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In this final episode of Crash Course Chemistry, Hank takes us on a tour of the The Global Carbon Cycle and how it all works. From Carbon Fixation to Redox Reactions, it’s all contained within!
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Transcript Provided by YouTube:
00:00
You and I began this journey a year ago when I introduced you to this spectacular method for understanding the world.
00:07
While physics explains the mechanics of the universe,
00:09
and biology describes how life functions and interacts with itself, chemistry lies between.
00:14
In our next course we are excited to announce that we’re gonna to be exploring psychology,
00:18
the study of human mental functions and behavior.
00:21
Chemistry has brought us great insights into even such complicated sciences as psychology,
00:26
like how neurotransmitters allow us to feel anxiety, and hormones give us our desires.
00:30
But before we drill down into the level of your brain and blood stream,
00:33
I want to close out this course with a talk about chemistry on a global scale.
00:38
Certainly larger than your mind – though I’m not sure it’s more complicated!
00:41
It is complicated though. And also terrifying.
00:44
Let’s talk about a little thing called “Global Warming”
00:46
and the spiking atmospheric CO2 concentrations that are causing it.
00:50
I’m sure you are aware.
00:51
But awareness isn’t the same thing as understanding,
00:53
and if I had one final attempt to persuade you that chemistry is important, which I do,
00:58
I’d tell you about the carbon cycle, which is what I’m about to do.
01:02
Of all the chemistry that happens on this planet the carbon cycle is the thing that makes Earth “earthy”,
01:06
unique among planets that we know and understand.
01:09
The early Earth’s atmosphere was made up of Ammonia, Methane, Nitrogen, and Carbon Dioxide
01:13
among some other things, but essentially there was no free oxygen.
01:17
Many of these gasses, including CO2, are what we call “Greenhouse Gases”,
01:20
because they absorb heat energy thus increasing the average temperature on earth.
01:24
When plants came along with their fancy photosynthesis,
01:27
they used up most of the carbon dioxide in the atmosphere.
01:30
They used that carbon to make life, turning carbon dioxide into a rare commodity,
01:35
while producing huge amounts of oxygen to create a whole new atmosphere.
01:39
These changes served not only to open up the earth’s atmosphere,
01:42
allowing huge amounts of head to radiate back into space,
01:45
they also created a huge reservoir of highly energetic organic forms of carbon that are used as fuel by living things.
01:52
Including us.
01:54
Organic carbon, or in the language of chemists “reduced carbon”, is natures equivalent of a battery.
01:59
Organisms can draw power by feeding on organic carbon compounds
02:03
and then oxidizing the reduced carbon back to carbon dioxide.
02:06
Lots of organisms are good at this. No organisms are as good at this as we have become.
02:12
Carbon’s role in the biology and physics of our planet is unparalleled.
02:15
It’s the main route for the transfer of energy between organisms,
02:19
and it’s the main way that we trap heat in our atmosphere,
02:23
and that’s why when we talk about global warming we talk about carbon:
02:26
carbon footprints, carbon credits, carbon loading.
02:28
So, let’s wrap up our study of the chemical universe by going not all the way back to square 1, but square 6…
02:33
on the periodic table! That’s right we have a periodic table designed by Thought Café.
02:38
It’s a beautiful Crash Course: Chemistry period table, it’s quite large, and you want it,
02:42
and it’s available for $15 at dftba.com
02:45
[Theme Music]
02:54
Since carbon is the stuff of life, one of the best ways of understanding the carbon cycle is
02:58
just a whole bunch of things living and dying, and in the process swapping carbon.
03:02
Here’s the 30 second version:
03:04
Plants use the carbon in the atmospheric CO2 to make sugars and other carbohydrates to grow and reproduce.
03:08
Lots of those plants end up being eaten by other organisms supplying them with the building
03:12
blocks for other biological molecules and fuel.
03:15
After being metabolized the carbon returns to the environment in one of several different ways,
03:19
ending up in the air, water, or the earth itself.
03:21
From that point it’s released naturally or is extracted by humans;
03:25
in either case returning carbon dioxide to the atmosphere and it starts all over again.
