Problem 26 - Chemistry The Science in Context

Question

Aluminum-Air Batteries In recent years engineers have been working on an aluminum-air battery as an alternative energy source for electric vehicles. The battery consists of an aluminum anode, which is oxidized to solid aluminum hydroxide, immersed in an electrolyte of aqueous KOH. At the cathode oxygen from the air is reduced to hydroxide ions on an inert metal surface. Write the two half-reactions for the battery and diagram the cell. Use the generic \(\mathrm{M}(s)\) symbol for the metallic cathode material.

Step-by-Step Solution

Answer

Answer: The half-reactions for an aluminum-air battery are: Anode: Al (s) → Al(OH)₃ (s) + 3e⁻ Cathode: O₂ (g) + 2H₂O (l) + 4e⁻ → 4OH⁻ (aq) The cell diagram is: Anode: Al (s) → Al(OH)₃ (s) + 3e⁻ _ Al (s) |-------------→| M (s) Electrolyte: Aqueous KOH O₂ (g) + 2H₂O (l) + 4e⁻ → 4OH⁻ (aq) : Cathode

Step 1: Write the half-reactions for the anode and cathode

For the anode, aluminum (Al) is being oxidized to form solid aluminum hydroxide (Al(OH)3). The oxidation half-reaction is written as: Al (s) → Al(OH)₃ (s) + 3e⁻ For the cathode, oxygen (O₂) from the air is reduced to form hydroxide ions (OH⁻) on the inert metal surface (M). The reduction half-reaction is: O₂ (g) + 2H₂O (l) + 4e⁻ → 4OH⁻ (aq)

Step 2: Diagram the cell

To diagram the cell, follow these steps: 1. Label the left side as the anode (oxidation) and the right side as the cathode (reduction). 2. Represent the aluminum anode as "Al" on the left side. 3. Represent the metallic cathode material with the generic symbol "M(s)" on the right side. 4. Draw an arrow pointing from the aluminum anode to the metallic cathode to indicate the flow of electrons. 5. Label the electrolyte as "aqueous KOH" in the middle, with the half-reactions written above and below the cell diagram. 6. Indicate that oxygen from the air is being introduced at the cathode. The final diagram should look like this: Anode: Al (s) → Al(OH)₃ (s) + 3e⁻ _ Al (s) |-------------→| M (s) Electrolyte: Aqueous KOH O₂ (g) + 2H₂O (l) + 4e⁻ → 4OH⁻ (aq) : Cathode

Key Concepts

Half-Reactions

When discussing batteries, it's crucial to understand half-reactions. These are the two parts of the overall chemical reaction happening in an electrochemical cell. Each half-reaction represents either an oxidation or a reduction process.

In an aluminum-air battery, the half-reaction at the anode involves oxidation. Aluminum ( Al(s) ) is converted to aluminum hydroxide ( Al(OH)_3(s) ). This releases electrons, as shown:

• Al (s) → Al(OH)₃ (s) + 3e⁻

At the cathode, the half-reaction involves reduction. Oxygen from the air gains electrons to form hydroxide ions ( OH⁻ ), depicted as:

• O₂ (g) + 2H₂O (l) + 4e⁻ → 4OH⁻ (aq)

Remember, these reactions complement each other. The electrons released at the anode are used at the cathode, completing an important cycle in the battery.

Oxidation-Reduction

Oxidation-reduction, often referred to as redox, is a fundamental concept in chemistry. It involves the transfer of electrons between two substances. Understanding this helps in grasping how batteries work.

In the aluminum-air battery, oxidation occurs at the anode. Oxidation means the loss of electrons. The aluminum ( Al ) loses electrons and changes into aluminum hydroxide ( Al(OH)_3 ). Conversely, reduction occurs at the cathode. Reduction is the gaining of electrons. Oxygen ( O_2 ) from the air accepts electrons to form hydroxide ions ( OH⁻ ). This complementary exchange is essential for the battery's operation.

Think of redox reactions like a balancing act: one side loses electrons (oxidation), while the other gains them (reduction). Together, they allow the battery to generate electrical energy.

Electrolyte

Electrolytes are substances that conduct electricity by dissolving into ions. They are fundamental in electrochemical cells since they allow the flow of charge. In an aluminum-air battery, the electrolyte is an aqueous solution of potassium hydroxide ( KOH ).

This electrolyte plays several critical roles:

• It helps facilitate the movement of ions within the battery, allowing the redox reactions to occur at both the anode and cathode.
• It ensures electrical neutrality by balancing the charges as ions move through the solution.

Moreover, the electrolyte keeps the anode and cathode reactions separated while allowing the necessary ions to transfer. This separation is crucial for the battery to maintain a consistent voltage over time. Understanding the role of electrolytes is key to seeing how such batteries function efficiently.