19.8 Energy Changes During Nuclear Reactions

CHM 51Chapter 19

19.8 – Energy Changes During Nuclear Reactions:

*Recall: Activation energy is the potential energy barrier that must be surmounted before reactants can be converted to products. The height of the barrier controls the rate of the reaction

Mass Defect: The loss in mass that occurs when protons and neutrons combine to form a nucleus. The loss in mass is converted into energy that is released during the nuclear reaction and is thus a direct measure of the binding energy holding the nucleons together

Δm = mass of nucleons– actual mass

Find the mass defect of a copper-63 nucleus if the actual mass of a copper-63 nucleus is 62.914 amu.

Copper has 29 protons and copper-63 also has (63 - 29) 34 neutrons.
The mass of a proton is 1.00728 amu and a neutron is 1.00867 amu.

• The combined mass or mass of nucleons is calculated:

• Calculate the mass defect.

Δm =

Nuclear Binding energy of a nucleus is the energy needed to break a nucleus into its individual protons and neutrons. The larger binding energy, the more stable the nucleus

Conversion of Mass Defect into Energy

To convert the mass defect into energy:

• Convert the mass defect into kilograms (1 amu = 1.6606 x 10-27 kg)
• Convert the mass defect into its energy equivalent using Einstein's equation, ΔE = Δmc2.

Example: Determine the binding energy of the copper-63 atom.

• Convert the mass defect (calculated in the previous example) into kg.

• Convert this mass into energy using ΔE = Δmc2, where c = 3.00 x 108 m/s.

The energy calculated in the previous example is the nuclear binding energy. However, nuclear binding energy is often expressed as kJ/mol of nuclei or as MeV/nucleon.

• To convert the energy to kJ/mol of nuclei we will simply employ the conversion factors for converting joules into kilojoules (1 kJ = 1000 J) and for converting individual particles into moles of particles (Avogadro's Number).

To convert the binding energy to MeV (megaelectron volts) per nucleon we will employ the conversion factor for converting joules into MeV (1 MeV = 1.602 x 10-13 J) and the number of nucleons (protons and neutrons) which make up the nucleus.

Determine the binding energy per nucleon for Osmium190.

mass of 190Os = 189.95863 amumass of neutron = 1.008664 amumass of proton = 1.007276 amu

Example: Consider the following nuclear reaction and determine how much energy gain or lost when U-238 undergoes alpha emission.

The nuclei in the reaction have the following masses

U-238 = 238.0003 amu, Th-234 = 233.9942 amu and 42He = 4.0015 amu

The binding energy per nucleon for most nuclei falls around 1.3 x 10-12 J (8 MeV). A plot of binding energy per nucleon vs. mass number shows that the most stable nuclei occur around A = 50 to 60. Fe56 is the most stable nucleus in the universe. The existence of a maximum in this curve indicates that energy is released in either a fission or fusion process in which more stable nuclei (i.e., closer to 56Fe) are produced.

Binding energy per nucleon generally increases from small atoms to atoms with mass number around 56. Thus fusing atoms to form medium-size atoms (nuclear fusion) release energy.

Binding energy per nucleon generally decreases from atoms with mass number around 56 and larger. Thus splitting larger atoms to form medium-sized atoms (nuclear fission) also releases energy.

19.9-19.10 Nuclear Fission and Nuclear Fusion

Nuclear fission and nuclear fusion are reactions in which nuclei attain size closer to the intermediate range (A = 50)

In nuclear fission,

•a large nucleus is bombarded with a small particle.

•the nucleus splits into smaller nuclei and several neutrons.

•large amounts of energy are released.

When a neutron bombards 235U,

•an unstable nucleus of 236U undergoes fission (splits).

•smaller nuclei are produced, such as Kr-91 and Ba-142.

•neutrons are released to bombard more 235U.

The result is a chain reaction that continues to occur even if the external supply of bombarding neutrons is cut off.

If the sample is too small, many of the neutrons leave the sample before they have a chance to be absorbed

-Critical mass is the smallest mass of fissionable material in which a chain reaction can be sustained

Nuclear fusion

It’s the fusion reaction of hydrogen nuclei to produce helium that powers our sun and other stars. To get the nuclei to react, the bombarding nucleus must have enough kinetic energy to overcome the repulsion of electric charges of the nuclei.

Advantages: Relatively cheap fuel source and the products are nontoxic.

Disadvantage: A temperature of 40 millionKelvin is needed to initiate the process.

19.7 – Nuclear Transmutation

Nuclear Transmutation: The change of one element into another. Such transmutation is often brought about by bombardment of an atom with a high energy particle such as a proton, neutron or α-particle. In the ensuing collision energy between particle and atom, an unstable nucleus is momentarily created, a nuclear change occurs, and a different element is produced. The first nuclear transmutation was accomplished in 1917 by Ernest Rutherford

Examples:

a)Plutonium-241 can be made by bombarding uranium-238 with alpha particles:

b)In another fission reaction, U-235 bombarded with a neutron produces Sr-94, another small nucleus, and 3 neutrons. Write the complete series of equation for the fission reaction

c)Silver-117 atoms undergo three beta emissions before they reach a stable nuclide. What is the final product?

d) Cobalt-60 is used in radiation therapy for cancer patients. The overall preparation process can be written as

19.4 – Detecting and Measuring Radioactivity

Radioactive emissions are invisible. We can’t see, hear, smell, touch or taste them, no matter how high the dose. We can, however, detect radiation by measuring its ionizing properties.

Ionizing Radiation: Radiation that knocks an electron from a molecule, thereby ionizing it.

-Including not only α-particle, β-particle, γ-particle but also X-ray and cosmic ray

Radiation intensity is expressed in different ways, depending on what is being measured. See Table 22.3 for some units measure the number of nuclear decay events.

19.9 – Application of Nuclear Chemistry

Medical Application:

Radioisotopes with short half-lives are used in nuclear medicine because

•They have the same chemistry in the body as the nonradioactive atoms.

•In the organs of the body, they give off radiation that exposes a photographic plate (scan), giving an image of an organ.

•In Vivo Procedures

•Determination of whole-blood volume using red blood cells labeled with chromium-51.

•Therapeutic Procedures

•Irradiation of tumors using gamma rays emitted from cobalt-60.

•Beta emission of iodine-131 to treat thyroid disease.

•Imaging Procedures

•Use of a radiopharmaceutical agent known to concentrate in a specific tissue or organ. These target areas show up as “hot” spots compared to a “cold” background. Technetium-99m is the most widely used radioisotope.

Dang1