Question

1. Which one describes not about a nuclear reaction but about a chemical reaction?

		Protons, neutrons, electrons, and other elementary particles may be involved.
		Rates of reaction normally are not affected by temperature, pressure, and catalysts.
		Only electrons in atomic or molecular orbitals are involved in the breaking and forming of bonds.
		Elements are converted from one to another. 2. If 12% of the original amount of a certain radioactive isotope is left after 5.2 years of its radioactive decay, what is the half-life of thisisotope? 28.1 years 14.05 years 1.699 years 7.025 years 3. Which statement is false? No chemical treatments can make radioactive elements harmless.    The rate of decay (disintegration) of U-238 is proportional to its sample size.    Changes in temperature or pressure cannot affect the rate of decay (disintegration) of radioactive elements.     Decay constant maintains its constancy only at the specific temperature.

Answer 1

1. Which one describes not about a nuclear reaction but about a chemical reaction?

a) Proton, neutrons, electron and other elementary particles may be involved.

Verdict: This describes a nuclear reaction.

Reason:  In simple chemical reactions, the the subatomic nucleons like neutrons do not participate. You need to provide a very high energy to take a nucleon out from the nucleus as they are boung by strong nuclear force and in simple chemical reaction that kind of external energy is not used.

b) Rate of reaction normally is nnot affected by temperature, pressure and catalyst.

Verdict: This also refers to the nuclear reaction.

Reason: Rate of a chemical reaction increases with addition of a catalyst as it lowers the activation barrier. Increase in pressure and temperature also increases rate because of higher rate of collision between reactants.

c) Only electrons in atomic or molecular orbitals are involved in breaking or forming of bonds.

Verdict: This describes the chemical reaction.

Reason: In most chemical reactions, atomic orbitals and molecular orbitals are involved in the formation of new bonds and breaking of existing bond via electron transfers. Whereas, in nuclear reactions, the nucleus of the reactant undergoes changes, sometimes by combining with another nuclei and other times breaking down into two different/same nuclei.

d) Elements are converted from one to another.

Verdict: This describes a nuclear reaction.

Reason: Addition of a proton to a nucleus changes the number of protons in it. As number of protons determines the identity of an atom,in nuclear reactions we see elements changing from one to another via addition or subtraction of one or more protons from their nucleus.

2. Radioactive decay follows first order kinetics.

The given isotopes decays to 12% of its original value in 5.2 years.

The first order rate equation can be written as

where k = rate constant of the decay

t = time , here 5.2 years.

a = 12%

a_0 = 100%

Hence, putting the values we can determine the rate constant of the decay.

The rate constant is related to the ahlf life by the relation

hence, the correct option is option c) 1.699 yr.

3.

a) No chemical treatment can make radioactive elements harmless.

Verdict: TRUE.

Reason: Chemical treatment does not affect the nuclear levels of a radioactive element, thus the radioactive material continue to be as dangerous as it is. Changing the state of the material or mixing it with other chemical can make it even more dangerous.

b) The rate of decay of U-238 is proportional to its sample size.

verdict: TRUE.

Reason: All the raadioactive decays follow first order kinetics. According to which the rate of decay is proportional to the current amount of the material that is present. It is evident from the following relation where k is the rate of decay.

c) Changes in temperature and pressure cannot affect the rate of decay of radioactive material.

Verdict: TRUE.

Reason: Again, change in temperature and pressure only result in change of the physical state of the material. It does not affect the nuclear energy levels or amount of radioactive material. Hence, it does not affect the rate of decay.

d) Decay constant maintains its constancy only at the specific temeperature.

Verdict: FALSE.

Reason: Decay constant is always constant irrespective of the value of temperature as given in the equation in part b). It only depends on the existing amount of radioactive material that is there.