Chemistry 5070/42 — October/November 2020

Walkthrough

For part (a), we are electrolyzing two aqueous solutions using inert carbon electrodes.

1. Dilute sulfuric acid: The ions present are H⁺, OH⁻, and SO₄²⁻. At the anode (+), OH⁻ is discharged in preference to SO₄²⁻ because sulfate ions are not discharged in dilute solutions. The OH⁻ ions lose electrons to form oxygen gas and water. Observation: bubbles of colourless gas. At the cathode (–), H⁺ ions are discharged to form hydrogen gas. Observation: bubbles of colourless gas.

2. Aqueous copper(II) sulfate: The ions present are Cu²⁺, H⁺, SO₄²⁻, and OH⁻. At the anode (+), OH⁻ is discharged to form oxygen gas. Observation: bubbles of colourless gas. At the cathode (–), Cu²⁺ ions are discharged in preference to H⁺ ions because copper is below hydrogen in the reactivity series. Cu²⁺ gains electrons to form solid copper metal. Observation: a pink, brown, or orange solid deposits on the cathode.

Key Takeaways

• In the electrolysis of dilute aqueous acids with inert electrodes, oxygen is produced at the anode and hydrogen at the cathode.
• In the electrolysis of aqueous solutions containing metals below hydrogen in the reactivity series (like copper), the metal is deposited at the cathode instead of hydrogen.
• The physical state of the metal deposit must be described correctly as a solid.

Common Mistakes

• Writing "hydrogen" or "sulphur" at the anode for dilute sulfuric acid. SO₄²⁻ is not discharged in dilute solutions; OH⁻ from water is discharged to form oxygen.
• Describing the copper deposit as a "solution" or "liquid" instead of a "solid". It is a solid metal depositing on the electrode.
• Confusing the observations at the anode and cathode.

Things to Be Careful About

• Ensure the table columns are matched correctly: anode (+) vs cathode (–).
• For the copper deposit, acceptable observations include "pink solid", "brown solid", "orange solid", or "pink / brown / orange solid". Do not just write "solid" without a colour description.

Walkthrough

Hydrogen gas is identified using the standard "squeaky pop" test. A lighted (burning) splint is brought near the mouth of a test tube containing the gas. The hydrogen ignites and burns rapidly, producing a characteristic popping sound.

Key Takeaways

• The test for hydrogen gas is a lighted splint, not a glowing splint (which is for oxygen).
• The observation must include the word "pops" or "squeaky pop" to score the mark.

Common Mistakes

• Using a "glowing splint" instead of a "lighted splint". A glowing splint relights in oxygen but does not pop in hydrogen.
• Saying the gas "burns" without mentioning the "pop". The pop is the specific observation that confirms hydrogen.

Things to Be Careful About

• Ensure the test and observation are clearly separated. The test is the action (lighted splint), the observation is the result (pops).