PROPOSITIONAL KNOWLEDGE STATEMENTS

The propositional knowledge statements were identified from common textbooks used by the students. These statements were written for the content and concepts relevant to the topic on electrolysis. The statements were written at a level of sophistication appropriate to the chemistry students in Form Four. There were 26 propositional knowledge statements in the first draft (Figures 3.3a and 3.3b).

1. Electrolytes are substances that conduct electricity either in the molten state or in aqueous solution, and undergo chemical changes in the process.

2. Non-electrolytes are substances that do not conduct electricity either in the molten state or in aqueous solution.

3. Electrolytes consist of positively charged ions (cations) and negatively charged ions (anions).

4. The electrical conductivity of electrolytes is due to the presence of free moving ions in the electrolytes.

5. Electrolysis is a process in which an electrolyte is decomposed when an electric current passes through it.

6. An electrolytic cell consists of a battery, an electrolyte, and two electrodes.

7. In a complete closed electric circuit, electrons flow from the negative terminal to the positive terminal of the battery.

8. Electrodes are conductors in the form of wires, rods or plates that enable electricity to pass through an electrolyte during electrolysis.

9. Electrodes that do not take part in chemical reactions during electrolysis are known as inert electrodes. Examples are graphite and platinum electrodes.

10. Electrodes that take part in chemical reactions during electrolysis are known as active electrodes. Examples are metal electrodes such as copper and silver.

11. The electrode that is connected to the positive terminal of a battery is called the anode while the electrode that is connected to the negative terminal of a battery is called the cathode.

12. During electrolysis, cations (positive ions) move towards the cathode, the negatively- charged electrode whereas anions (negative ions) move towards the anode, the positively-charged electrode.

Figure 3.3a List of propositional knowledge statements (1-12) developed for the topic of electrolysis

13. Cations are discharged at the cathode by accepting electron(s) from the cathode, whereas anions are discharged at the anode by donating electron(s) to the anode. 14. Reduction occurs at the cathode and oxidation occurs at the anode during electrolysis. 15. Electrolysis of aqueous copper(II) sulphate using inert carbon electrodes will produce

copper at the cathode, and oxygen gas at the anode.

16. Electrolysis of dilute sulphuric acid using inert carbon electrodes will produce hydrogen gas at the cathode, and oxygen gas at the anode.

17. The selective discharge of ions depends on the positions of ions in the electrochemical series, the concentration of ions in the electrolyte, and the nature of electrodes used in the electrolysis.

18. The electrochemical series is a list of ions arranged in order of their tendency to be discharged during electrolysis. The lower the position of an ion in the electrochemical series, the higher is the tendency for the ion to be discharged.

19. Industrial applications of electrolysis include extraction, purification, and electroplating of metals.

20. Highly reactive metals can only be extracted from their ores by electrolysis.

21. In the purification of metals using electrolysis, the impure metal is the anode, the pure metal is the cathode, and the electrolyte is an aqueous salt solution of the metal. 22. Electroplating is the process of depositing a layer of metal on another substance using

electrolysis.

23. Objects are electroplated to protect them from corrosion and to give them an attractive appearance.

24. To electroplate an object with a metal, the cathode is the object to be plated, the anode is a piece of the pure plating metal and the electrolyte shall contain ions of the plating metal.

25. Electrolysis is an expensive process because it requires a large amount of energy. 26. The industrial use of electrolysis may cause pollution as a result of the production of

waste chemicals from the electroplating industry that are released into the environment.

Figure 3.3b List of propositional knowledge statements (13-26) developed for the topic of electrolysis

Changes were made to the original list of propositional knowledge statements to improve the accuracy of the statements required for the understanding by the students. Fifteen additional propositional knowledge statements were needed, six

statements were split up into more statements, some minor word and phrase changes were made to various propositions. None of the original propositions were deleted. The revised list of propositions containing forty one propositional knowledge statements is shown in Figures 3.4a, 3.4b and 3.4c.

1. Electrolytes are substances that conduct electricity either in the molten state or in aqueous solution, and undergo chemical changes in the process.

2. Non-electrolytes are substances that do not conduct electricity either in the molten state or in aqueous solution.

3. The electrical conductivity of electrolytes is due to the presence of free moving ions in the electrolytes.

4. Electrolysis is a process in which an electrolyte is decomposed when an electric current passes through it.

5. An electrolytic cell consists of a battery, an electrolyte, and two electrodes.

6. Electrodes are conductors in the form of wires, rods or plates that enable electricity to pass through an electrolyte during electrolysis.

