2 00:00:10,449 --> 00:00:16,183 To understand how organic carbon saves energy, stores energy, 3 00:00:16,183 --> 00:00:21,165 we have to know something about what's called the oxidation 4 00:00:21,165 --> 00:00:26,420 and reduction of the carbon. There are some simple rules. 5 00:00:27,810 --> 00:00:30,530 1. When you put together a molecule, that has an 6 00:00:30,530 --> 00:00:35,680 oxygen in it, that oxygen is greedy for two electrons 7 00:00:35,680 --> 00:00:39,170 in the chemical bond. It pulls them very tightly towards itself. 8 00:00:39,170 --> 00:00:42,810 Those electrons have a negative charge and so, 9 00:00:42,810 --> 00:00:46,714 this oxygen has whats called, an Oxidation state 10 00:00:46,714 --> 00:00:50,180 of -2 equal, to the -2 charge of 11 00:00:50,180 --> 00:00:52,850 those two electrons that it's pulling close to itself. 12 00:00:54,020 --> 00:00:57,630 2. Hydrogen, in contrast, is very generous with 13 00:00:57,630 --> 00:01:00,290 one electron, when it forms a chemical bond. 14 00:01:00,290 --> 00:01:04,080 Hydrogen in say, water, where its bond is with 16 00:01:04,080 --> 00:01:07,490 oxygen, will have a deficit of one electron. 17 00:01:07,490 --> 00:01:09,420 And so that means that it has an 18 00:01:09,420 --> 00:01:13,900 oxidation state, kind of like a charge, of plus one. 19 00:01:13,900 --> 00:01:15,490 However, there's an exception to this. 20 00:01:15,490 --> 00:01:20,890 If you have two identical atoms together in a bond, like O2, two oxygens 21 00:01:20,890 --> 00:01:24,760 are the same as each other, in that case they have to share equally. 22 00:01:24,760 --> 00:01:26,320 In that case, the oxidation 23 00:01:26,320 --> 00:01:29,480 state of those oxygens is each equal to zero. 24 00:01:31,160 --> 00:01:35,460 And, then the other thing you have to know is that, the sum of the oxidation states 25 00:01:35,460 --> 00:01:40,180 of all of the atoms in a molecule has to equal the charge in the molecule. 26 00:01:40,180 --> 00:01:44,630 Using these simple rules, we can look at the various forms of 27 00:01:44,630 --> 00:01:47,480 carbon that we're talking about and understand 28 00:01:47,480 --> 00:01:51,660 how its oxidation state changes, when the photosynthesis 29 00:01:51,660 --> 00:01:55,990 reaction happens. This is how the energy is stored. 30 00:01:55,990 --> 00:01:57,660 You can understand the energy 31 00:01:57,660 --> 00:02:00,680 storage, by looking at this bookkeeping of electrons. 32 00:02:04,880 --> 00:02:08,630 The most oxidized form of carbon is CO2. 33 00:02:10,480 --> 00:02:15,280 Each oxygen here has minus two oxidation state, 34 00:02:15,280 --> 00:02:20,010 because each oxygen is greedy for those two electrons and there are two of 35 00:02:20,010 --> 00:02:25,490 them, so there's a total of minus four oxidation state, in those oxygens. 36 00:02:25,490 --> 00:02:30,170 The total charge on CO2 is zero, so carbon has to fill in the gap. 37 00:02:30,170 --> 00:02:34,290 And it has to have an oxidation state of plus four, in order to make this work. 38 00:02:36,810 --> 00:02:43,420 Methane, CH4, is the most reduced of the carbon species. That's because 39 00:02:43,420 --> 00:02:50,790 each hydrogen that it's bonded to is donating one electron, so that's a charge 40 00:02:50,790 --> 00:02:55,620 on the hydrogen of plus one, and there are four hydrogens, so then the carbon has to 41 00:02:55,620 --> 00:03:01,840 be minus four, in order to have the overall charge on the molecule be zero. 42 00:03:01,840 --> 00:03:07,230 Going from CO2, the oxidation state of carbon is plus four. 43 00:03:08,340 --> 00:03:09,470 Here it's minus four. 44 00:03:11,160 --> 00:03:13,970 The word oxidation kind of makes 45 00:03:13,970 --> 00:03:16,220 sense, just by looking at where the oxygens 46 00:03:16,220 --> 00:03:20,710 are. Carbon CO2 is oxidized carbon, Oxidized iron 47 00:03:20,710 --> 00:03:23,740 is rust which is an iron oxide. 