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Chapter 20: Problem 20

Explain why water cannot be used in the electro-chemical cell during the production of aluminum.

Short Answer

Expert verified
Water gets reduced to hydrogen before aluminum, lowering efficiency and purity.

Step by step solution

01

- Understand the Electrochemical Series

Water is placed higher in the electrochemical series compared to aluminum, meaning it is more likely to be reduced. In aluminum production, a more easily reducible substance will be reduced at the cathode before aluminum ions.
02

- Reaction at the Cathode

In an electrochemical cell, the species with a higher reduction potential gets reduced at the cathode. If water is used, it will be reduced to form hydrogen gas, instead of aluminum ions being reduced to form aluminum metal.
03

- Competing Reactions

The reduction of water to hydrogen gas (2H₂O + 2e⁻ → H₂ + 2OH⁻) is preferred over the reduction of aluminum ions (Al³⁺ + 3e⁻ → Al) because water has a higher reduction potential.
04

- Efficiency and Product Purity

Using water would lower the efficiency of the aluminum production process and reduce the purity of the aluminum produced, as hydrogen gas would be a by-product.
05

- Conclusion

Therefore, water cannot be used in the electrochemical cell during the production of aluminum because it would interfere with the reduction process necessary to produce pure aluminum.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Electrochemical Series
The electrochemical series is a list that ranks elements and compounds by their ability to gain electrons in an electrochemical reaction. This ranking helps us understand which materials are more likely to be reduced or oxidized.

Here's why it's important:
  • Elements higher in the series have higher reduction potentials and are more easily reduced.
  • Those lower have lower reduction potentials and are more likely to be oxidized.
In the production of aluminum, water is positioned higher in the electrochemical series, making it more likely to be reduced than aluminum ions. Thus, if water is present, it will be reduced to hydrogen gas, interfering with the reduction of aluminum ions.
Reduction Potential
Reduction potential refers to the tendency of a substance to gain electrons and be reduced. It is measured in volts and is crucial for understanding electrochemical reactions.

Key points:
  • Species with higher reduction potentials get reduced preferentially in an electrochemical cell.
  • The reduction potential of water is higher compared to aluminum ions, meaning water is more likely to gain electrons and form hydrogen gas.
In the context of aluminum production, if water is present, it will be reduced instead of aluminum ions, leading to hydrogen gas formation and inefficiency in the desired production of pure aluminum.
Aluminum Production
In industrial settings, aluminum is produced through a process called electrolytic reduction using an electrolytic cell. This method ensures high purity and efficiency.

The typical process includes:
  • The use of molten salt (cryolite) and aluminum oxide instead of water to avoid unwanted reactions.
  • Applying a high electric current to reduce aluminum ions at the cathode to form aluminum metal.
  • Ensuring efficient and pure aluminum production while preventing the formation of by-products like hydrogen gas.
By avoiding water, manufacturers ensure that aluminum ions are preferentially reduced, which is crucial for obtaining pure aluminum metal and maintaining the efficiency of the production process.

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