7. The electrode that is connected to the positive terminal of a battery is called the anode. 8. The electrode that is connected to the negative terminal of a battery is called the

cathode.

9. Electrodes that do not take part in chemical reactions during electrolysis are known as inert electrodes.

10. Examples of inert electrodes are carbon (graphite) and platinum electrodes.

11. Electrodes that take part in chemical reactions during electrolysis are known as active electrodes.

12. Examples of active electrodes are metal electrodes such as copper and silver.

13. Electrolytes consist of positively charged ions (cations) and negatively charged ions (anions).

14. In a complete closed electric circuit, electrons flow from the negative terminal to the positive terminal of the battery.

15. During electrolysis, cations (positive ions) move towards the cathode, the negatively- charged electrode whereas anions (negative ions) move towards the anode, the positively-charged electrode.

Figure 3.4a Revised list of propositions and content knowledge statements (1-15) of electrolysis

16. Cations are discharged at the cathode by accepting electron(s) from the cathode, whereas anions are discharged at the anode by donating electron(s) to the anode. 17. In the electrolysis of molten magnesium oxide using carbon electrodes, magnesium is

produced at the cathode and oxygen at the anode.

18. In the electrolysis of molten lead(II) bromide using graphite electrodes, lead is produced at the cathode and bromine gas at the anode.

19. Reduction occurs at the cathode and oxidation occurs at the anode during electrolysis. 20. An aqueous solution of a compound is a solution produced when the compound is

dissolved in water.

21. An aqueous solution of a compound contains anions and cations of the compound, hydrogen ions, and hydroxide ions from the partial dissociation of water molecules. 22. During electrolysis of an aqueous solution of a compound, two different types of

cations move towards the cathode, which are cations of the compound and hydrogen ions.

23. During electrolysis of an aqueous solution of a compound, two different types of anions move towards the anode, which are anions of the compound and hydroxide ions.

24. The selective discharge of ions depends on the positions of ions in the electrochemical series, the concentration of ions in the electrolyte, and the nature of electrodes used in the electrolysis.

25. The electrochemical series is a list of ions arranged in order of their tendency to be discharged during electrolysis. The lower the position of an ion in the electrochemical series, the higher is the tendency for the ion to be discharged.

26. Electrolysis of dilute sulphuric acid using inert carbon electrodes will produce hydrogen gas at the cathode, and oxygen gas at the anode.

27. The concentration of sulphuric acid increases gradually as water is decomposed to hydrogen gas and oxygen gas.

28. Electrolysis of aqueous copper(II) sulphate using inert carbon electrodes will produce copper at the cathode, and oxygen gas at the anode.

29. The aqueous solution of copper (II) sulphate consists of copper(II) ions, sulphate ions, hydrogen ions, and hydroxide ions that move freely.

30. During electrolysis of aqueous solution of copper(II) sulphate, the copper(II) and hydrogen ions move to the cathode. Copper(II) ions are selectively discharged to form copper metal.

Figure 3.4b Revised list of propositions and content knowledge statements (16-30) of electrolysis

31. During electrolysis of aqueous solution of copper(II) sulphate, the sulphate and hydroxide ions move to the anode. The hydroxide ions are selectively discharged to form oxygen and water.

32. The intensity of the blue colour of the electrolyte decreases as the concentration of blue copper(II) ions decreases when more copper is deposited on the cathode.

33. The electrolyte becomes more acidic because of the hydrogen and sulphate ions left. 34. Industrial applications of electrolysis include extraction, purification, and

electroplating of metals.

35. Highly reactive metals can only be extracted from their ores by electrolysis.

36. In the purification of metals using electrolysis, the impure metal is the anode, the pure metal is the cathode, and the electrolyte is an aqueous salt solution of the metal. 37. Electroplating is the process of depositing a layer of metal on another substance using

electrolysis.

38. Objects are electroplated to protect them from corrosion and to give them an attractive appearance.

39. To electroplate an object with a metal, the cathode is the object to be plated, the anode is a piece of the pure plating metal and the electrolyte shall contain ions of the plating metal.

40. Electrolysis is an expensive process because it requires a large amount of energy. 41. The industrial use of electrolysis may cause pollution as a result of the production of

waste chemicals from the electroplating industry that are released into the environment.

Figure 3.4c Revised list of propositions and content knowledge statements (31-41) of electrolysis