Nuclear Chemistry Part 2 - Fusion and Fission: Crash Course Chemistry #39

Summary

This Crash Course Chemistry episode delves into nuclear chemistry, explaining the pursuit of stability in atomic nuclei through binding energy. It introduces Einstein's E=mc² to calculate energy from mass defect, highlighting the significance of nuclear energy in both fission and fusion reactions. The video contrasts the controlled use of fission in power plants with its uncontrolled potential in nuclear weapons, while discussing the challenges and applications of fusion, including the sun's energy production. It concludes by encouraging further exploration in nuclear chemistry to harness its potential safely and efficiently.

Takeaways

• 🔬 Chemistry, like life, seeks stability, and in nuclear chemistry, this stability is about maintaining the nucleus' integrity.
• ⚛️ Radioactive decay is a process where atomic nuclei shed particles to achieve greater stability.
• 🧲 Binding energy is the force that holds protons and neutrons together in an atom's nucleus, and it's a fundamental concept in nuclear chemistry.
• 🌟 Einstein's E=mc² formula is used to calculate the binding energy of an atom, demonstrating that mass can be converted into energy.
• 🔍 The mass of an atom's nucleus is less than the sum of its individual nucleons, and this 'missing mass' is the mass defect, which is equivalent to energy.
• ⚡ The energy released in nuclear reactions, such as fission and fusion, is immense, making nuclear energy a potent source of power.
• 📉 Elements with higher binding energies, like iron-56, are very stable and less likely to undergo nuclear reactions.
• 💥 Fission is a nuclear reaction where a large nucleus splits into smaller ones, releasing energy, while fusion is the combination of light nuclei to form a heavier nucleus.
• ⚙️ Nuclear power plants use controlled fission reactions to generate electricity, but they must manage the risk of uncontrolled chain reactions.
• ♻️ Nuclear reactions produce radioactive waste with varying half-lives, posing long-term storage and disposal challenges.
• 🌞 Fusion reactions, like those in the sun, release vast amounts of energy, but controlling them for practical use on Earth remains a significant challenge.

Q & A

What is the concept of stability in nuclear chemistry?

In nuclear chemistry, stability refers to the ability of an atomic nucleus to remain intact without breaking apart. It is associated with the balance of forces that hold the nucleus together.

What is binding energy and why is it significant?

Binding energy is the amount of energy that holds protons and neutrons together in an atomic nucleus. It is significant because it is a fundamental principle of nuclear chemistry and is directly related to the stability of the nucleus. The binding energy is released or absorbed during nuclear reactions, such as fission or fusion.

How is the binding energy of an atom calculated?

The binding energy of an atom is calculated using Einstein's mass-energy equivalence formula, E=mc^2, where 'm' is the mass defect (the difference between the mass of individual nucleons and the mass of the nucleus), 'c' is the speed of light, and 'E' is the binding energy.

What is the mass defect and how does it relate to energy?

The mass defect is the difference in mass between the individual nucleons (protons and neutrons) of an atom and the mass of the nucleus as a whole. This 'missing mass' is converted into energy, which is the binding energy that holds the nucleus together.

What are the two general types of nuclear reactions that release energy?

The two general types of nuclear reactions that release energy are fission, where a large nucleus splits into two lighter ones, and fusion, where two light nuclei join together to form a heavier one.

How does the binding energy curve relate to the stability of elements?

The binding energy curve shows that elements with higher binding energies per nucleon, such as iron-56, are more stable and less likely to undergo nuclear reactions. Elements with lower binding energies per nucleon are less stable and more likely to undergo fission or fusion reactions to achieve greater stability.

What is the role of uranium-235 in nuclear reactions?

Uranium-235 is a common fuel for nuclear fission reactions. It can be triggered to split into smaller atoms by absorbing neutrons, releasing energy, more neutrons, and other byproducts.

How do nuclear power plants control the fission chain reaction?

Nuclear power plants control the fission chain reaction by using control rods made of materials that absorb neutrons. These rods can be inserted or removed to regulate the rate of the reaction, thus controlling the release of energy.

What are the challenges associated with the waste products of nuclear fission?

The waste products of nuclear fission are radioactive and can remain hazardous for very long periods, ranging from a few years to millions of years. These radioactive materials need to be stored safely to prevent environmental contamination.

How does fusion differ from fission in terms of energy release and control?

Fusion reactions release even more energy than fission due to the larger energy changes involved in combining light nuclei. However, fusion reactions are harder to control because they require extremely high temperatures and pressures to overcome the electrostatic repulsion between nuclei.

