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Lecture Comments (3)

0 answers

Post by Torrey Poon on July 26, 2014

Thank you sir!

1 answer

Last reply by: Professor Franklin Ow
Fri Jul 25, 2014 2:43 PM

Post by Torrey Poon on July 24, 2014

Hi Dr. Ow

I was assigned this problem from my instructor: "If you begin with a 7.52g sample of Uranium-238 and it decays for 8 half-lifes, how much of the original sample will remain?"  If you could tell me how to get this problem started I'd greatly appreciate it.

Nuclear Chemistry

  • Nuclear reactions involve a change of chemical composition and can be accompanied by substantial amounts of energy.
  • The energy released can be in the form of alpha, beta, and gamma rays, with gamma rays being the most harmful and penetrating.
  • Writing and balancing a nuclear equation follows the law of conservation of mass and charge.
  • Radioactive decay processes follow first-order kinetics.

Nuclear Chemistry

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Lesson Overview 0:06
  • Introduction 0:40
    • Introduction to Nuclear Reactions
  • Types of Radioactive Decay 2:10
    • Alpha Decay
    • Beta Decay
    • Gamma Decay
    • Other Types of Particles of Varying Energy
  • Nuclear Equations 6:47
    • Nuclear Equations
  • Nuclear Decay 9:28
    • Nuclear Decay and the First-Order Kinetics
  • Summary 11:31
  • Sample Problem 1: Complete the Following Nuclear Equations 12:13
  • Sample Problem 2: How Old is the Rock? 14:21

Transcription: Nuclear Chemistry

Hi, welcome back to

Today's lesson from general chemistry is on nuclear chemistry. Here is the lesson overview.0002

We are going to start off with a brief introduction as to exactly what we mean by nuclear reaction.0010

Then after that, we are going to go into the types of nuclear reactions, basically the different types of what we call radioactive decay.0015

We are going to learn how to write and balance nuclear equations followed by studying how fast a nuclear reaction can occur.0022

We are going to go ahead and wrap up the lesson with a summary as always followed by some sample problems.0033

Nuclear reactions are different than what we have been referring to all this whole time as a chemical reaction.0042

In a chemical reaction, chemical identity never changes.0051

Carbon remains carbon; oxygen remains oxygen.0055

But in nuclear reactions, the chemical identity of the reactant actually changes.0058

We actually change one element to something totally different.0063

In other words, we are changing the chemical composition.0067

We are changing the number of protons of the original starting material during the reaction.0072

In general, a nuclear reaction involves starting with an isotope that is relatively unstable.0078

The isotope can then undergo successive decay reactions or successive decompositions forming additional unstable isotopes or nuclei of different chemical identity.0086

We also call these daughter nuclei.0103

Because this is nuclear chemistry after all, you think of some type of energy associated with it.0109

Each decay is often associated with the release of energy in the form of what you and I commonly refer to as radiation.0119

Let's now discuss the different types of radioactive decay that can occur.0132

The first one is called alpha decay.0137

Alpha decay is the weakest type of radioactive decay.0139

In alpha decay, energy is released in the form of what we call an alpha particle.0144

An example will be the decomposition of this isotope here going to 222-86-Rn plus 4-2-He.0151

Let's go ahead and do a brief review.0177

The top number is equal to Z... excuse me, the top number is equal to A.0179

We talked about this a long time ago.0185

That is going to be the atomic mass.0187

The bottom number is of course what we call Z.0192

That is just basically your atomic number.0195

An alpha particle is essentially a helium atom.0199

Sometimes you see people write it also like that.0204

An alpha decay is the weakest type of radioactive decay.0209

You can essentially stop alpha particles in its path with simple paper.0212

A stronger type of decay, and therefore a little more dangerous, is called beta decay0221

where energy is released in the form of a beta particle.0226

A beta particle is essentially an electron.0229

Let's go ahead and look at this isotope of hydrogen which is what is called tritium.0233

That is going to decay to a helium isotope releasing also an electron.0241

An electron has 0 mass but an overall charge of -1.0247

That is how it represents an electron or also a beta particle.0252

0-(-1)-beta is sometime how it is also represented.0257

Because a beta particle is more dangerous, you require something a little tougher to stop it.0264

Something like a piece of aluminum metal is sufficient to stop a beta particle in its path.0273

However the strongest type of decay of course is going to be what we call gamma decay.0280

Gamma rays are pure energy; there is no mass.0285

All of the energy that is released is pure energy.0289

Nothing is transferred to another atom as mass.0294

The example would be an excited cobalt isotope going to a lower state cobalt isotope and releasing just a ridiculous amount of energy.0298

A gamma ray is often referred to as 0-0-γ or just γ for short.0312

Again this is pure energy.0321

Of course you need something a lot more dense to stop gamma radiation.0322

This is what is coming out of nuclear facilities after all.0327

Something like concrete or something like lead is going to be the only suitable objects.0331

There are other types of particles of varying energy that can also be released.0341

One type of decay is what we call positron emission.0347

A positron is essentially the positive equivalent of an electron.0353

That is going to be represented as Β1+ or as 0-1-e.0357

You can also have nuclear decay that releases a proton; proton emission.0366

Of course proton emission is basically just your hydrogen.0376

That is going to be 1-1-p.0380

Finally you can also have the release of neutrons which is going to be represented as 1-0-n.0385

