5 Oxidation and reduction: redox (Topic 3)

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1 Page 91 Questions 1 a) Cr 2+ to Cr 3+ is oxidation because the Cr 2+ ion loses an electron to become a Cr 3+ ion. b) HI to I 2 is oxidation. The I ion in HI loses an electron to become an I atom, which then joins with another I atom to form an I 2 molecule. c) Sn 4+ to Sn 2+ is reduction. The Sn 4+ ion gains two electrons to become an Sn 2+ ion. d) Cl to Cl 2 is oxidation because each Cl ion loses an electron. [e] If a species loses an electron, it becomes more positive or less negative. For example: Cr 2+ Cr 3+ or Cl ½Cl 2 If a species gains electrons, it becomes less positive. For example: Sn 4+ Sn 2+ 2 a) Cr 2+ (aq) Cr 3+ (aq) + e b) 2HI(aq) I 2 (s) + 2H + (aq) + 2e or HI(aq) ½I 2 (s) + H + (aq) + e c) Sn 4+ (aq) + 2e Sn 2+ (aq) d) 2Cl (aq) Cl 2 (g) + 2e or Cl (aq) ½Cl 2 (g) + e [e] Note that all the equations balance for charge. This is why it helps to put the minus on the symbol for the electron. 3 a) IO 3 (aq) + 6H + (aq) + 5e ½I 2 (s) + 3H 2 O(l) or 2IO 3 (aq) + 12H + (aq) + 10e I 2 (s) + 6H 2 O(l) The oxidation number of iodine changes from +5 to 0, so there must be five electrons for each IO 3 ion. The change is reduction, so the electrons are on the left-hand side of the equation. Six hydrogen ions are needed to form three molecules of water with the three oxygen atoms from the IO 3 ion. Note that the overall charge on each side of the equation is zero. b) MnO 2 (s) + 4H + (aq) + 2e Mn 2+ (aq) + 2H 2 O(l) The oxidation number of manganese changes by two from +4 to +2. Therefore, it is reduced. There must be two electrons on the left-hand side of the equation. Four hydrogen ions are needed to form two molecules of water with the two oxygen atoms from MnO 2. Note that the overall charge on each side of the equation is +2. c) VO + 2 (aq) + 2H + (aq) + e VO 2+ (aq) + H 2 O(l) The oxidation number of vanadium in VO + 2 is +5; (x + 2 ( 2) = +1; x = +5). In VO 2+, it is +4; (x + ( 2) = +2; x = +4).

2 Therefore, vanadium is reduced. The change in oxidation number is 1, so one electron is needed on the left-hand side of the equation. Two hydrogen ions are required to form one molecule of water with the oxygen atom removed from the VO + 2 ion. 4 a) The two half-equations are: MnO 4 (aq) + 8H + (aq) + 5e Mn 2+ (aq) + 4H 2 O(l) 2HI(aq) I 2 (s) + 2H + (aq) + 2e The overall equation is found by multiplying the first equation by 2 and the second equation by 5 and then adding them together: 2MnO 4 (aq) + 6H + (aq) + 10HI(aq) 2Mn 2+ (aq) + 5I 2 (s) + 8H 2 O(l) Manganate(VII) ions are reduced to Mn 2+ ions, which is a change in oxidation number of five per manganese. The oxidation number of each iodine atom changes by one, so 5HI are needed per MnO 4. Two MnO 4 ions will produce 8H 2 O, so the equation needs six hydrogen ions to go with the ten hydrogen atoms from 10HI to form the eight molecules of water. Note that the overall charge on each side is +4. [e] The following equation is also acceptable: MnO 4 (aq) + 3H + (aq) + 5HI(aq) Mn 2+ (aq) + 2½I 2 (s) + 4H 2 O(l) b) 2Fe 3+ (aq) + Sn 2+ (aq) 2Fe 2+ (aq) + Sn 4+ (aq) The oxidation number of tin changes by two going from +2 to +4. Therefore, two Fe 3+ ions are needed to balance the change. Note that the equation balances for charge, both sides being +8. c) The two half-equations are: Cr 2 O 2 7 (aq) + 14H + (aq) + 6e 2Cr 3+ (aq) + 7H 2 O(l) NO 2 (g) + H 2 O(l) NO 3 (aq) + e + 2H + (aq) The overall equation is found by multiplying the second equation by 6 and then adding it to the first equation: Cr 2 O 2 7 (aq)+ 2H + (aq) + 6NO 2 (g) 2Cr 3+ (aq) + 6NO 3 (aq) + H 2 O(l) The dichromate(vi) is in an acid solution to provide H +. Dichromate(VI) ions are reduced to Cr 3+ ions, a change in oxidation number of three per chromium atom or six per Cr 2 O 2 7 ion. The oxidation number of nitrogen changes from +4 in NO 2 to +5 in NO 3, so there must be six NO 2 molecules in the equation to balance the change in oxidation numbers. There are 19 oxygen atoms on the left-hand side of the equation, so there must be one water molecule on the right-hand side and, hence, two hydrogen ions on the left. Note that the overall charge on each side is zero. 5 a) The oxidation number of iodine in I 2 O 7 is +7.