48 00:03:23,740 --> 00:03:25,030 That makes sense, but if 49 00:03:25,030 --> 00:03:27,080 you didn't know about the electrons, you wouldn't 50 00:03:27,080 --> 00:03:29,670 make any sense out of the word reduced. 51 00:03:29,670 --> 00:03:35,040 The word reduced comes from the oxidation state on the carbon. 52 00:03:35,040 --> 00:03:37,910 This number is reduced, relative to that one. 53 00:03:37,910 --> 00:03:41,330 This is a reduced carbon atom. 54 00:03:42,470 --> 00:03:45,380 CH2O is actually right in the middle of these two. 55 00:03:45,380 --> 00:03:50,810 It's half way between the oxidized and the reduced ends of the spectrum. 56 00:03:50,810 --> 00:03:52,260 Oxygen has a charge 57 00:03:52,260 --> 00:03:56,750 of minus two, there's two positive charges on the hydrogens, 58 00:03:56,750 --> 00:03:59,780 so, the oxidation state of this carbon here is zero. 59 00:04:00,820 --> 00:04:05,760 Now the reason why we're doing all this is because, the oxidized form of 60 00:04:05,760 --> 00:04:11,470 carbon is the lowest energy form of carbon, if you have oxygen around. 61 00:04:11,470 --> 00:04:14,720 Carbon in CO2 is like a dead battery. 62 00:04:14,720 --> 00:04:17,400 You couldn't get any energy out of it, because it's already dead. 63 00:04:19,320 --> 00:04:24,450 The more reduced the carbon gets, the more energy it's storing. 64 00:04:24,450 --> 00:04:27,020 This will be important for fossil fuels, which we'll learn 65 00:04:27,020 --> 00:04:32,270 about next because some fossil fuels are more reduced, like natural gasses. 66 00:04:32,270 --> 00:04:34,810 Methane is very reduced. 67 00:04:34,810 --> 00:04:39,140 Oil is in between, and coal is about here. 68 00:04:39,140 --> 00:04:41,670 There are different amounts of energy 69 00:04:41,670 --> 00:04:44,710 per carbon, in the different types of fossil fuels, 70 00:04:44,710 --> 00:04:48,540 because of the different oxidation states of the carbon. 71 00:04:48,540 --> 00:04:52,190 You can also see why the different fossil fuels yield different 72 00:04:52,190 --> 00:04:56,420 amounts of energy, just by looking at the reaction of combustion. 73 00:04:56,420 --> 00:05:03,260 Coal is mostly just elemental carbon and it combines with O2 to make CO2. 74 00:05:03,260 --> 00:05:08,336 The formation of the CO2 is what yields the energy. 75 00:05:08,336 --> 00:05:10,584 Organic Carbon 76 00:05:10,584 --> 00:05:15,510 is the same oxidation state as coal, but it has got hydrogens and oxygens 77 00:05:15,510 --> 00:05:19,850 in it, the H2 and the O here balance each other In the 78 00:05:19,850 --> 00:05:25,270 oxidation state because it's 79 00:05:25,270 --> 00:05:29,290 pulling two electrons closer, and two electrons are donating to electron. 80 00:05:29,290 --> 00:05:35,810 This part washes, when you burn that, you get CO2 plus the water 81 00:05:35,810 --> 00:05:38,350 but there is also water here, so you are not 82 00:05:38,350 --> 00:05:42,810 really getting energy from H2O on the two sides there. 83 00:05:42,810 --> 00:05:44,610 This is like coal. 84 00:05:44,610 --> 00:05:48,030 You just have this H2O kind of tagging along on both sides. 85 00:05:48,030 --> 00:05:53,190 It doesn't really effect things. But methane now, CH4, when you combust 86 00:05:53,190 --> 00:05:59,200 that, you need more oxygen and you make this water that didn't exist before. 87 00:05:59,200 --> 00:06:00,910 And so the extra energy 88 00:06:00,910 --> 00:06:06,357 you get from methane is basically the energy you get from making water.