Question

Alumina, \(\mathrm{Al}_{2} \mathrm{O}_{3},\) 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, \(\operatorname{Ca}\left(\mathrm{AlO}_{2}\right)_{2}\)

Short answer

(a) \(\mathrm{Al}_2\mathrm{O}_3 + 3\mathrm{SiO}_2 \rightarrow \mathrm{Al}_2(\mathrm{SiO}_3)_3\); (b) \(\mathrm{Al}_2\mathrm{O}_3 + \mathrm{CaO} \rightarrow \mathrm{Ca(AlO}_2)_2\).

Step-by-step solution

Identify the Reaction Components for Reaction A

The first task is to identify the reactants and products for the reaction between aluminum oxide (\(\mathrm{Al}_2\mathrm{O}_3\)) and silica (\(\mathrm{SiO}_2\)) to form aluminum metasilicate (\(\mathrm{Al}_2(\mathrm{SiO}_3)_3\)). The reactants are \(\mathrm{Al}_2\mathrm{O}_3\) and \(\mathrm{SiO}_2\), and the product is \(\mathrm{Al}_2(\mathrm{SiO}_3)_3\).

Write a Balanced Equation for Reaction A

To balance the equation \(\mathrm{Al}_2\mathrm{O}_3 + \mathrm{SiO}_2 \rightarrow \mathrm{Al}_2(\mathrm{SiO}_3)_3\), balance the number of silicon and oxygen atoms:- From three \(\mathrm{SiO}_2\), we get three \(\mathrm{Si}\) and six \(\mathrm{O}\).- This means one mole of \(\mathrm{Al}_2\mathrm{O}_3\) (two \(\mathrm{Al}\) and three \(\mathrm{O}\)) reacts with three moles of \(\mathrm{SiO}_2\) to form one mole of \(\mathrm{Al}_2(\mathrm{SiO}_3)_3\).- The balanced reaction is: \[ \mathrm{Al}_2\mathrm{O}_3 + 3\mathrm{SiO}_2 \rightarrow \mathrm{Al}_2(\mathrm{SiO}_3)_3 \]

Identify the Reaction Components for Reaction B

For the second reaction, identify the reactants and products. Alumina (\(\mathrm{Al}_2\mathrm{O}_3\)) reacts with calcium oxide (\(\mathrm{CaO}\)) to form calcium aluminate (\(\mathrm{Ca(AlO}_2)_2\)). Thus, the reactants are \(\mathrm{Al}_2\mathrm{O}_3\) and \(\mathrm{CaO}\), and the product is \(\mathrm{Ca(AlO}_2)_2\).

Write a Balanced Equation for Reaction B

To balance the equation \(\mathrm{Al}_2\mathrm{O}_3 + \mathrm{CaO} \rightarrow \mathrm{Ca(AlO}_2)_2\), balance the reactants and products:- One \(\mathrm{Ca}\) is needed for every \(\mathrm{Ca(AlO}_2)_2\) molecule formed.- Thus, one mole of \(\mathrm{CaO}\) reacts with one mole of \(\mathrm{Al}_2\mathrm{O}_3\) to produce one mole of \(\mathrm{Ca(AlO}_2)_2\).- The balanced reaction is: \[ \mathrm{Al}_2\mathrm{O}_3 + \mathrm{CaO} \rightarrow \mathrm{Ca(AlO}_2)_2 \]

Key concepts

Alumina

Alumina, also known as aluminum oxide (\(\mathrm{Al}_2\mathrm{O}_3\)), is a versatile compound primarily found in bauxite, the chief ore of aluminum. One of the fascinating aspects of alumina is its amphoteric nature, meaning it can react with both acids and bases. This dual behavior makes alumina essential in various applications, from the production of aluminum metal to its use in ceramics and refractories.
Amphoteric oxides like alumina can neutralize both acidic and basic oxides, illustrating a unique chemical property that few compounds possess. This makes it invaluable in chemical processes where such dual reactivity is required.

Chemical Reactions

In chemistry, a chemical reaction involves transformation of reactants into products. For alumina, its ability to react with both acidic and basic oxides underlines its amphoteric property. By engaging in such reactions, alumina forms new compounds, showcasing its chemical versatility.

• When alumina reacts with an acidic oxide like silica (\(\mathrm{SiO}_2\)), it results in the formation of aluminum metasilicate (\(\mathrm{Al}_2(\mathrm{SiO}_3)_3\)).
• Conversely, when alumina interacts with a basic oxide such as calcium oxide (\(\mathrm{CaO}\)), it produces calcium aluminate (\(\mathrm{Ca(AlO}_2)_2\)).

These reactions not only demonstrate alumina's amphoteric nature but also illustrate its role in creating valuable compounds used in various industrial applications.

Balancing Equations

Balancing chemical equations is crucial to ensure that a reaction follows the law of conservation of mass, meaning matter is neither created nor destroyed in a chemical reaction. To balance an equation, you must have an equal number of each type of atom on both sides of the equation.
For example, when balancing the equation for the reaction between alumina and silica, we start with the unbalanced equation:
\(\mathrm{Al}_2\mathrm{O}_3 + \mathrm{SiO}_2 \rightarrow \mathrm{Al}_2(\mathrm{SiO}_3)_3\)

• The number of silicon atoms is balanced by adjusting the moles of silica to three (\(3\mathrm{SiO}_2\)), matching the calculation for oxygen and aluminum as well.
• This gives the balanced equation:
  \(\mathrm{Al}_2\mathrm{O}_3 + 3\mathrm{SiO}_2 \rightarrow \mathrm{Al}_2(\mathrm{SiO}_3)_3\)

Similarly, for the reaction between alumina and calcium oxide:
\(\mathrm{Al}_2\mathrm{O}_3 + \mathrm{CaO} \rightarrow \mathrm{Ca(AlO}_2)_2\), we need to have equal quantities of all elements to achieve balance.

Acidic and Basic Oxides

Oxides are compounds of oxygen with another element. They can be categorized based on their chemical behavior as acidic or basic. Acidic oxides, such as silica (\(\mathrm{SiO}_2\)), react with bases to form salts. They generally consist of non-metals bonded with oxygen and can act as acids by providing protons when dissolved in water.
On the other hand, basic oxides like calcium oxide (\(\mathrm{CaO}\)) react with acids and can act as bases, neutralizing acids to form salts. Such oxides typically originate from metals and easily yield hydroxide ions when in solution.
Understanding these general characteristics is essential for predicting their reactions. When these oxides engage with amphoteric oxides like alumina, they form compounds such as aluminum metasilicate and calcium aluminate, as discussed in the reactions here.