These eight questions are original and move from recall to comparison. Write your answer first, then open the model answer.
Use the timed original practice session builder to attempt them against the clock. The lessons are listed in electrochemistry.
Questions 1 to 4: conduction and electrolysis
Question 1. Solid potassium iodide does not conduct electricity, but its solution does. Explain.
Answer
In the solid, the K⁺ and I⁻ ions are held in fixed positions and cannot move. In solution, the ions are free to move, so they carry charge through the liquid.
Question 2. A student says, “Electrons travel through the solution to complete the circuit.” State what is wrong.
Answer
Electrons travel in the wire and electrodes. In the electrolyte, ions carry the charge. Electrons are handed to and from ions at the electrodes.
Question 3. Copper(II) sulfate solution is electrolysed with carbon electrodes. Name the product at the cathode and state one observation.
Answer
Cu²⁺ ions are attracted to the cathode and gain electrons: Cu²⁺ + 2e⁻ → Cu. The product is copper, seen as a pink-brown coating on the cathode.
Question 4. Dilute sulfuric acid is electrolysed. Twice as much gas collects at one electrode as at the other. Name the gas collected in the larger volume and the electrode.
Answer
Hydrogen collects in the larger volume, at the cathode. Water contains 2 hydrogen atoms for every 1 oxygen atom, so the ratio of volumes of hydrogen to oxygen is 2 : 1.
Questions 5 to 8: cells and applications
Question 5. An invented table lists cell voltages with the same electrolyte: zinc and copper 1.1 V; iron and copper 0.7 V; magnesium and zinc 0.9 V. Identify the negative terminal in the magnesium and zinc cell.
Answer
Magnesium is the negative terminal, because it is more reactive than zinc and loses electrons more readily.
Question 6. Using the table in Question 5, state which cell gives the smallest voltage and one thing the table does not show.
Answer
The iron and copper cell gives the smallest voltage, at 0.7 V. The table does not show how long each cell lasts or how much each costs.
Question 7. A nickel key ring is to be coated with zinc by electroplating. State what it is connected to and which ion is in the electrolyte.
Answer
The key ring is connected to the negative terminal, so it is the cathode. The electrolyte contains Zn²⁺ ions, for example zinc sulfate solution.
Question 8. In Question 7, the zinc bar at the positive terminal gets smaller over time. Explain why, and why the concentration of the solution stays about the same.
Answer
Zinc atoms at the anode lose electrons and dissolve as Zn²⁺ ions. These replace the Zn²⁺ ions that are discharged at the cathode, so the concentration of the solution stays nearly constant.
If you got these wrong, where do you go next?
Questions 1 and 2 point to explaining current flow in an electrolyte. Questions 3 and 4 point to interpreting electrolysis observations.
Questions 5 and 6 point to comparing simple cell designs, and questions 7 and 8 point to explaining applications. If the same type keeps returning, see online one-to-one Science tuition.