M1 Part 3 - Oxidation States and Redox Reactions.pdf

A B M DULE 1  transfer of a proton = ACID-BASE reaction HCl (aq) + H2O (l) → H3O+ (aq) + Cl– (aq) formation of a solid = PRECIPITATION reaction NaI (aq) + AgNO3 (aq) → NaNO3 (aq) + AgI (s) transfer of electrons = REDOX reaction Zn (s) + Cu2+ (aq)...

A B M DULE 1  transfer of a proton = ACID-BASE reaction HCl (aq) + H2O (l) → H3O+ (aq) + Cl– (aq) formation of a solid = PRECIPITATION reaction NaI (aq) + AgNO3 (aq) → NaNO3 (aq) + AgI (s) transfer of electrons = REDOX reaction Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s) CHM1311 Oxidation States and Redox Reactions 2  an oxidation-reduction, or REDOX, reaction involves the transfer of electrons causes changes to the OXIDATION STATE oxidation state (or oxidation number) = form of electron bookkeeping = charge on an atom IF the bonding electrons were theoretically taken away (not shared) CHM1311 Oxidation States and Redox Reactions 3  Charge = measurable parameter = #protons – #electrons Oxidation state = theoretical parameter – what if we took away the bonding electrons? Na+ H-Cl charge = +1 no charge oxidation state of H = +1 oxidation state = +1 oxidation state of Cl = –1 CHM1311 Oxidation States and Redox Reactions 4  Ag(s) OS = 0 * Na+ OS = +1 * * ∑OS = charge 1 +1 2 +2 H +1 F –1 O –2 X –1 CHM1311 Oxidation States and Redox Reactions 5  if two rules contradict each other, follow the rule that appears higher in the list for a multi-atom species, label the easy oxidation states, and solve for the unknown atoms CHM1311 Oxidation States and Redox Reactions 6 What are the oxidation states of the elements in each of the following? a) S8 S= b) P2O5 P= O= c) CrO42– Cr = O= d) CaH2 Ca = H= CHM1311 Oxidation States and Redox Reactions 7 What are the oxidation states of the elements in each of the following? 1 x 2 a) S8 S= H F 8x = 0 O X CHM1311 Oxidation States and Redox Reactions 8 What are the oxidation states of the elements in each of the following? 1 x -2 2 b) P2O5 P= O= H F 2x + 5(–2) = 0 O x = +5 X CHM1311 Oxidation States and Redox Reactions 9 What are the oxidation states of the elements in each of the following? 1 x -2 2 c) CrO42– Cr = O= H F x + 4(–2) = –2 O x = +6 X CHM1311 Oxidation States and Redox Reactions 10 What are the oxidation states of the elements in each of the following? 1 +2 x Ca is +2 2 d) CaH2 Ca = H= H H is +1 F (+2) + 2x = 0 O Compounds in which x = –1 hydrogen has a –1 ox. state X are called HYDRIDES CHM1311 Oxidation States and Redox Reactions 11 What are the oxidation states of each element in the following? a) SO2 b) XeF4 c) BaO2 d) K2CO3 CHM1311 Oxidation States and Redox Reactions 12 In which of the following species does carbon have an oxidation state of +4? 1. CO 2. CO32– 3. C2O42– 4. CH4 CHM1311 Oxidation States and Redox Reactions 13  2 e– Zn + Cu2+ Zn2+ + Cu the Zn atom lost electrons OXIDATION the Cu2+ ion gained electrons REDUCTION If something has been oxidized, then something else has also been reduced. CHM1311 Oxidation States and Redox Reactions 15  oxidation is loss OIL RIG reduction is gain LEO says GER loss of electrons is oxidation gain of electrons is reduction CHM1311 Oxidation States and Redox Reactions 16 Some terminology: An oxidizing agent (oxidant): ox state – is the reactant that induces oxidation in the other reactant – its own oxidation state decreases in a redox reaction (it gets reduced) A reducing agent (reductant): – is the reactant that induces reduction in the other ox state reactant – its own oxidation state increases in a redox reaction (it gets oxidized) CHM1311 Oxidation States and Redox Reactions 17  We can determine which species is oxidized and which species is reduced by looking at the oxidation states. +3 –2 +2 –2 0 +4 –2 Fe2O3 + 3 CO → 2 Fe + 3 CO2 Fe is reduced C is