5 Steps to a 5 AP Chemistry 2019

Nuclear Chemistry ❮ 265

of matter, as in ordinary chemical reactions.) The amount of energy that is produced can

be calculated by using Einstein’s equation E = mc^2 , where E is the energy produced, m is

the mass converted into energy (the mass defect), and c is the speed of light. The amount of

matter that is converted into energy is normally very small, but when it is multiplied by the

speed of light (a very large number) squared, the amount of energy produced is very large.

For example: When 1 mol of U-238 decays to Th-234, 5 × 10 -^6 kg of matter is con-

verted to energy (the mass defect). To calculate the amount of energy released:

Emc

E

E

(5 10 kg)(3.00 10 m/s)

51 0kgm/s 51 0J

2

682

11 22 11

=

=× ×

=× ×=×

−

If the mass is in kilograms, the answer will be in joules.

Common Mistakes to Avoid

1. Make sure your answer is reasonable. Don’t just write down the answer from your calculator.


2. Make sure your units cancel in your calculations, leaving the unit you want.


3. Make sure that in alpha, beta, gamma, and positron emissions the particle being emitted
   is on the right-hand side of the reaction arrow. In electron capture, the electron should
   be on the left side of the arrow.


4. In half-life problems, don’t omit the minus sign. Watch your units.


5. In half-life problems, be sure to use the amount of isotope still radioactive as Nt and not
   the amount decayed.

❯ Review Questions

Use these questions to review the content of this chapter and practice for the AP Chemistry

exam. First are 10 multiple-choice questions similar to what you will encounter in Section

I of the AP Chemistry exam. Following those is a long free-response question like the ones

in Section II of the exam. To make these questions an even more authentic practice for the

actual exam, time yourself following the instructions provided.

Multiple-Choice Questions

Answer the following questions in 15 minutes. You may not use a calculator. You may use

the periodic table and the equation sheet at the back of this book.

1. When^22688 Ra decays, it emits 2 a particles, then
   a a particle, followed by another a particle. The
   resulting nucleus is:
   (A)^21283 Bi
   (B)^22286 Rn
   (C)^21482 Pb
   (D)^21483 Bi
   2. The formation of^23090 Th from^23492 U occurs by:
   (A) electron capture
   (B) a decay
   (C) b decay
   (D) positron decay

TIP