What are the applications and limitations of nuclear fusion on Earth?

On Earth, controlled nuclear fusion has not yet been achieved for practical energy generation due to the technical challenges of reaching and maintaining the necessary conditions. However, uncontrolled fusion reactions are used in thermonuclear weapons.

Outlines

00:00

🔬 Nucleus Stability and Binding Energy

This paragraph introduces the concept of stability in nuclear chemistry, emphasizing the importance of binding energy in holding atomic nuclei together. It explains that stability equates to the ability of an element to exist without breaking apart. The paragraph delves into Einstein's famous equation E=mc^2, illustrating how it is used to calculate the binding energy of an atom. The process of calculating mass defect and its conversion to energy is discussed, using the example of an oxygen nucleus. The paragraph concludes by contrasting the immense energy released in nuclear reactions with traditional energy sources like coal, highlighting the potential of nuclear energy.

05:02

⚡ Nuclear Reactions: Fission and Fusion

Paragraph 2 explores the two primary types of nuclear reactions: fission and fusion. Fission involves the splitting of a large nucleus into smaller ones, while fusion is the combination of light nuclei to form a heavier nucleus. The paragraph explains that these reactions result in more stable products than the starting materials, which is the driving force behind them. The discussion includes a comparison of binding energies of various elements and their mass numbers, highlighting the stability of elements like iron-56. It also touches on the practical applications of fission, particularly in power plants and weapons, and the challenges associated with controlling these reactions. The paragraph also addresses the issue of radioactive waste produced by fission, emphasizing the need for safe storage and disposal methods. Finally, it contrasts fusion's potential with the current lack of control over these reactions, noting the vast energy output and the technical challenges in harnessing fusion for peaceful purposes.

10:02

🌟 Harnessing Nuclear Energy: Challenges and Potential

The final paragraph of the script wraps up the discussion on nuclear chemistry by highlighting the potential benefits and challenges of harnessing nuclear energy. It points out the vast energy that fusion could provide, with helium as a byproduct, which is non-radioactive and thus less hazardous. The paragraph poses thought-provoking questions about the efficient use of radioactive materials and the technical hurdles to achieving controlled fusion reactions. It encourages viewers to consider these challenges and perhaps contribute to the field with innovative ideas. The paragraph ends by summarizing key points from the episode, including the calculation of binding energy using Einstein's formula, the differences between fission and fusion, and their real-world applications. It also credits the team behind the production of the episode, emphasizing the collaborative effort in creating educational content.

Keywords

💡Stability

In the context of the video, 'stability' refers to the state of an atomic nucleus where it is not prone to decay or disintegration. This is a fundamental concept in nuclear chemistry as it relates to the nucleus's ability to remain intact. The video explains that atoms seek stability, which is essentially a state of being able to exist without breaking apart. For instance, when discussing radioactive decay, the video mentions that atomic nuclei shed particles to achieve greater stability.

💡Binding Energy

Binding energy is the energy that holds protons and neutrons (nucleons) together in an atomic nucleus. The video describes it as a fundamental principle in nuclear chemistry and equates it with nuclear energy. It is the amount of energy required to disassemble a nucleus into its constituent parts. The video uses the oxygen atom as an example to illustrate how the mass of the nucleus is less than the sum of the masses of its individual nucleons, indicating the presence of binding energy.

💡E = mc2

This famous equation, introduced by Albert Einstein, is the mass-energy equivalence formula and is central to the video's discussion of nuclear energy. It states that mass can be converted into energy and vice versa, with 'm' representing mass, 'c' representing the speed of light, and 'E' representing energy. The video uses this equation to calculate the binding energy of an atom, demonstrating how the 'missing mass' in a nucleus is actually energy that holds the nucleons together.

💡Mass Defect

Mass defect is the difference between the total mass of individual nucleons and the actual mass of a nucleus. The video explains that this 'missing mass' is not actually lost but is converted into energy, which is a form of binding energy. The concept is used to calculate the binding energy of an oxygen nucleus, showing that the mass defect is negative, indicating the release of energy.

💡Nucleons

Nucleons are the collective term for protons and neutrons, which are the particles that make up an atomic nucleus. The video mentions nucleons when discussing the composition of the oxygen nucleus, emphasizing that the stability of the nucleus is directly related to the binding energy that holds these nucleons together.

