), we start by writing the unbalanced equation:

• \(\text{Al(s)} + \text{Br}_2(l) \rightarrow \text{AlBr}_3(s)\)

In this equation:

• Aluminium and bromine combine to form aluminium bromide.
• The equation initially shows one \(\text{Al}\) atom and two \(\text{Br}\) atoms on the left, and one \(\text{Al}\) and three \(\text{Br}\) atoms on the right.

To balance it, we adjust coefficients:

• \(2\text{Al(s)} + 3\text{Br}_2(l) \rightarrow 2\text{AlBr}_3(s)\)

Here:

• Each side now has two aluminium atoms and six bromine atoms.
• This process ensures the conservation of mass is upheld in the reaction.

Balancing equations requires practice but becomes easier as you gain familiarity with atomic symbols and stoichiometry.

Thermite Reaction

The thermite reaction is a process involving a metal, typically aluminium, as a reducing agent to extract another metal from its oxide. A classic example is the reaction of chromium (III) oxide (\(\text{Cr}_2\text{O}_3\)) with aluminium to produce chromium:

• \(\text{Al(s)} + \text{Cr}_2\text{O}_3(s) \rightarrow \text{Al}_2\text{O}_3(s) + \text{Cr}(s)\)

This reaction is unbalanced initially. To balance it:

• \(2\text{Al(s)} + \text{Cr}_2\text{O}_3(s) \rightarrow \text{Al}_2\text{O}_3(s) + 2\text{Cr}(s)\)

In the balanced equation:

• Two aluminium atoms and one chrome oxide yield two chromium and one aluminium oxide molecule.
• The aluminium atoms reduce the chromium oxide, facilitating the extraction of chromium.

Thermite reactions are highly exothermic, meaning they release a significant amount of heat, often enough to produce molten metal. This type of reaction has practical applications in welding and metal reclamation.

Understanding this reaction can help in comprehending similar processes where one metal is used to strip another metal from an oxide.

Aluminium Bromide

Aluminium bromide (\(\text{AlBr}_3\)) is a chemical compound formed through the reaction of aluminium with bromine. In the equation:

• \(2\text{Al(s)} + 3\text{Br}_2(l) \rightarrow 2\text{AlBr}_3(s)\)

The formation of aluminium bromide highlights a classic synthesis reaction:

• Two reactants combine to form a single product.
• Aluminium, a metal, reacts with bromine, a halogen.
• This results in a compound where aluminium bonds with three bromine atoms.

Aluminium bromide is used in organic synthesis, serving as a catalyst or a halogenating agent. Its dry form is highly hygroscopic, meaning it readily absorbs water. When working with this compound, cautious handling is essential due to its reactive nature.

The study of aluminium bromide helps illustrate broader chemical principles, such as synthesis reactions and the role of halogens in forming reactive compounds.