Problem 38 - Chemistry and Chemical Reactivity

Question

Alumina, \(A l_{2} O_{3},\) is amphoteric. Among examples of its amphoteric character are the reactions that occur when \(\mathrm{Al}_{2} \mathrm{O}_{3}\) is heated strongly or "fused" with acidic oxides and basic oxides. (a) Write a balanced equation for the reaction of alumina with silica, an acidic oxide, to give aluminum metasilicate, \(\mathrm{Al}_{2}\left(\mathrm{SiO}_{3}\right)_{3}\) (b) Write a balanced equation for the reaction of alumina with the basic oxide CaO to give calcium aluminate, \(\mathrm{Ca}\left(\mathrm{AlO}_{2}\right)_{2}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{Al}_{2} \mathrm{O}_{3} + 3\mathrm{SiO}_{2} \rightarrow \mathrm{Al}_{2}\left(\mathrm{SiO}_{3}\right)_{3}\); (b) \(\mathrm{Al}_{2} \mathrm{O}_{3} + \mathrm{CaO} \rightarrow \mathrm{Ca}\left(\mathrm{AlO}_{2}\right)_{2}\).

Step 1: Understanding the Reaction with Silica

When alumina (\(\mathrm{Al}_{2} \mathrm{O}_{3}\)) reacts with silica (\(\mathrm{SiO}_{2}\)), which is an acidic oxide, they form aluminum metasilicate (\(\mathrm{Al}_{2}\left(\mathrm{SiO}_{3}\right)_{3}\)). We need to balance this chemical equation by making sure that the number of each type of atom on the reactant side equals the number on the product side. Begin by writing the unbalanced equation:\[ \mathrm{Al}_{2} \mathrm{O}_{3}(s) + \mathrm{SiO}_{2}(s) \rightarrow \mathrm{Al}_{2}\left(\mathrm{SiO}_{3}\right)_{3}(s) \]

Step 2: Balancing the Equation with Silica

In the unbalanced equation:- We have 2 aluminum (\(\mathrm{Al}\)) atoms on both sides, so aluminum is balanced.- Oxygen (\(\mathrm{O}\)) needs to be balanced next. We have 3 atoms from \(\mathrm{Al}_{2} \mathrm{O}_{3}\) and 2 from \(\mathrm{SiO}_{2}\) (5 total on the reactant side). On the product side, we have 9 oxygen in \(\mathrm{Al}_{2}\left(\mathrm{SiO}_{3}\right)_{3}\). We need to adjust the coefficients to have 9 oxygens on both sides.- Silicon (\(\mathrm{Si}\)) is initially in \(\mathrm{SiO}_{2}\), and after balancing, there should be three on both sides.Thus, adjust the coefficients as follows:\[ \mathrm{Al}_{2} \mathrm{O}_{3}(s) + 3\mathrm{SiO}_{2}(s) \rightarrow \mathrm{Al}_{2}\left(\mathrm{SiO}_{3}\right)_{3}(s) \]

Step 3: Understanding the Reaction with Calcium Oxide

Next, alumina will react with calcium oxide (\(\mathrm{CaO}\)) to form calcium aluminate (\(\mathrm{Ca}\left(\mathrm{AlO}_{2}\right)_{2}\)). Again, we begin with the unbalanced chemical equation:\[ \mathrm{Al}_{2} \mathrm{O}_{3}(s) + \mathrm{CaO}(s) \rightarrow \mathrm{Ca}\left(\mathrm{AlO}_{2}\right)_{2}(s) \]

Step 4: Balancing the Equation with Calcium Oxide

In the unbalanced equation:- We have 2 \(\mathrm{Al}\) on the reactant side and 2 in \(\mathrm{Ca}\left(\mathrm{AlO}_{2}\right)_{2}\) on the product side, so aluminum is balanced.- Calcium (\(\mathrm{Ca}\)) has 1 atom on both sides, so it is balanced.- Oxygen needs to be checked last. On the left, 3 oxygen from alumina and 1 from CaO equals 4. On the right, the product has 2x2=4 oxygens.Thus, the equation is already balanced as:\[ \mathrm{Al}_{2} \mathrm{O}_{3}(s) + \mathrm{CaO}(s) \rightarrow \mathrm{Ca}\left(\mathrm{AlO}_{2}\right)_{2}(s) \]

Key Concepts

Balancing Chemical Equations

Balancing chemical equations is an essential skill in chemistry that ensures the law of conservation of mass is followed. Each side of the equation must have the same number of atoms for every element. The process might seem complex initially, but with practice, it becomes intuitive.
Here's a step-by-step guide for balancing equations:

• Write down the unbalanced equation to identify the reactants and products.
• List the number of atoms of each element on both sides of the equation.
• Start balancing with the element that appears in the least compounds.
• Use coefficients to balance atoms, never alter subscripts in chemical formulas.
• Continue to adjust coefficients until all elements are balanced.
• Finally, double-check each element to ensure the equation is balanced.

In the provided exercise, balancing the reaction of alumina with silica involved adjusting the coefficient of silica to ensure both the silicon and oxygen atoms were balanced. Similarly, alumina's reaction with calcium oxide was already balanced upon initial inspection.

Reactions with Acidic Oxides

Acidic oxides, sometimes referred to as non-metal oxides, are compounds that react with bases to form salts and water. These oxides typically include elements from the p-block of the periodic table such as sulfur, phosphorus, and silicon.

• Acidic oxides react with basic substances to form salts.
• They play a significant role in environmental chemistry, contributing to processes like acid rain formation.

In our exercise, silica (\(\text{SiO}_{2}\)) acts as an acidic oxide. When combined with alumina (\(\text{Al}_{2}\text{O}_{3}\)), itself exhibits amphoteric behavior, the two form aluminum metasilicate. This specific reaction highlights the acidic nature of silica as it reacts with a base-like substance (alumina) to produce a complex silicate.

Reactions with Basic Oxides

Basic oxides consist mainly of metals combined with oxygen. They typically react with acids to yield salts and water, demonstrating their basicity through neutralization reactions.

• Common basic oxides include \(\text{Na}_2\text{O}\), \(\text{CaO}\), and \(\text{MgO}\).
• They feature prominently in industrial applications, such as the manufacture of cement and glass.

In the example given, alumina (\(\text{Al}_{2}\text{O}_3\)) reacts with calcium oxide (\(\text{CaO}\)), which is a basic oxide. Their combination forms calcium aluminate (\(\text{Ca(AlO}_{2})_{2}\)). This reaction further showcases alumina's amphoteric character, as alumina reacts with both acidic and basic oxides, forming different compounds based on the nature of the oxide.