💡Fission

Fission is a type of nuclear reaction where a large atomic nucleus splits into two or more smaller nuclei, releasing energy in the process. The video explains that fission is used in nuclear power plants and nuclear weapons, with uranium-235 being a common fuel. The video also discusses how fission reactions can lead to chain reactions if not controlled properly.

💡Fusion

Fusion is the opposite of fission, where two light nuclei combine to form a heavier nucleus, releasing energy. The video uses the sun's nuclear reactions as an example of fusion, where hydrogen atoms combine to form helium, releasing energy in the form of gamma radiation. The video also touches on the challenges of harnessing fusion for energy production on Earth due to the extreme conditions required for the reaction.

💡Chain Reaction

A chain reaction in nuclear chemistry refers to a series of nuclear reactions where each reaction triggers additional reactions. The video explains that both fission and fusion can be chain reactions, but control is crucial, especially in fission, to prevent runaway reactions that could lead to a nuclear explosion or meltdown.

💡Control Rods

Control rods are used in nuclear reactors to control the rate of the fission reaction by absorbing neutrons. The video describes how these rods, made of materials that readily absorb neutrons, can be inserted or removed to regulate the reaction, thus controlling the release of energy and preventing a meltdown.

💡Radioactive Waste

Radioactive waste refers to the byproducts of nuclear reactions that remain radioactive and can be hazardous. The video discusses the challenges of dealing with the waste produced by fission reactions, which includes a variety of radioactive isotopes with half-lives ranging from years to millions of years. The video highlights the need for safe storage and management of these wastes.

Highlights

Chemistry is about a search for stability, including in nuclear chemistry where atomic nuclei strive for stability.

Radioactive decay occurs as atomic nuclei shed particles to become more stable.

Binding energy holds protons and neutrons together in the nucleus and is released when a nucleon is removed.

The famous equation E=mc², introduced by Einstein, demonstrates the relationship between mass and energy.

The 'mass defect' refers to the difference in mass between a nucleus and its individual nucleons, representing the binding energy.

Nuclear energy comes from the binding energy released during fission or fusion reactions.

Fission is the splitting of a large nucleus into smaller nuclei, while fusion is the joining of light nuclei to form a heavier one.

Uranium-235 is a common fuel for fission reactions, where neutrons initiate a chain reaction releasing significant energy.

Control rods in nuclear reactors absorb neutrons to regulate the fission process and prevent dangerous meltdowns.

Nuclear waste from fission reactions remains radioactive for years to millions of years, posing significant environmental challenges.

Fusion, as seen in the sun, releases more energy than fission but is difficult to control on Earth.

The fusion process requires extreme temperatures (100 million kelvin) and pressures to overcome the repulsion between atomic nuclei.

Fusion reactions generate energy by combining hydrogen atoms to form helium, with gamma radiation as a byproduct.

While fusion reactions have the potential to be an enormous energy source, they remain impractical for controlled energy generation today.

The future of nuclear chemistry holds promise in finding new ways to control fusion and utilize radioactive materials efficiently.

Transcripts

As I said before chemistry is, like many aspects of your own life, all about a search for stability.

Last week we talked about radioactive decay

and how atomic nuclei get rid of various particles in order to become more stable.

But what is this illusive stability that all things seem to be striving for exactly?

In nuclear chemistry it simply has to do with keeping the nucleus together.

If the nucleus is going to break apart, then that's not going to be something that lasts very long.

Stability really, is kind of just a way of saying, it can exist.

And the amount of energy, that holds each proton or neutron in an atom's nucleus,

is the same amount that's released when it's removed.

This is known as binding energy and it's one of the fundamental principles of nuclear chemistry.

It's actually what we mean when we talk about nuclear energy.

Now I'm not going to lie to you, nuclear chemistry is terribly complicated,

but we have a way of understanding it that, while not exactly simple, is one I'm sure you've heard of.

The binding energy of an atom is calculated with the formula E = mc2.

Probably the most famous equation in the world,

and since it was first hit upon by a young patent clerk in 1905 it has become synonymous with scientific genius.

Part of why it's so famous, I like to think, is because the logic behind it is so elegantly simple,

and yet totally counter intuitive, but it's probably also famous because it's important.

It explains one of the most powerful sources of energy known to humanity.

[Theme Music]

E = mc2 is formally known as the mass-energy equivalence formula

and it states that mass is interchangeable with energy.

OK, there is a lot there, in what I just said.

To tease it apart consider the nucleus of an atom of oxygen: 8 protons and 8 neutrons.

