Answers to Redox Reactions III. MCQ D D B D C C. 7. C 8. D 9. D 10. A 11. D 12. D. Section B, Question 1 i) -3 ii) 0 iii) +2 iv) +4 v) +5 vi) -3 i) 0 ii) +2 iii) +3 iv) +6 v) +6 i) 0 ii) +4 iii) +6 iv) +6 +6 vi) –2 (Fe 2+ S 2- )
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Read question, you must underline !! Always underline the reactant, NOT product !
a) Zn (s) + 2H+ (aq) Zn2+ (aq) + H2 (g)
The oxidation no. of zinc increases from
0 (in Zn) to +2 (in Zn2+).
b) 2I- (aq) + Cl2 2Cl- (aq) + I2 (s)
The oxidation no. of iodine increases from -1
(in I-) to 0 (in I2).
[Cannot write “from -1 (in 2I-) to …
2 is NOT part of formula!!, write as -1 (in I-)
f)FeO4- + MnO2 + 4H+ Fe3+ + MnO4- 2H2O
The oxidation no. of manganese increases from +4 (in MnO2) to +7 (in MnO4-)
The following are strictly required for calculations (more so for SPA assessment in term 3):
d)20.0 cm3 of solution contain 0.00500 mol of Na2S2O3.
Hence concentration of Na2S2O3
= 0.00500 20/1000 = 0.250 mol/dm3
e) Concentration of Na2S2O3 in g/dm3
= concentration in mol/dm3 x molar mass
= 0.250 x (2x 23 + 2x32 + 3x16) = 39.5 g/dm3 (3 sf)
f) Iodine. The oxidation no. of iodine decreases from 0 (in I2) to -1 (in NaI).