Problem 35 - General Chemistry: Principles and Modern Applications

Question

A description for preparing potassium aluminum alum calls for dissolving aluminum foil in KOH(aq). The solution obtained is treated with \(\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq}),\) and the alum is crystallized from the resulting solution. Write plausible equations for these reactions.

Step-by-Step Solution

Answer

The chemical reactions for the described process are: dissolving of aluminum in KOH solution \[2\mathrm{Al} + 2\mathrm{KOH} + 6\mathrm{H}_{2}\mathrm{O} \rightarrow 2\mathrm{KAlO_{2}} + 3\mathrm{H}_{2}\], and the reaction of the resulting solution with \(\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})\) to form alum \[\mathrm{KAlO_{2}} + \mathrm{H_{2}SO_{4}} \rightarrow \mathrm{KAl(SO_{4})_{2}} + 2\mathrm{H_{2}O}\]. The alum then crystallizes upon evaporation of the solution.

Step 1: Dissolving of aluminum foil in KOH(aq)

Aluminum foil reacts with aqueous solution of KOH to form potassium aluminate, \(\mathrm{KAlO_{2}}\), and hydrogen gas, \(H_{2}\). The balanced chemical equation for this reaction is: \[2\mathrm{Al} + 2\mathrm{KOH} + 6\mathrm{H}_{2}\mathrm{O} \rightarrow 2\mathrm{KAlO_{2}} + 3\mathrm{H}_{2}\]

Step 2: Treating the obtained solution with \(\mathrm{H}_{2}\mathrm{SO}_{4}(\mathrm{aq})\)

Potassium aluminate reacts with \(\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})\) to form potassium aluminum sulfate, also known as alum, and water. This reaction is represented by the balanced chemical equation: \[ \mathrm{KAlO_{2}} + \mathrm{H_{2}SO_{4}} \rightarrow \mathrm{KAl(SO_{4})_{2}} + 2\mathrm{H_{2}O}\]

Step 3: Crystallisation of Alum

The chemical reaction in step 2 will produce the desired potassium aluminum alum in solution. The next goal is to isolate the alum. This is normally achieved through a process known as crystallisation. The process itself does not demand a specific chemical reaction to detail, but it is pivotal to state that the solution is allowed to slowly evaporate, enabling the alum particles to form a structured pattern and thus becoming crystals.

Key Concepts

Dissolving Aluminum Foil in KOH

The process of dissolving aluminum foil in an aqueous solution of potassium hydroxide (KOH) is a fascinating chemical reaction that initiates the synthesis of potassium alum. Students sometimes find it challenging to visualize, so let's simplify it.

Imagine the aluminum foil as tiny metal boats navigating in a KOH solution sea. When these 'boats' come into contact with KOH, they react to form a new product, much like the boats transforming into rafts named potassium aluminate, with the chemical formula \(\mathrm{KAlO_{2}}\). During this transformation, a gas escapes like bubbles from the sea—the hydrogen gas, \(H_{2}\). The disappearance of the 'boats' (aluminum foil) is noticeable because they dissolve into the KOH solution, leaving behind a clear solution that has the 'rafts' (potassium aluminate) fully dissolved in it.

KOH is an alkali that provides the necessary aggressive environment for aluminum to dissolve. This step is crucial for creating potassium alum and is described by the balanced equation:
\[2\mathrm{Al} + 2\mathrm{KOH} + 6\mathrm{H}_{2}\mathrm{O} \rightarrow 2\mathrm{KAlO_{2}} + 3\mathrm{H}_{2}\]
It's a great demonstration of a redox reaction, where aluminum donates electrons to hydrogen ions in the water present in the KOH solution. Understanding this foundational step is crucial for grasping the overall chemical process of creating potassium alum.

Potassium Aluminate Formation

After the aluminum has dissolved, we're left with a solution containing potassium aluminate. This formation is an intermediate step in the process of making potassium aluminum sulfate, the compound commonly known as alum.

Why does this happen? Because the KOH solution acts as a base, it pulls apart the aluminum from the foil and allows it to bond with potassium and oxygen ions, forming potassium aluminate. The newly created compound has unique properties; it is very soluble in water, which means it readily dissolves, and it sets the stage for the next step in the aluminum sulfate synthesis.

Understand that the initial violent reaction between the foil and KOH settles down to a clear solution of \(\mathrm{KAlO_{2}}\). It generally remains in solution unless we change the conditions—this is where sulfuric acid comes into play to lead us to the next stage.

Crystallization Process

The crystallization process is like a nature-inspired art in the lab. After our potassium aluminate solution is ready, it must be turned into solid, lustrous crystals of potassium aluminum sulfate. But how?

Simply put, crystallization is a method where we force a chemical substance to form a solid structure with a definite geometric shape. In this case, by evaporating some of the water from our solution or by cooling it down, we make the environment less friendly for the potassium aluminum sulfate to stay dissolved. This unfriendly environment encourages the alum molecules to cling to one another, forming a solid network—a crystal.

During the crystallization process, molecules align in a specific repeating pattern, which is why crystals have such precise shapes. The key to beautiful crystals is patience; slow and controlled evaporation or cooling allows for the orderly formation of crystal lattices. Fast evaporation might lead to small, poorly formed crystals. In essence, crystallization turns the chaos of a dissolved substance into the ordered beauty of a solid crystal.

Potassium Aluminum Sulfate Synthesis

The synthesis of potassium aluminum sulfate is the goal of dissolving aluminum in KOH. This compound, also known as alum, has a myriad of uses—from water purification to the culinary arts. So how do we make it from our solution of potassium aluminate?

When we add sulfuric acid (\(\mathrm{H_{2}SO_{4}(aq)}\)) to the mix, it combines with the potassium aluminate to form potassium aluminum sulfate (the alum). This is described by the chemical equation:
\[\mathrm{KAlO_{2}} + \mathrm{H_{2}SO_{4}} \rightarrow \mathrm{KAl(SO_{4})_{2}} + 2\mathrm{H_{2}O}\]
But the magic really happens during the crystallization. After adding sulfuric acid, the solution contains the alum in a dissolved state. By lowering the temperature or allowing water to evaporate, we progressively get closer to the saturation point. Then, alum starts to form stunning geometric crystals—a sure sign of successful synthesis.

In classrooms or labs, synthesizing alum serves as an excellent example of chemical transformation, where students can observe the journey from metallic aluminum to a crystalline compound. It's chemistry in action and brings theory to sparkling life!