03:29
So let’s talk about those green plants and what is is exactly that they do.
03:32
Photosynthesis is an extremely complex process,
03:35
and it can happen in a couple of different ways, depending on the organism.
03:38
But the main point is to take in carbon dioxide from the environment
03:41
and use a process called carbon fixation to convert it to organic compounds such as sugars.
03:46
It’s called fixation because it takes carbon in the form of a gas, carbon dioxide,
03:51
and solidifies it into solid carbon compounds. A general reaction looks like this.
03:55
Note that CH2O isn’t actually a thing, it’s a reduced formula that stands for a simple carbohydrate;
04:00
a compound that’s composed of carbon, and hydrogen, and oxygen at a ratio of 1 to 2 to 1.
04:05
In real life there are all kinds of carbohydrates and they’re usually a lot larger,
04:10
but chemists use this streamlined version to illustrate the basic reaction.
04:14
And the reaction is basically the same with any hydrocarbon,
04:16
but you’ve probably seen it written most often with glucose.
04:18
In this form we say that 6 moles of carbon dioxide plus 6 moles of water plus some light energy
04:23
yields a mole of glucose plus 6 moles of oxygen.
04:26
Either way you might have noticed something important: this is a redox reaction!
04:30
Carbon is reduced, going from an oxidation number of plus 4 to 0,
04:34
and oxygen goes from negative 2 to 0, meaning that oxygen is oxidized. It can happen!
04:40
And the carbohydrates produced by these carbon fixation reactions are the most sought after currency among earths living things.
04:46
Because organisms can use them in 2 different ways: as building material, and as fuel.
04:51
Some carbohydrates go through additional reactions to become even more complex,
04:55
like starches, fats, proteins, nucleic acids, and all the other stuff that makes up living things.
04:59
Meanwhile other carbohydrates, the ones that are used to produce energy,
05:02
go through a process called cellular respiration.
05:05
Like carbon fixation, respiration is an extremely complex cluster bomb of reactions,
05:09
so we typically condense it too down to a fairly simple reaction.
05:12
The overall reaction for respiration is essentially the reverse of carbon fixation.
05:17
A carbohydrate and some oxygen react to produce carbon dioxide, water, and energy.
05:21
Again, this is most commonly written with glucose, so we’ll do that here too.
05:25
And like carbon fixation it’s also a redox reaction.
05:28
In this case the oxygen is reduced from an oxidation number of 0 to -2, and the carbon is oxidized from 0 to +4.
05:34
That’s right, the cells of living things re-oxidize the reduced carbon and re-reduce the oxidized oxygen
05:40
to produce all the energy that keeps living things alive.
05:43
But despite the fact that these two reactions look like the exact opposites of each other,
05:46
even down to the changes in their oxidation numbers, it’s important to remember that they’re just summaries.
05:50
The overall processes actually include lots of steps that are completely different from each other.
05:54
Now after organisms metabolize carbohydrates the carbon can be re-released back into the world in several ways.
05:59
For one thing lots of carbon dioxide is released as a direct product of cellular respiration:
06:04
for you and me that happens mostly when we exhale,
06:07
and that carbon dioxide can be recycled immediately by photosynthesizers.
06:11
For organisms that live in water the carbon that they release largely ends up dissolved in that water,
06:15
and oceans and other surface water can also dissolve carbon dioxide directly from the air.
06:20
No matter how it gets there, carbon dioxide reacts with water to form carbonic acid;
06:24
a weak acid that dissociates into hydrogen and bicarbonate ions.
06:28
The hydrogen ions can react with various rocks and other substances in fairly typical neutralization reactions.
06:33
This results in weathering which, besides melting the faces off of statues, produces stuff that forms clays.
06:38
But the carbonate ions have a whole bunch of functions,
06:41
especially in marine animals, which use them to build their shells and skeletons in the form of calcium carbonate.
06:47
The carbon remains trapped there until those animals die and their structures break down,
06:52
which happens on huge, very slow, geologic time scales.
