4. Electrochemistry (Part 2) (2/3) (Cambridge IGCSE Chemistry 0620 for 2023, 2024 & 2025)

Summary

This video provides a clear explanation of electrolysis, focusing on both molten and aqueous solutions. It covers key examples like the electrolysis of molten lead(II) bromide and aqueous sodium chloride, detailing the role of inert electrodes and the ions involved. The video also explains how the reactivity series helps predict the discharge of ions at the cathode and anode. Key processes such as the formation of chlorine, hydrogen, oxygen, and lead are discussed, along with the effect of concentration on electrolysis. Viewers are encouraged to engage with the content and share feedback to support future educational efforts.

Takeaways

• 😀 Inert electrodes (platinum or carbon graphite) are used in electrolysis to prevent interference with chemical reactions.
• 😀 Molten compounds have free ions that can carry charge during electrolysis.
• 😀 In the electrolysis of molten lead(II) bromide (PbBr2), bromine gas forms at the anode and lead metal forms at the cathode.
• 😀 In aqueous electrolysis, the electrolyte contains ions from both the dissolved substance and water, including hydrogen and hydroxide ions.
• 😀 At the cathode, hydrogen or less reactive metal ions are discharged. Hydrogen ions are discharged if less reactive metals are present.
• 😀 The reactivity series helps predict which ions will be discharged at the cathode, with less reactive metals or hydrogen being favored.
• 😀 At the anode, halide ions are discharged first to form halogens. In the absence of halides, hydroxide ions are discharged to produce oxygen gas.
• 😀 In concentrated aqueous sodium chloride (NaCl) electrolysis, hydrogen gas is produced at the cathode, and chlorine gas forms at the anode.
• 😀 In the electrolysis of dilute sulfuric acid (H2SO4), hydrogen gas forms at the cathode, while oxygen gas is released at the anode.
• 😀 The concentration of the electrolyte influences whether halogen or oxygen gas is produced at the anode during electrolysis.
• 😀 The video emphasizes the importance of understanding the behavior of ions in different electrolytic solutions to predict the products of electrolysis.

Q & A

What is the purpose of using inert electrodes in electrolysis?

Inert electrodes, such as platinum or carbon graphite, are used in electrolysis because they do not participate in the chemical reactions. This allows the focus to be on the ions being discharged at the electrodes without interference from the electrodes themselves.

What does the term 'molten' mean in the context of electrolysis?

'Molten' refers to a substance that is in a liquid state due to being heated above its melting point. When a compound is molten, it has free ions that can carry charge during electrolysis.

What are the ions present in molten lead(II) bromide (PbBr₂)?

The ions present in molten lead(II) bromide are Pb²⁺ (lead ions) and Br⁻ (bromide ions).

What happens at the anode during the electrolysis of molten lead(II) bromide?

At the anode, bromide ions (Br⁻) are attracted and oxidized. Each bromide ion loses one electron to form bromine molecules (Br₂), which can be observed as brown gas bubbles.

What is formed at the cathode during the electrolysis of molten lead(II) bromide?

At the cathode, lead ions (Pb²⁺) gain electrons to form solid lead metal, which accumulates as a gray substance on the electrode.

What is the difference between molten and aqueous electrolytes?

Molten electrolytes are substances in a liquid state due to being heated above their melting points, while aqueous electrolytes are substances dissolved in water, resulting in ions from both the compound and water itself, such as hydrogen (H⁺) and hydroxide (OH⁻) ions.

How can we predict which ions are discharged during electrolysis of aqueous solutions?

At the cathode, either a metal ion or hydrogen ion will be discharged, with the less reactive ion being chosen based on the reactivity series. At the anode, either a halide ion or a hydroxide ion will be discharged, with halide ions taking priority if present.

What happens during the electrolysis of concentrated aqueous sodium chloride (NaCl)?

In the electrolysis of concentrated aqueous sodium chloride, hydrogen ions (H⁺) are discharged at the cathode to form hydrogen gas, while chloride ions (Cl⁻) are discharged at the anode to form chlorine gas. Sodium (Na⁺) and hydroxide ions (OH⁻) remain in the solution, forming sodium hydroxide (NaOH).

Why is oxygen produced at the anode during the electrolysis of dilute sulfuric acid?

During the electrolysis of dilute sulfuric acid, there are no halide ions present to be discharged. Instead, hydroxide ions (OH⁻) are discharged at the anode, losing electrons to form oxygen gas and water.

What is the reactivity series, and how does it help predict ion discharge during electrolysis?

The reactivity series is a list of metals ranked by their reactivity. It helps predict which ion will be discharged at the cathode during electrolysis. The less reactive ion, whether metal or hydrogen, will be discharged at the cathode.