Asia-Pacific Forum on Science Learning and Teaching, Volume 17, Issue 1, Article 8 (Jun., 2016) Anita Yung Li BONG and Tien Tien LEE Form four students’ misconceptions in electrolysis of molten compounds and aqueous solutions Previous Contents

Appendix B

Percentage of misconceptions occurring among the respondents for each question in the test

Item no.	Question	Scientific answers	Misconceptions	Percentage (%)
1 (a)	Identify anode and cathode.	Anode: Electrode X Cathode: Electrode Y	Anode: Electrode Y Cathode: Electrode X	5.0
1 (b)	How to identify anode and cathode?	Positive terminals of batteries is connected to anode while negative terminals to cathode.	Longer battery to anode, shorter to cathode. Left is anode, right is cathode. Oxidation anode, reduction cathode	8.3
1 (c)	Does polarity of batteries affect the position of anode and cathode in diagram?	Yes	No	11.7
1 (d)	Can carbon electrode take place in this electrolysis reaction? Why?	No. It is inert.	Carbon electrode can electrolyze compound. Carbon electrode can conduct electricity. Carbon electrode is universal electrode. Cheap and good conductor. Stable electrode Active electrode Carbon ions can move freely Only carbon electrode react in molten	25.0
1 (e) (i)	Ions move to electrode X and Y during the electrolysis process.	Electrode X: I- ions Electrode Y: K+ ions	Electrode X: K+ ions Electrode Y: I- ions Electrode X: OH- ions Electrode Y: H+ ions	18.3
1 (e) (ii)	Explain your answer in e (i).	Anions, I- ions are negatively charged and they are attracted to anode whereby the positive ends of batteries is attached. Cations, K+ ions are positively charged and they are attracted to cathode whereby the negative end of batteries.	K+ ions and I- ions are the only ions present OH- ion is in lower position then I- ion in electrochemical series. H+ ion is lower position then K+ ion in electrochemical series.	10.0
1 (f)	Write the half equation for anode and cathode.	Anode: 2I- (aq) → I2(g) + 2e- Cathode: K+ (aq) + e- → K(s)	Anode: K+ (aq) + e- → K(s) Cathode: 2I- (aq) → I2(g) + 2e- Anode: 4OH- (aq) → O2 (g) + 2H2O(l) + 4e- Cathode: 2H+ + 2e- → H2 Potassium ion is written as Na+ ion. Wrong equation I- → I2 + 2e- K+ + e- = K	25.0
1 (g) (i)	If the electrolyte is changed to aqueous potassium iodide, what are the ions involved in electrolysis process?	K+ ions, H+ ions, I- ions, OH- ions	Only K+ ions, I- ions. Oxygen Wrong chemical symbol Potassium ions written as Na+ ions Iodide ions written as I2- ions	18.3
1 (g) (ii)	Which ion is selectively discharged to anode and cathode?	Anode: OH- ions Cathode: H+ ions	Anode: H+ ions Cathode: OH- ions Anode: I- ions Cathode: K+ ions Anode: I- ions Cathode: H+ ions	25.0
2 (a) (i)	Identify anode and cathode.	Anode: Electrode X Cathode: Electrode Y	Anode: Electrode Y Cathode: Electrode X	5.0
2 (a) (ii)	How to identify anode and cathode?	Positive terminals of batteries is connected to anode while negative terminals to cathode.	Longer battery to anode, shorter to cathode. Anode at right, cathode at left.	5.0
2 (b) (i)	What ions are present in the electrolysis process above?	Cu2+ ions, H+ ions, Cl- ions, OH- ions	Cu2+ ions, Cl- ions only Cu2+ ions, OH- ions only Wrong chemical symbol Copper ions written as Ca2+	26.7
2 (b) (ii)	What are the sources for the ions you have listed in b (i)?	Copper (II) chloride contributes Cu2+ ions and Cl- ions while water contributes to H+ and OH- ions.	Answers not complete, not related or no answers	0.0
2 (c)	Does water take place in this reaction?	Yes	No	15.0