oxidized Fe2O3 is the OXIDIZING AGENT CO is the REDUCING AGENT CHM1311 Oxidation States and Redox Reactions 18  Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s) Zn: 0 to +2 Cu2+: +2 to 0 ox. state increased ox. state decreased ∴ it got oxidized ∴ it got reduced ∴ it is the reducing agent ∴ it is the oxidizing agent CHM1311 Oxidation States and Redox Reactions 19 A redox reaction is represented by two half-reactions. Oxidation: Zn(s) → Zn2+(aq) + 2 e– Reduction: Cu2+(aq) + 2 e– → Cu(s) Overall: Cu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq) Balancing redox reactions depends on whether it is an acidic or basic medium… CHM1311 Oxidation States and Redox Reactions 20 The Half-Equation Method: – the overall reaction is separated into two half-reactions: an oxidation and a reduction – each half-reaction is balanced – the balanced half-reactions are then added to give the overall reaction CHM1311 Oxidation States and Redox Reactions 21 STEP (1): Write out the net ionic form of the reaction. STEP (2): Separate into two half-reactions. STEP (3): Balance all elements except O and H in the half-reactions. STEP (4): Add enough molecules of H2O to balance the O atoms. STEP (5): Add enough H+ ions to balance the H atoms. STEP (6): Balance the net charge on each side by adding electrons. STEP (7): If necessary, multiply the half-reactions by the LCD. STEP (8): Add the two half-reactions together and simplify. STEP (9): Verify your final reaction. CHM1311 Oxidation States and Redox Reactions 22 Balance the following redox reaction. 0 +5 –2 +2 +2 –2 Cu(s) + NO3 –(aq) ® Cu2+(aq) + NO(g) Cu is oxidized N is reduced Oxidation: Cu → Cu2+ + 2 e– X3 0 +2 Reduction: 3 e– + 4 H+ + NO3– → NO + 2 H2O X2 +3 0 CHM1311 Oxidation States and Redox Reactions 23 Balance the following redox reaction. 0 +5 –2 +2 +2 –2 Cu(s) + NO3 –(aq) ® Cu2+(aq) + NO(g) Cu is oxidized N is reduced Oxidation: 3 Cu → 3 Cu2+ + 6 e– Reduction: 6 e– + 8 H+ + 2 NO3– → 2 NO + 4 H2O Overall: 8 H+ + 2 NO3– + 3 Cu → 2 NO + 4 H2O + 3 Cu2+ CHM1311 Oxidation States and Redox Reactions 24 Is the following half-reaction an oxidation or reduction? ClO2 (g) ® ClO3– (aq) (acidic solution) 1. oxidation 2. reduction 3. I’m not sure CHM1311 Oxidation States and Redox Reactions 25 STEP (1): Write out the net ionic form of the reaction. STEP (2): Separate into two half-reactions. STEP (3): Balance all elements except O and H in the half-reactions. STEP (4): Add enough molecules of H2O to balance the O atoms. STEP (5): Add enough H+ ions to balance the H atoms. STEP (6): Balance the net charge on each side by adding electrons. STEP (7): If necessary, multiply the half-reactions by the LCD. STEP (8): Add the two half-reactions together and simplify. STEP (9): Verify your final reaction. CHM1311 Oxidation States and Redox Reactions 27  when the reaction is in basic solution, STEP (5) is modified as follows: – for every H+, add an OH– on each side of the half-reaction – where H+ and OH– appear on the same side, combine these two ions to make H2O CHM1311 Oxidation States and Redox Reactions 28 Balance the following redox reaction in basic medium. Cu2O (s) + MnO4– (aq) à Cu2+ (aq) + Mn2+ (aq) CHM1311 Oxidation States and Redox Reactions 29  oxidation states reduction vs. oxidation half-reactions oxidizing vs. reducing agents balancing redox reactions CHM1311 Oxidation States and Redox Reactions 30 In an environmental test for lead (207.2 g/mol) in soil, the lead is oxidized using permanganate ions in acidic solution, according to the following (unbalanced) reaction: Pb (s) + MnO4– (aq) à Pb2+ (aq) + Mn2+(aq) A 25.0 g sample of soil was found to react with 14.92 mL of a 0.150 M KMnO4 solution. What was the percent composition by mass of lead in the soil sample? CHM1311 Oxidation States and Redox Reactions 31