8–1 john a. schreifels chemistry 212 chapter 21-1 chapter 21 nuclear chemistry

John A. SchreifelsChemistry 212

Chapter 21-1

8–1

Chapter 21

Nuclear Chemistry


Chapter 21-2

8–2

Overview

• Radioactivity and Nuclear Bombardment Reactions– Radioactivity– Nuclear Bombardment Reactions– Radiations and Matter: Detection and Biological Effects– Rate of Radioactive Decay– Applications of Radioactive Isotopes

• Energy of Nuclear Reactions– Mass – Energy Calculations– Nuclear Fission and Nuclear Fusion


Chapter 21-3

8–3

Nuclear Chemistry

• In this chapter we will look at two types of nuclear reactions.

– Radioactive decay is the process in which a nucleus spontaneously disintegrates, giving off radiation.

– Nuclear bombardment reactions are those in which a nucleus is bombarded, or struck, by another nucleus or by a nuclear particle.


Chapter 21-4

8–4

Nuclear Reactions and their characteristics

• Nuclear Chemistry: study of changes in structure of nuclei and subsequent changes in chemistry.

• Radioactive nuclei: spontaneously change structure and emit radiation.

• Differences between nuclear and chemical reactions:– Much larger release in energy in nuclear reaction.– Isotopes show identical chemical reactions but different

nuclear reactions.– Nuclear reactions not sensitive to chemical environment.– Nuclear reaction produces different elements.– Rate of nuclear reaction not dependent upon temperature.


Chapter 21-5

8–5

NUCLEAR STRUCTURE & Stability

• nucleon: any nuclear particle, e.g. protons, p, and neutrons, n. • Nucleus held together by strong attractive forces; but electrostatic

repulsion causes large atoms (>83 protons) to be unstable.• Let Z = atomic # (# of protons) and A = Z + # of neutrons. Isotopes

represented as .

– has 8 p, 8 e, and 8 n;

– has 8 p, 8 e, and 9 n;

– has 8 p, 8 e, and 10 n.

• Structure deduced from emission of radiation from unstable particles:– .ray = attracted towards negatively charged plate Positively charged.– . ray = attracted towards positively charged plate Negatively charged.– . ray = not attracted to either plate Neutral.

XAZ

O168

O178

O188


Chapter 21-6

8–6

NUCLEAR REACTIONS

• Radioactivity: nucleus unstable and spontaneously disintegrates.

• Nuclear Bombardment: causes nuclei to disintegrate due to bomdarbment with very energetic particles.

• Particles in nuclear reactions:

• Positron: positively charged particle with same mass as electron.

• Gamma ray: Very high energy photon ( = 1012 M; Visible: = 107M).

• Nuclear reaction written maintaining mass and charge balance.

E.g.

1. Proton H11 or p1

1 2. Neutron n1

0 3. Electron e0

1 or 0 1

4. Positron e0 1 or

0 1

5. Gamma ray 00

C146 N14

7 e0 1


Chapter 21-7

8–7

RADIOACTIVITY

• Types of Radioactive decay:– Beta emission: Converts neutron into a proton by emission of

energetic electron; atomic # increases:

E.g. Determine product for following reaction: – Alpha emission: emits He particle.E.g. Determine product:– Positron emission: Converts proton to neutron: E.g. Determine product of

• Gamma emission: no change in mass or charge but usually part of some other decay process.E.g.

• Electron capture: electron from electron orbitals captured to convert proton to neutron.

E.g. Determine product:

epn 0 1

11

10

?K 0 1

4019

He?Ra 42

22688

enp 0 1

10

11

e?Tc 01

9453

eNC 01

147

14 6

nep 10

0 1

11

?eK 0 1

4019


Chapter 21-8

8–8

NUCLEAR STRUCTURE and STABILITY

• Shell model of nucleus: protons and neutrons exist in energy levels which have optimum # of each in each shell.

