4.1 Electrolysis
1. What is the definition of electrolysis?
The decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
2. In an electrolytic cell, what is the name of the positive electrode?
The anode.
3. In an electrolytic cell, what is the name of the negative electrode?
The cathode.
4. Define the term "electrolyte" in the context of an electrolytic cell.
The molten or aqueous substance that undergoes electrolysis.
5. Describe how charge is transferred through the external circuit during electrolysis.
By the movement of electrons.
6. Describe the transfer of charge that occurs specifically at the electrodes.
Charge is transferred through the loss or gain of electrons at the electrodes.
7. Describe how charge is transferred through the electrolyte during the process of electrolysis.
By the movement of ions.
8. Identify the products formed at the anode and cathode during the electrolysis of molten lead(II) bromide.
Lead is formed at the cathode and bromine is formed at the anode.
9. What observations are made during the electrolysis of molten lead(II) bromide using inert electrodes?
A silvery liquid (lead) forms at the cathode and a brown gas (bromine) is evolved at the anode.
10. Identify the products formed during the electrolysis of concentrated aqueous sodium chloride.
Hydrogen is formed at the cathode and chlorine is formed at the anode.
11. What observations are made during the electrolysis of concentrated aqueous sodium chloride using inert electrodes?
Colorless gas bubbles (hydrogen) form at the cathode and pale yellow-green gas bubbles (chlorine) form at the anode.
12. Identify the byproducts formed during the electrolysis of concentrated aqueous sodium chloride using inert electrodes?
aqueous sodium hydroxide.
13. Identify the products formed during the electrolysis of dilute sulfuric acid.
Hydrogen is formed at the cathode and oxygen is formed at the anode.
14. What observations are made during the electrolysis of dilute sulfuric acid using inert electrodes?
Colorless gas bubbles (hydrogen) form at the cathode and colorless gas bubbles (oxygen) form at the anode.
15. What materials are typically used to make inert electrodes for these electrolytic processes?
Platinum or carbon/graphite.
16. State the general rule for which substances are formed at the cathode during electrolysis.
Metals or hydrogen are formed at the cathode.
17. State the general rule for which substances are formed at the anode during electrolysis.
Non-metals (other than hydrogen) are formed at the anode.
18. How do you predict the identity of the products at each electrode for the electrolysis of a binary compound in the molten state?
The metal cation will form the metal at the cathode, and the non-metal anion will form the non-metal at the anode.
19. State two primary reasons why metal objects are electroplated.
To improve their appearance and to improve their resistance to corrosion.
20. Describe the general process of how a metal object is electroplated.
The object to be plated is made the cathode, the plating metal is made the anode, and the electrolyte is a solution containing the ions of the plating metal.
21. Identify the products formed during the electrolysis of aqueous copper(II) sulfate when using inert carbon/graphite electrodes.
Copper is formed at the cathode and oxygen is formed at the anode.
22. What observations are made during the electrolysis of aqueous copper(II) sulfate using inert electrodes?
A pink-brown solid (copper) coats the cathode, bubbles of colorless gas (oxygen) form at the anode, and the blue color of the solution fades.
23. Describe the results and observations when aqueous copper(II) sulfate is electrolyzed using copper electrodes.
Copper is transferred from the anode to the cathode; the anode decreases in mass while the cathode increases in mass, and the blue color of the solution remains constant.
24. Predict the anode product for the electrolysis of a halide compound in a concentrated aqueous solution.
The halogen (e.g., chlorine, bromine, or iodine).
25. Predict the anode product for the electrolysis of a halide compound in a dilute aqueous solution.
Oxygen.
26. Construct the reduction half-equations (showing the gain of electrons at the cathode) for lead(II) ions (Pb2+), copper(II) ions (Cu2+), and hydrogen ions (H+).
Lead(II) reduction: Pb2+ + 2e- → Pb, Copper(II) reduction: Cu2+ + 2e- → Cu, Hydrogen reduction: 2H+ + 2e- → H2
27. Construct the oxidation half-equations (showing the loss of electrons at the anode) for hydroxide ions (OH-), chloride ions (Cl-), and copper metal (Cu).
Hydroxide oxidation: 4OH- → O2 + 2H2O + 4e- (standard for dilute aqueous solutions), Chloride oxidation: 2Cl- → Cl2 + 2e-, Copper oxidation: Cu → Cu2+ + 2e- (occurs when using copper anodes)
4.2 Hydrogen–oxygen fuel cells
28. What reactants does a hydrogen–oxygen fuel cell use to produce electricity?
Hydrogen and oxygen.
29. What is the only chemical product formed by a hydrogen–oxygen fuel cell?
Water.
30. State the advantages of using hydrogen–oxygen fuel cells compared with gasoline/petrol engines in vehicles.
They are quiet in operation, more efficient than internal combustion engines, and produce no harmful pollutants or greenhouse gases at the point of use, only water.
31. State the disadvantages of using hydrogen–oxygen fuel cells compared with gasoline/petrol engines in vehicles.
Fuel cells are expensive to manufacture, hydrogen gas is difficult and dangerous to store (requires high pressure or low temperature), and hydrogen is often currently produced from non-renewable fossil fuels.