06:55
And if the conditions are just right like in the super high pressures and temperatures in the earth’s crust,
06:59
large deposits of calcium carbonate can eventually form rocks like limestone.
07:03
And before you start thinking rock equals boring,
07:06
you should know that about 80% of all the carbon on the planet is locked up this way,
07:11
as inorganic carbonates in the earths crust and mantle or lithosphere.
07:15
Most of the remaining 20% is spread throughout the lithosphere too, as organic carbon from buried organic matter.
07:20
So, those pools of carbonate everyone’s so obsessed with, those fossil fuels that we use to power all the things?
07:26
They account for only 0.006% of all the carbon on earth.
07:31
Those carbon deposits got there from the dead bodies of other organisms,
07:35
like plants that managed not to be eaten or have their carbon released some other way.
07:39
Hence the name fossil fuels – the dead things that you know today as coal, petroleum, and natural gas.
07:45
The general chemical reaction for the combustion of those fuels is
07:48
just a hydrocarbon reacting with molecular oxygen to form carbon dioxide and water vapour.
07:52
For example each mole of methane, the simplest hydrocarbon, reacts to form 1 mole of carbon dioxide.
07:57
And of course this too is a redox reaction, where carbon is oxidized all the way from -4 to +4,
08:04
and oxygen is reduced from 0 to -2.
08:06
Combustion is an extremely common reaction type.
08:09
The only change that occurs from one example to the next is the hydrocarbon,
08:12
and obviously as larger more complex hydrocarbons are used the CO2 production per mole of fuel increases.
08:16
What about gasoline for example?
08:18
The combustion of octane, a main ingredient of gasoline, produces 8 moles of carbon dioxide for every 1 mole of fuel used.
08:24
A mole of octane is only about 0.15 liters or 0.041 gallons,
08:30
but those 8 moles of CO2 take up nearly 180 liters of space at standard pressure and temperature.
08:35
This carbon dioxide is released back into the atmosphere where it re-enters the carbon cycle.
08:40
The problem with all this is that the environment can only really re-absorb about 40% of the 30 million tons,
08:45
or giga-tons, that humans produce annually,
08:48
so we have a surplus of about 18 giga-tons of CO2 every single year,
08:53
and that number goes up every single year.
08:57
The excess remains in the atmosphere trapping the sun’s heat causing temperatures to rise.
09:02
And yes, there are other greenhouse gases too; methane, ozone, even water vapor, to name a few,
09:06
but none of these are increasing in abundance at nearly the rate that carbon dioxide is.
09:10
The good news is that now you understand the problem better than you did 10 minutes ago,
09:14
and understanding about situations in the first step to making it better.
09:17
That goes for all situations really, not just chemistry ones,
09:20
so continue to pay attention to what’s happening in the world around you,
09:23
keep learning science, and never stop caring.
09:26
And thank you for the difference you have made by watching this episode of Crash Course: Chemistry,
09:29
and this entire Crash Course: Chemistry course, which I hope you watched.
09:33
In this episode you learned that the carbon cycle includes carbon fixation,
09:37
both cellular and macroscopic respiration in living things, and deposition in limestone and fossil fuels,
09:42
and that many of the chemical reactions involved in the carbon cycle are redox reactions.
09:47
And finally, you learned, from the chemistry perspective,
09:49
exactly why the excessive burning of fossil fuels harms the environment.
09:53
And I’d especially like to thank all of our Subbable subscribers,
09:56
without whom we would literally not be able to do this.
09:59
Would you like a personally signed giant Crash Course periodic table?
10:03
Or even to see yourself animated in one of our future episodes?
10:06
To find out about these and other perks go to Subbable.com.
10:09
And thanks too to our skilled writer Edi Gonzalez, script editor Blake de Pastino,
10:13
and our chemistry consultant Dr. Heiko Langner.
10:15
This episode was filmed, edited, and directed by Nicholas Jenkins,
10:18
the script supervisor was Michael Aranda who was also our sound designer,
10:21
and our graphics team is Thought Cafe.
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This post was previously published on YouTube.
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Photo credit: Screenshot from video