2 (d) (i)	Which ions move to anode and cathode?	Anode: Cl- ions , OH- ions Cathode: Cu2+ ions , H+ ions	Anode: Cl- ions Cathode: Cu2+ ions Wrong chemical symbol Chloride ions written as I- ions	3.3
2 (d) (ii)	Explain how do you identify the ions which move to anode and cathode?	Anions, which are OH- ions and Cl- ions, are negatively charged thus they are attracted to anode. Cations which are Cu2+ ions and H+ ions are positively charged thus they are attracted to cathode.	Cu2+ only attracted to anode because it is positive charge. Cl- ions only attracted to cathode because it is negative charge.	6.7
2 (e) (i)	Write the half equations for reactions at electrode X and electrode Y.	Anode: 4OH- (aq) → O2 (g) + 2H2O(l) + 4e- Cathode: Cu2+ (aq) + 2e- → Cu (s)	Anode: 2Cl- (aq) → Cl2 (g) + 2e- Anode: Cu2+ (aq) + 2e- → Cu (s) Cathode: 4OH- (aq) → O2 (g) + 2H2O(l) + 4e- Cathode: 2H+ (aq) + 2e- → H2 (s) Wrong equation 2OH- (aq) → O2 (g) + H2(l) + 2e- OH- (aq) → 2H (g) + 2H2O(l) + 4e- Cu2+ (aq) + 2e- = Cu (s)	65.0
2 (e) (ii)	Explain how do you choose which ions is selectively discharged at anode and cathode based on the half equation you have written in e (i).	OH- ions are selectively discharged at anode due to its lower position in electrochemical series as compared to Cl- ions. Cu2+ ions are selectively discharged at cathode due to its lower position in electrochemical series as compared to H+ ions	Cl- ions are selectively discharged at anode due to its higher concentration. H+ ions are selectively discharged at cathode due to its lower position in electrochemical series	33.3
2 (f) (i)	0.05 mol dm-3 of copper (II) chloride solution is replaced with 2 mol dm-3 of copper (II) chloride solution. Predict what will be observed at anode.	A green gas is released at anode and there is pungent smell.	Brown solid is deposited Anode covered by brown solid Gas bubble produce Colourless gas produces.	16.7
2 (f) (ii)	Explain why chlorine is selectively discharged.	Cl- ions is selectively discharged due to higher concentration of Cl- ions in electrolyte as compared to OH- ions.	The solution is dilute. Copper is selectively discharged. Hydrogen is produced. Hydroxide ions is discharged Copper is selected because there are more copper ions in solution. Copper atom is concerted to copper ions.	15.0
2 (f) (iii)	Write a half equation for the observation in f (i).	2 Cl- (aq) → Cl2 (g) + 2e-	Wrong equation Cl- (aq) → Cl2 (g) + 2e- Incorrect end product Cu2+ (aq) + 2e- → Cu (s) 4OH- (aq) → O2 (g) + 2H2O(l) + 4e- Half equation is written as full equation 2Cl- + Cu2+ → Cl2 + Cu	23.3
2 (g) (i)	If both of the carbon electrodes are replaced with 2 copper electrodes, what can you observe on physical appearances of the two copper electrodes after the electrolysis process?	Anode becomes thinner while cathode becomes thicker. Mass of anode decreases while mass of cathode increases.	Anode becomes thicker, cathode becomes thinner. No change.	6.7
2 (g) (ii)	Explain answer in g (i)	Copper anode dissolves to form Cu2+ ions while Cu2+ ions will be selectively discharged at cathode to form copper metal.	Anode loses electrons. Cathode dissolved to release electrons. Chlorine displaces copper.	8.3
2 (g) (iii)	Write half equation for anode and cathode to support your answer in g (i) and g (ii).	Anode: Cu(s) → Cu2+ (aq) + 2e- Cathode: Cu2++ (aq) + 2e- → Cu (s)	Anode: Cu2+ (aq) + 2e- → Cu (s) Cathode: Cu(s) → Cu2+ (aq) + 2e- Wrong equation Cu (s) → Cu+ (aq) + e-	15.0