Department of Chemistry CHEM1010 General Chemistry *********************************************** Instructor: Dr. Hong Zhang Foster Hall, Room 221 Tel:

CHEM1010/General Chemistry _________________________________________ Chapter 8. (L31)-Oxidation and Reduction Today’s Outline..Review of oxidants and reductants..Hydrogen and reducing agents (reductants) Hydrogen Other common reductants..Redox chemistry of living things Photosynthesis Respiration

Chapter 8. (L31)-Oxidation and Reduction Oxidants and reductants Oxidation and reduction happen together (they are coupled). Example: CuO + H 2 = Cu + H 2 O Cu: +2 0Cu is reduced H: 0 +1 H is oxidized CuO oxidizes H 2, so CuO is the oxidant (oxidizing agent) H 2 reduces CuO, so H 2 is the reductant (reducing agent)

Chapter 8. (L31)-Oxidation and Reduction Oxidants and reductants Oxidation and reduction happen together (they are coupled). Example: C + O 2 = CO 2 C: 0 +4C is oxidized O: 0 -2 O is reduced O 2 oxidizes C, so O 2 is the oxidant (oxidizing agent) C reduces O 2, C is the reductant (reducing agent)

Chapter 8. (L31)-Oxidation and Reduction Hydrogen: An important reductant..Hydrogen is 0.9% of outer accessible portion of the Earth by mass..Unlike oxygen gas, hydrogen is seldom found as a free gas, but always combined with some elements, as in coal, petroleum, water, biomass as hydrocarbon..Hydrogen is special, in Group IA, but not a metal, a unique element

Chapter 8. (L31)-Oxidation and Reduction Hydrogen: An important reductant..Small amounts of hydrogen can be made by Zn(s) +2HCl = ZnCl 2 + H 2..Commercial quantities of hydrogen are made as by-products of petroleum refining or by the reaction of natural gas with steam..~12 billion kg of hydrogen is produced each year in the USA, with 2/3 is used to produce NH 3

Chapter 8. (L31)-Oxidation and Reduction Hydrogen: An important reductant..Hydrogen can be a new fuel for powering cars: O 2 + 2H 2 = 2H 2 O no pollution from the exhaust pipe Textbook: ppxxvii hydrogen fuel cell cars, the future

Chapter 8. (L31)-Oxidation and Reduction Hydrogen: An important reductant..Hydrogen is a very good reductant to free metals from their ores (e.g., metals oxides) Example: WO 3 + 3H 2 = W + 3H 2 O W:+6 0 H: e - transfer from 3H 2 to W

Chapter 8. (L31)-Oxidation and Reduction Hydrogen: An important reductant..Hydrogen can also reduce many organic compounds Example: C 2 H 4 + H 2 = C 2 H 6 ethylene ethane C:-2 -3 H: e - transfer from H 2 to 2C

Chapter 8. (L31)-Oxidation and Reduction Hydrogen: An important reductant..Hydrogen can also reduce inorganic gases Example: N 2 + 3H 2 = 2NH 3 N:0 -3 H: e - transfer from 3H 2 to 2N

Chapter 8. (L31)-Oxidation and Reduction Hydrogen: An important reductant..Hydrogen can also reduce inorganic gases Example: O 2 + 2H 2 = 2H 2 O O:0 -2 H: e - transfer from 2H 2 to 2O Reactions of hydrogen with gases often needs catalysts such as Pt, Ni, because they can adsorb a large amount of hydrogen gas on their surfaces.

Chapter 8. (L31)-Oxidation and Reduction Other common reducing agents (reductants)..Carbon (e.g., in coal), commonly used to free metals from their oxides: SnO 2 + C = Sn + CO 2 Sn: +4 0 C: e - transfer from C to Sn reductant: C oxidant: Sn in SnO 2

Chapter 8. (L31)-Oxidation and Reduction Other common reducing agents (reductants)..Metals can also be made by heating metal oxides with another more active metal: Cr 2 O 3 + 2Al = 2Cr + Al 2 O 3 Cr: +3 0 Al: e - transfer from 2Al to Cr reductant: Al oxidant: Cr in Cr 2 O 3

Chapter 8. (L31)-Oxidation and Reduction Reduction in photography.. Developer used in black-and-white photography: Film: AgBr coated, upon exposed to light, the following reaction occurs: C 6 H 4 (OH) 2 + 2Ag + + light = C 6 H 4 O 2 + Ag + 2H + hydroquinonesilver metal The film is then treated with ‘hypo’ a solution of sodium thiosulfate (Na 2 S 2 O 3, an O is replaced by a S in Na 2 SO 4 ) to remove the AgBr unexposed to get the negative Then, light will be pass through the negative to induce reaction on a light-sensitive paper to obtain the pictures.

Chapter 8. (L31)-Oxidation and Reduction Antioxidants..In food chemistry, certain reducing agents are called antioxidants. Example: ascorbic acid (Vitamin C) is used to react with oxygen to avoid its reaction with food Browning of apples and pears: oxidation of the fruit by oxygen Vitamins are considered to act as antioxidants in living systems for protection

Chapter 8. (L31)-Oxidation and Reduction Redox chemistry living things..Respiration C 6 H 12 O 6 + 6O 2 = 6CO 2 + 6H 2 O + energy C 6 (H 2 O) 6 + 6O 2 = 6CO 2 + 6H 2 O + energy..Photosynthesis 6CO 2 + 6H 2 O + sunlight energy = C 6 (H 2 O) 6 + 6O 2 Source of oxygen on earth: photosynthesis The “old” earth did not have oxygen in the air before photosynthesis bacteria came into being.

Chapter 8. (L31)-Oxidation and Reduction Quiz Time Hydrogen is (a) a metal; (b) the lightest and unique element; (c) an oxidant; (d) none no above is right.

Chapter 8. (L31)-Oxidation and Reduction Quiz Time Which of the following reactions is the one for hydrogen fuel cell: (a) Cr 2 O 3 + 2Al = 2Cr + Al 2 O 3 ; (b) N 2 + 3H 2 = 2NH 3 ; (c) C + O 2 = CO 2 ; (d) O 2 + 2H 2 = 2H 2 O.

Chapter 8. (L31)-Oxidation and Reduction Quiz Time When a picture is taken, light causes which of the following to happen: (a) damage of the film; (b) drying of water in the film; (c) reduction the Ag + cation in the AgBr salt on the film to Ag particles; (d) burning of the film.

Chapter 8. (L31)-Oxidation and Reduction Quiz Time Which of the following is an antioxidant: (a) water; (b) baking soda; (c) vinegar; (d) Vitamin C.

Chapter 8. (L31)-Oxidation and Reduction Quiz Time Vitamin C is a common name for: (a) phosphoric acid; (b) sulfuric acid; (c) ascorbic acid; (d) hydrochloric acid.

Chapter 8. (L31)-Oxidation and Reduction Quiz Time Vitamin C is: (a) an oxidant; (b) non redox agent; (c) a reductant; (d) none of above is right.

Chapter 8. (L31)-Oxidation and Reduction Quiz Time Which of the following reactions is photosynthesis: (a) C 2 H 4 + H 2 = C 2 H 6 ; (b) O 2 + 2H 2 = 2H 2 O; (c) C 6 (H 2 O) 6 + 6O 2 = 6CO 2 + 6H 2 O + energy; (d) 6CO 2 + 6H 2 O + sunlight energy = C 6 (H 2 O) 6 + 6O 2.