Have a mass of 1 and a relative charge of 0.0396

Now that we know the different types of radioactive decay, let's go ahead and learn how to express nuclear reactions.0401

Just how we have seen this entire session on general chemistry, a chemical equation expresses a chemical reaction.0408

Similarly we are going to have what is called a nuclear equation to express a nuclear reaction.0417

The same principles hold though, especially the conservation of mass and the conservation of charge.0422

Basically the sum of the superscripts must equal each other on both sides of the equation.0433

The sum of the subscripts must equal also each other on both sides of the equation.0453

It is essentially we are balancing the equation essentially.0463

Let's just go ahead and take a look at three examples here.0467

We can have polonium-84 going to an alpha particle plus blank.0471

Let's go ahead and fill in what we can.0483

This subscript here has to be 82; subscript here has to be 207.0485

When you look up element 82 on the periodic table, you get lead.0491

Let's go ahead and do another one.0496

We can have sodium-11 undergoing decay to form magnesium-12 plus blank.0498

The subscript here has to be -1; superscript here is going to be 0.0509

That is going to be therefore an electron.0519

Excuse me... this is a typo here; let's make this 22; there we go.0522

Finally we can have calcium-20 combining this time with an electron.0529

When that happens, we are going to get 41 on top on the0540

right side and 19 on the bottom on the right side too.0543

That is element 19 which is potassium; it is relatively straightforward.0547

Nothing too difficult really when writing and balancing a nuclear equation.0552

Let's now go ahead and examine how fast a nuclear reaction can occur, basically the rate of nuclear decay.0559

The rate at which an unstable isotope decays is a matter of kinetics.0569

If you recall, kinetics as we saw was either zero, first, or second order which are the common ones.0574

It turns out that all radioactive decay processes follow first order kinetics.0582

Let's go ahead and refresh our memories and exactly what we mean by first order.0589

The integrated rate law for first order kinetics was the following.0594

The natural log of the concentration of A at some time t divided by the initial concentration of A is equal to -kt.0599

The half-life is equal to natural log of 2 over k.0612

If you all can recall what we mean by half-life, let's plot the amount on the y-axis.0620

On the x-axis, let's plot time.0630

Let's say this dot represents the initial concentration of A right here.0632

As it gets consumed, it is going to follow the following profile.0640

At some point, we are going to reach half of this initial amount.0652

The time that is required to reach half the initial amount, that is what we call the half-life.0658

We can come up with a nice equation in terms of mass where the mass0666

at time t is equal to the initial mass times e raised to the ?kt.0672

That is an equation we are going to be using when we do some sample problems.0680

Once again radioactive decay processes follow first order kinetics.0686

To summarize this very short session, nuclear reactions involve a change of chemical0693

composition and can be accompanied by substantial amounts of energy which is why0698

when nuclear reactors melt down, that is why they are so tragic.0704

Number two, the energy release can be in the form of what we call alpha, beta, and gamma rays,0709

with gamma rays of course being the most harmful and penetrating.0715

Writing and balancing a nuclear equation follows our most fundamental laws of conservation of mass and charge.0719

Finally radioactive decay processes follow first order kinetics.0727

Let's go ahead and tackle some sample problems.0734

Complete the following nuclear equations.0737

14-7-n plus something is going to go on to form oxygen-8 and a proton.0740

Let's go ahead and look at this.0752

Here my subscript is going to be 2; my superscript is 4.0754

That essentially is going to be my alpha particle.0758

Let's go ahead and do a second example.0763

Here we can have something plus a neutron going on to form Bk-97 plus an electron.0765

My subscript here is 96; my superscript is 248.0780

This is going to be a curium, Cm.0785

Another one can be something plus a neutron going on to form 244-96-Cm plus a gamma ray plus blank.0790

This should be americium-95... excuse me about that.0812

Now on the right side here, this is going to be 0 on top, -1 on bottom, and an electron.0816

Finally the last example is going to be carbon-6 reacting with a neutron.0823

That is going to go on to form something plus a gamma ray plus carbon-6.0835

I'm sorry, excuse me... the question mark is the carbon-6 to balance out everything.0851

That is just some straightforward examples on balancing nuclear equations.0856

Now finally onto sample problem two.0862

Potassium-40 can be used to date materials because it is presumed to have existed at the formation of the earth.0865

If three-fifths of the original K-40 exists in a rock, how old is the rock?0872

Three-fifths is the fraction remaining.0878

Once again three-fifths is the fraction remaining of K-40 in the rock.0887

We can look up the half-life of potassium-40.0900

The half-life of potassium-40 is 1.26 times 109 years.0905

The rate constant is going to be equal to natural log of 2 over the half-life.0915

We are going to get 5.50 times 10-10 reciprocal years.0922

We can set up and use the equation M at some time t is equal to M0 times e raised to the ?kt.0932

Three-fifths is equal to the M0... which is what we call 1 because at the initial point,0941

we have 1 if we are dealing with fractions... times e raised to the ?kt.0953

Solving for t, we are going to get 9.29 times 108 years.0959

Once again when using fractions, don't forget, when using fractions,0967

the initial amount can always be represented as simply 1; 1.0.0976

That is our lecture on nuclear chemistry.0993

I will see you next time on