Question

Aluminum oxide(s) decomposes when electricity passes through it.

Short answer

Aluminum oxide (Al2O3) decomposes into aluminum (Al) and oxygen (O2) through a process called electrolysis. We set up an electrolytic cell with a graphite anode, a molten aluminum oxide electrolyte, and a molten aluminum cathode. When electric current passes through, aluminum ions gain three electrons at the cathode to become aluminum atoms, and oxide ions lose two electrons at the anode to become oxygen gas: \[ 2Al_2O_3(s) \rightarrow 4Al(l) + 3O_2(g) \] The aluminum is collected as a molten layer, and the oxygen gas is vented away.

Step-by-step solution

Understand the chemical reaction

Aluminum oxide (Al2O3) is an ionic compound that can be decomposed into its elements, aluminum (Al) and oxygen (O2), by passing an electric current through it. This process is known as electrolysis. The chemical reaction for the decomposition of aluminum oxide is: \[ 2Al_2O_3(s) \rightarrow 4Al(l) + 3O_2(g) \] This shows that two moles of aluminum oxide decompose to produce four moles of aluminum and three moles of oxygen gas.

Electrolysis setup

To perform the electrolysis and decompose aluminum oxide, we need to set up an electrolytic cell with an inert (nonreactive) anode and a molten aluminum oxide electrolyte. 1. Place a graphite anode, typically made of carbon, into the electrolyte, which is molten aluminum oxide (Al2O3). 2. Place a molten aluminum cathode at the bottom of the electrolyte container. Aluminum oxide needs to be heated at high temperature (\(>2000^{\circ}C\)) to become a molten state.

Performing electrolysis

Once the electrolysis setup is prepared, follow these steps: 1. Connect the graphite anode to the positive terminal of a DC power source (e.g., a battery) and the molten aluminum cathode to the negative terminal. 2. Switch on the power source to let the electric current pass through the molten aluminum oxide. This current causes the aluminum ions to move toward the cathode and the oxide ions to move toward the anode. 3. At the cathode, aluminum ions gain three electrons to become aluminum atoms. The reaction at the cathode can be represented as: \[ Al^{3+} + 3e^{-}\rightarrow Al \] 4. At the anode, the oxide ions lose two electrons to become oxygen gas. The reaction at the anode can be represented as: \[ 2O^{2-} \rightarrow O_{2}(g) + 4e^{-} \]

Analyze the results

As a result of electrolysis, aluminum is reduced at the cathode, forming a layer of molten aluminum that can be extracted. At the anode, oxygen gas is released and can be safely vented away. The overall reaction is: \[ 2Al_2O_3(s) \rightarrow 4Al(l) + 3O_2(g) \] By understanding and performing the steps of electrolysis, we have successfully decomposed aluminum oxide into its constituent elements, aluminum and oxygen.

Key concepts

Understanding Chemical Reactions

A chemical reaction is a process where reactants are transformed into products through the breaking and forming of chemical bonds. In the case of aluminum oxide, or \( Al_2O_3 \), the compound is composed of aluminum and oxygen atoms bonded together. During the electrolysis process, a chemical reaction occurs that decomposes this ionic compound into its elemental forms: aluminum metal and oxygen gas. The balanced chemical equation, \[ 2Al_2O_3(s) \rightarrow 4Al(l) + 3O_2(g) \], succinctly represents this transformation, indicating that two moles of solid aluminum oxide are converted into four moles of liquid aluminum and three moles of gaseous oxygen. It's crucial to understand the stoichiometry of the reaction which helps to predict the amount of each product formed from a given quantity of reactant.

To facilitate and comprehend the process fully, we should dive into the nature of ionic compounds and the operations of an electrolytic cell, which are the underlying principles of electrolysis.

Ionic Compounds and Their Properties

Ionic compounds, such as aluminum oxide, are substances composed of positively charged ions called cations and negatively charged ions, anions, which are held together by strong electrostatic forces known as ionic bonds. These compounds have high melting and boiling points due to the strength of the ionic bonds, and that's why aluminum oxide must be heated to over \(2000^\circ C\) to become molten and conductive. In its molten state, the ions are free to move, making it possible to conduct electricity and engage in the reactions of electrolysis.

When applying electricity, cations and anions move to their respective electrodes - cations to the cathode and anions to the anode. This movement is critical for the exchange of electrons and the resultant formation of elemental substances. Understanding the behaviour of these ions within an electrolytic cell is essential for mastering the principle of electrolysis.

The Role of the Electrolytic Cell in Electrolysis

An electrolytic cell is a device that uses electrical energy to drive a non-spontaneous chemical reaction, such as the decomposition of aluminum oxide. The cell is typically made up of two electrodes: the anode, which is connected to the positive terminal of the power source, and the cathode, connected to the negative terminal.

In the electrolysis of aluminum oxide, the molten aluminum serves as the cathode and graphite acts as the anode. When the DC power source is switched on, the aluminum ions \( Al^{3+} \) move towards the cathode, where they gain electrons, a process called reduction. They transform into neutral aluminum atoms and accumulate as molten aluminum. Conversely, at the anode, oxide ions \( O^{2-} \) release electrons, a process known as oxidation, and exit as oxygen gas. Together, these half-reactions occur simultaneously to sustain a continuous process that ultimately leads to the separation of the original ionic compound into elemental forms.