• Magic # : # of nuclear particles in particular shell (similar to 2,8,18 etc. for electrons.)– Protons : 2, 8, 20, 28, 50, 82– Neutrons: 2, 8, 20, 28, 50, 82 and 126.

E.g. -particles ( ) & are doubly magic.• Nuclei with even # of protons and neutrons most stable.

( Largest # of stable isotopes).• Nuclei with odd # of protons and neutrons least stable. (Least #

of stable isotopes).

42 Pb208

82


Chapter 21-9

8–9

Band of Stability

• Band of stability = stable isotopes. (above Z = 82: - or - emission.)

• above: beta emission; • below: electron or positron

emission


Chapter 21-10

8–10

NUCLEAR BOMBARDMENT (Transmutation)

• Bombard nuclei with nuclear particles to convert element to another one.

• Rutherford discovered:

E.g.1. Identify product for electron capture:

E.g.2. Identify products for neutron bombardment of Fe:

E.g.3 Identify the product of

HOHeN 11

178

42

147

?Cu 0 1

6429

e?nFe 0 1

5826 2

n?pAr 10

11

4018


Chapter 21-11

8–11

RATE OF DISINTEGRATION

• Rate of disintegration proportional to number of nuclei present.• Rate = k×N or

• Half-life-time required for half of original nuclei to undergo decay.– At t1/2 N = 1/2No and , t1/2 = 0.693/k or

E.g.1 The half-life of Cobalt-60 is 5.26 years how much of the original amount would be left after 21.04 years? E.g.2 Tritium decays by beta emission with a half-life of 12.3 years. How much of the original amount would be left after 30 years? E.g.3 If a 1.0 g sample of tritium is stored for 5.0 years, what mass of that isotope remains? k = 0.563/year.

e kt = No

N

n

tt.

o

/eNN

2

1216930


Chapter 21-12

8–12

RATE OF DISINTEGRATION2

• Dating ancient objects: Carbon-14 is generated naturally from cosmic rays. .

• is unstable with a half-life of 5730 yr.• Rate of disintegration measured and is proportional to the

concentration of 14C:

E.g. Charcoal from a tree killed by the volcanic eruption that formed the crater in Crater Lake (in Oregon) gave 7.0 disintegrations of 14C min.1g1 of total carbon. Present-day carbon (in living matter) gives 15.3 disintegrations min.1g1 of total carbon. Determine the date of the volcanic eruption.

HCnN 11

146

10

147

C146

1

2

1

2

1

2

mm

NkNk

RR

NkR


Chapter 21-13

8–13

RADIATION DETECTION

• Geiger counters detect charged particles produced from interaction of gas with particles emitted from radioactive material.

• Scintillation counters detect particles from radioactive material by measuring intensity of light when these particles hit phosphor.

• Units: 1 curie (Ci) = 3.7x1010 disintigrations×s1


Chapter 21-14

8–14

Energy Changes During Nuclear Reactions

• Most nuclear reactions give off a large amount of energy.

• The energy required to break an nucleus its individual protons and neutrons is called the binding energy, Eb.

• The total mass changes upon combination of protons and neutrons.E.g. determine the mass change during the formation of Helium nuclei.

Measured mass of He nuclei (excluding electrons) = 4.00150 amu (m = 0.03038 g/mol = called the mass defect).

• Energy change calculated from the mass change (decrease) using the Einstein equation: E = mc2.E.g. determine the binding energy for 1 mol He.E.g. determine the mass change during the combustion of butane 2878 kJ/mol

Protons 21.00728 amu = 2.01456 amu Neutrons 21.00866 amu = 2.01732 amu mass of He = 4.03188 amu

HenH 42

10

11 22

npHe 10

11

42 22


Chapter 21-15

8–15

Binding Energies

• 56Fe has highest Eb and is most stable isotope.

• Energy sources:– Fission for large radioactive

elements, such as U-235

– Fusion for two deuterium producing He. Not yet accomplished.

8–1 john a. schreifels chemistry 212 chapter 21-1 chapter 21 nuclear chemistry

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