Collectively, by the way, these particles are known as nucleons.

If you were to add up all the individual masses of all 16 nucleons separately

and then compare that to the total mass of an actual oxygen nucleus,

you'd find that there's a difference between the two.

Specifically the mass of the nucleus, exactly 15.99 atomic mass units,

is lower than the mass of its individual nucleons put together, in this case 16.13 amu's.

That mass went somewhere.

That "missing mass" in the nucleus, known as its mass defect, is actually present in the form of energy.

It's the energy that holds the nucleons together,

so for example the mass defect for an oxygen atom is negative 2.269 x 10^-28 kilograms.

To find out how much binding energy that missing mass amounts to, you can use it as the 'm' in Einstein's formula.

This ingenious little equation relates mass and energy by a simple proportionality constant,

and thanks to Einstein we know that constant is the square of the speed of light, or c2.

Solve for 'E' and you find that the binding energy in that oxygen nucleus is 2.04 x 10^-11 joules,

with the negative sign indicating that the energy is being released.

Now of course 2.04 x 10^-11 is a very small number.

That might surprise you, but hold the phone, that is just for one single nucleus.

If we multiply that by Avogadro's number to find the energy change for a whole mole of oxygen nuclei,

a mere 16 grams of oxygen, we get an amazing 1.23 x 10^13 joules of energy.

To produce that energy with coal, you would have to burn 420,000 kilograms, 420 metric tons of coal.

That energy is what we mean when we talk about nuclear energy,

the binding energy that's released when a nucleon is removed from its nucleus.

Now, to dislodge one of those nucleons and unleash that energy there are two general

types of nuclear reactions: fission and fusion.

Fission occurs when a large nucleus splits into two lighter ones.

Fusion is the opposite when two light nuclei join together to form a heavier one.

In both cases the products of the reactions are more stable than the starting materials,

and this is, as always, what drives the reaction.

This is a graph of the binding energies of various elements compared to their mass numbers.

Elements with very high binding energies such as iron-56 are very stable and rarely undergo nuclear reactions.

But elements with lower binding energies can react much more readily.

If the nucleus is heavier than iron-56 it will tend to break into two or more smaller nuclei; a fission reaction.

If it's lighter than iron-56 it will more likely participate in a fusion reaction,

joining two nuclei together to form a heavier one.

But the most important thing to notice here is that

with both fission and fusion stability increases as a result of the reaction.

Fission is the type of reaction that we use more often

because it's the one that we're better at initiating and controlling, at least so far.

And whether it's used in power plants or bombs, the most common fuel for fission is uranium-235.

There are several ways that it can react,

but the reaction is almost always triggered by hitting uranium with neutrons from another source.

When that happens the uranium splits into smaller atoms.

One such reaction produces krypton-92, yes krypton is a real thing,

along with barium-141, three free neutrons and lots of energy.

This energy is released mainly as the kinetic energy of the escaping particles,

which is immediately transferred to the surroundings as heat.

Some energy is also released in the form of electro-magnetic radiation such as

visible light, X-rays, and gamma radiation.

Nuclear power plants use the energy released by these reactions to convert water to steam,

which then is passed through turbines spinning a generator, powering cities and stuff.

Because of the enormous amounts of energy these reactions can release,

nuclear power plants can potentially produce lots of electricity,

but there's also, I think you may have heard, some serious draw backs.

For one thing, as you know, atoms rarely exist in isolation.

We write the equation of a fission reaction as it fits just one atom,

but in reality that one atom is surrounded by many, many more.

And if one little neutron can trigger the reaction and that reaction liberates three more neutrons,

well I think you can see where this is going.

If the reaction isn't controlled each reaction trigger three more,

and every reaction releases the same amount of energy, which adds up fast.

This is pretty much the definition of a chain reaction

and it is the basis of the remarkable power of the nuclear weapon.

The same type of reaction occurs in nuclear power plants,

but those reactions are controlled in several ways to keep them from getting out of hand.

The fact is these chain reactions have the potential to produce far more heat than the plant can use,

so much more that the temperature can easily rise to dangerous levels, enough to melt the uranium.

This is the meltdown that you hear about

and most reactor cores are immersed in water to disperse the heat and prevent this from happening.

But that, that's not enough on its own to control this thing.

If the chain reaction is allowed to run freely,

no amount of water can remove the heat fast enough to prevent a meltdown.

A real way we control nuclear reactions is with control rods.