3 Each oxygen is 2, so the seven oxygen atoms have a total of 14. Thus each of the two iodine atoms is +7. b) The oxidation number of iodine in IO 3 is +5. The three oxygen atoms each have the oxidation number 2, giving a total of 6. The charge on the ion is 1, so: x + 3( 2) = 1 x = +6 1 = +5 c) The oxidation number of iodine in KIO 4 is +7. The oxidation number of potassium is +1; each oxygen is 2, so: +1 + x + 4( 2) = 0 x = +8 1 = +7 d) The oxidation number of iodine in Ba(IO 2 ) 2 is +3. The oxidation number of barium is +2, each oxygen is 2, so: x + 4( 2) = 0 2x = +8 2 = +6 x = +3 6 a) The vanadium in V 3+ is in the +3 oxidation state. b) The vanadium in VO + 2 is in the +5 state (see the answer to Question 3c). c) The vanadium in VO 2+ is in the +4 state (see the answer to Question 3c). 7 a) The oxidation number of nitrogen in NO 2 is +4. The oxidation number of each oxygen atom is 2, so two oxygen atoms total 4. The molecule is neutral, so the oxidation number of nitrogen is +4. b) The oxidation number of nitrogen in NO 2 is +3. The oxidation number of oxygen is 2, so two oxygen atoms total 4. The ion is 1, so: x 4 = 1 x = +3 c) The oxidation number of nitrogen in N 2 O 5 is +5. The oxidation number of each oxygen atom is 2, so five oxygen atoms total 10. The molecule is neutral, so: 2x 10 = 0 2x = +10 x = +5 d) The oxidation number of nitrogen in N 2 H 4 is 2. The oxidation number of each hydrogen atom is +1, so four hydrogen atoms total +4. Therefore, the oxidation number of each nitrogen atom is 2.

4 8 a) The oxidation number of bromine in KBr is 1. In KBrO 3 the oxidation number of bromine is x + 3( 2) = 0 x = +6 1 = +5 In Br 2 the oxidation number of bromine is zero. b) An oxidising agent is reduced. The oxidation number of bromine changes from +5 in KBrO 3 to zero in Br 2. The oxidation number has decreased, so the bromine in the KBrO 3 has been reduced. Therefore, KBrO 3 is the oxidising agent. It has oxidised the bromine in KBr from the 1 state to zero. 9 The oxidation number of the carbon atoms in the C 2 O 2 4 ion is +3: 2x + 4( 2) = 2 so, 2x = = +6 x = +3 The carbon atom in CO 2 is in the +4 state, so each carbon atom changes by +1. The total change in oxidation number per C 2 O 2 4 ion is The molecular equation is: Br 2 + 2NaOH NaBr + NaOBr + H 2 O Converting this to ions gives: Br 2 (aq) + 2Na + (aq) + 2OH (aq) Na + (aq) + Br (aq) + Na + (aq) + OBr (aq) + H 2 O(l) Cancelling the spectator ions gives the final ionic equation: Br 2 (aq) + 2OH (aq) Br (aq) + OBr (aq) + H 2 O(l) [e] Water and bromine are not ionic, and so must be written as molecules. 11 a) Chlorine is reduced. The half-equation is: Cl 2 (g) + 2e 2Cl (aq) The bromide ions in NaBr are oxidised. The half-equation is: 2Br (aq) Br 2 (l) + 2e The chlorine gains these electrons, so it is the oxidising agent (becoming reduced to Cl ions). The bromine has given electrons to the chlorine, so the bromide ions are the reducing agent (becoming oxidised to Br atoms, which form Br 2 ). [e] Remember that, in a redox reaction, oxidising agents are reduced and reducing agents are oxidised. b) The iron(iii) ions are reduced to iron(ii) ions: 2Fe 3+ (aq) + 2e 2Fe 2+ (aq)

5 The hydrogensulfite(iv) ions, HSO 3, are oxidised to sulfate(vi) ions, SO 2 4 : HSO 3 (aq) + H 2 O(l) 3H + (aq) + SO 2 4 (aq) + 2e The Fe 3+ ions have been reduced, so they are the oxidising agent. The HSO 3 ions have been oxidised, so they are the reducing agent. 12 a) Fe Fe e and Cl 2 + 2e 2Cl b) 2Fe + 3Cl 2 2Fe Cl Page 92 Exam practice questions 1 a) i) Disproportionation is when an atom in a single species ( ) is simultaneously oxidised and reduced ( ). ii) The oxidation number of an atom in a molecule or ion is the charge that the atom would have ( ) if the substance were fully ionic ( ). b) C ( ) c) C ( ) [e] An element in two species going to being in one species is sometimes called reverse disproportionation. d) B ( ) [e] 2Na = +2, 6O = 12, so 4S = +10, S = +2½ e) A ( ) 2 a) Disproportionation is when an atom in a single species is simultaneously oxidised and reduced ( ). Manganese is in two species, one in the +4 and the other in the +7 state and goes to being in one species in the +6 ( ), so it is not disproportionation. b) i) +1 ( ) ii) +3 ( ) iii) +5 ( ) c) M r of AgCl = g mol 1 ( ) moles of AgCl = g/143.4 g mol 1 = mol = moles of Cl (aq) ( ) moles of Cl 2 (g) = dm 3 /24 dm 3 mol 1 = mol ( ) ratio Cl 2 :Cl = : = 0.6:1 = 3:5 ( ) Equation has 3Cl 2 producing 5Cl ( ) 3Cl 2 + 6OH 5Cl + ClO x + 3H 2 O x must be 3 ( ), so equation is 3Cl 2 + 6OH 5Cl + ClO 3 + 3H 2 O ( ) d) i) ClO (aq) + H 2 O(l) + e [or ] ½ Cl 2 (g) + 2OH (aq) [or twice this] ( )

6 Fe 2+ (aq) [or ] Fe 3+ (aq) ( ) ii) ClO (aq) + H 2 O(l) + Fe 2+ (aq) [or ] ½Cl 2 + Fe 3+ (aq) + 2OH (aq) ( ) [e] Penalise lack of state symbols once only.