A fission nuclear reaction is the splitting of a heavy nucleus into two or more lighter nuclei.

Example

U-235 is bombarded with slow neutrons to produce Ba-139, Kr-94, or other isotopes and 3 fast moving neutrons

A nuclear chain reaction is a reaction in which an initial step, such as the reaction above, leads to a succession of repeating steps that continues indefinitely. Nuclear chain reactions are used in nuclear reactors and nuclear bombs.

Example

A fusion nuclear reaction  is the combination of very light nuclei to make a heavier nucleus. Extremely high temperatures and pressures are required in order to overcome the repulsive forces of two nuclei.

Example

MU #1 Energy released during nuclear reactions is much greater than the energy released during chemical reactions. MU

Energy released in a nuclear reaction (fission or fusion) comes from the fractional amount of mass converted into energy. Nuclear changes convert matter into energy. 

Example 

Consider the fusion nuclear reaction taking place in stars.

Mass of reactants = 4 (mass of H-1) = 4 (1.00718 amu) = 4.02872 amu

Mass of products = 1 (mass of He-4) = 1 (4.00150amu) = 4.00150 amu

The difference between the mass of the reactants and products is  4.02872 amu - 4.00150 amu or  0.02722 amu

This difference  is called the mass defect and it is converted into energy according  to the formula E = mc²

E = energy

m= mass

c = speed of light (3 x 10⁸ m/s)

 Since the speed of light is a large number a small mass change corresponds to a large amount of energy.

Problem

Given the information below calculate the mass defect for the following nuclear reaction.

Am-241 = 241.0047 amu

Np-237 = 236.9972 amu

He-4 = 4.0015 amu

MU #2 There are benefits and risks associated with fission and fusion reactions.

Fission reactions

Benefits:

Large amount of energy is released

Production of electricity in nuclear plants (20% of the electricity produced in the US comes from nuclear power)

Development of nuclear weapons (Atomic bomb, depleted uranium bullets)

Creations of new elements in accelerators (Americium-241 used in smoke detectors)

Risks:

Dangerous nuclear waste are produced

Accidents can released radiations in the environment

Fusion reactions

Benefits:

Release of larger amount of energy than fission. However, they are not a practical source of energy because the technical problems of high temperature, pressure, and containment of reaction are enormous.

Risks:

Relatively low

TEST YOUR UNDERSTANDING

8/07

6/07

1/07

8/02

 Given the fusion reaction:

1/03 Given the nuclear equation:

a) State the type of nuclear reaction represented by the equation.

b) The sum of the masses of the products is slightly less than the sum of the masses of the reactants. Explain this loss of mass.

c) This process releases greater energy than an ordinary chemical reaction does. Name another type of nuclear reaction that releases greater energy than an ordinary chemical reaction.

1/04

46 Nuclear fusion differs from nuclear fission because nuclear fusion reactions
        (1) form heavier isotopes from lighter isotopes (2) form lighter isotopes from heavier isotopes
        (3) convert mass to energy (4) convert energy to mass

6/04

32 In a nuclear fusion reaction, the mass of the products is
        (1) less than the mass of the reactants because some of the mass has been converted to energy

        (2) less than the mass of the reactants because some of the energy has been converted to mass
        (3) more than the mass of the reactants because some of the mass has been converted to energy

        (4) more than the mass of the reactants because some of the energy has been converted to mass

1/05

30 Which change takes place in a nuclear fusion reaction?

                (1) Matter is converted to energy. (2) Energy is converted to matter.

                (3) Ionic bonds are converted to covalent bonds. (4) Covalent bonds are converted to ionic bonds.

6/05

31 Which equation represents positron decay? 

32 Which equation represents a fusion reaction? 

1/06

home