They're made of materials that readily absorb neutrons

and they're inserted between the fuel rods of uranium to slow the neutrons down and therefore slow the reaction.

They can be put in more to slow the reaction more, and lifted out more if you need more heat.

Now the other sticky wicket of fission reactions is the stuff that's left behind.

These reactions not only produce products that are still radioactive,

they produce tons of them, lots of different troublesome kinds.

Like we saw last week, uranium undergoes many different types of nuclear decay,

so not only does each uranium atom produce isotopes of krypton and bromine,

but that process also produces many other isotopes of other elements.

And as these various nuclei break down they release more neutrons

and more unstable products and the process continues for a long time.

All of these reactions eventually yield stable products

but they have half-lives ranging from a few years to tens of millions of years.

The products with shorter half-lives stabilize pretty quickly

but they release particles and energy like crazy during that time so they're extra dangerous.

The ones with longer half-lives decay more slowly, release less energy

but that means it takes a very, very long time for them to stabilize.

So long in fact, that for human purposes, it may as well be forever.

That means they'll always be an issue in our environment which is why we're always looking

for ways to store them, and keep them out of our way.

Fusion reactions, as you'd expect, are very different from fission.

For one thing, the energy released in many fusion reactions dwarfs even the huge amount released by fission.

You might be familiar, for example, with the wonderful work done by our sun.

The reactions that power the sun are like most fusion reactions,

in that they involve very small nuclei like isotopes of hydrogen and helium.

This reaction begins when two atoms of hydrogen,

accelerated by the sun's fantastically high temperatures and contained by its high pressures,

join to form an atom of deuterium, an isotope of hydrogen.

This fusion of particles releases a positron and some heat energy in the process.

Then another atom of hydrogen is joined to the deuterium to form helium-3.

This step also releases a lot of energy in the form of gamma radiation.

When two atoms of helium-3 are available they join together to form an atom of helium-4,

as well as two atoms of regular hydrogen which then can be used to begin the process all over again.

This final step also, as you might imagine, releases a large amount of energy in the form of mostly gamma radiation.

So this is a chain reaction too, but it's not a self-perpetuating one like we saw before.

This reaction requires a total input of 6 atoms of hydrogen but it only produces two,

in the end the remaining mass being released in the form of helium.

For this reason more fuel is always needed,

which is why our sun is going to run out of hydrogen in about 5 and a half billion years.

We can produce fusion reactions here on Earth too,

but they're not very useful for us because we haven't figured out how to control them.

They're super useful if you just want to blow up a big city though,

just to be clear, depending on your definition of use.

One reason is, as you can see on the mass-energy graph,

light nuclei that fuse together undergo a much larger energy change than heavy nuclei that break apart.

That means their reactions release far more energy than fission reactions do,

so much more that it's nearly impossible to contain and therefore use.

Also, because fusion involves joining nuclei,

the reaction has to overcome the really strong repulsion that naturally exists between their positive charges.

For this reason, fusion reactions can only occur when particles collide at very high speeds,

or under very high pressures.

At these mind-blowing speeds, the kinetic energy of the particles produces insane temperatures,

like in the 100 million kelvin range,

at which point, the material being accelerated actually exists in the form of plasma.

So not only are those speeds really hard to reach but material at that temperature, how do you control that?

Which is why we can't use fusion for things like generating electricity which would be super nice.

We've only found applications for it when we don't need to control it at all like in nuclear weapons.

So as you can tell, there is plenty of room for new ideas in nuclear chemistry.

Fusion would be really great because it would produce a lot of energy

and you'd just get helium out of the process and helium is awesome!

How can we use radioactive materials more efficiently?

Is there a way to achieve the speeds and manage temperatures that come with fusion?

And how can we do this stuff without blowing our faces off?

You've already taken the first step by learning the basics.

It's up to you how far you want to go from here.

Maybe you'll write the next totally crazy ingenious and counter intuitive equation that takes us to the next level.

For now though, thank you for watching this episode of Crash Course Chemistry.

If you paid attention, you learned how Einstein's famous formula helps us calculate

the binding energy of a nucleus from its mass defect.

You also learned the difference between fission and fusion.

You saw an example of each one.

and you learned about their applications in the real world.

This episode of Crash Course Chemistry was written by Edi González and edited by Blake de Pastino.

Our chemistry consultant is Dr. Heiko Langner.

It was filmed, edited and directed by Nicholas Jenkins. Our script supervisor is Caitlin Hofmeister.

Michael Aranda is our sound designer and our graphics team is Thought Café.