Heat Involved in Changes & Reactions
(a) Physical Change
- energy is added or removed to overcome or allow intermolecular forces to act; the particles are unchanged at the molecular level
- enthalpy range is 100 –102 kJ/mol
(b) Chemical Change
- energy changes overcome the bonds between particles in the molecules; new substances are formed
- enthalpy range is 102 – 104 kJ/mol
C(s) + O2(g) → CO2(g) + 394 kJ
(c) Nuclear Change
- energy changes overcome the forces between protons and neutrons in the nuclei; new atoms are formed
- enthalpy range is 1010 – 1012 kJ/mol
Nuclear Reactions
- the nucleus is held together by the Strong Nuclear Force
- the Law of Conservation of Mass & the Law of Conservation of Energy (the First Law of Thermodynamics) do not apply to nuclear reactions; instead the Law of Conservation of Mass-Energy holds
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| This equation is used in tandem with the above equation.* |
* m is the mass of a particle in motion, mo is the rest mass of a particle, v is the particle’s velocity and c is the speed of light
- as the particle approaches the speed of light
(3 x 108
m/s), the mass of the particle approaches _______________ (note that all fill-in-the-blank answers can be found at the end of the lesson)
Natural Radioactivity
(a) Alpha Particles (𝞪)
- alpha particles are helium nuclei, which are relatively heavy particles
- does great damage, but not very penetrating (stopped by skin)
(b) Beta Particles (𝛃)
- beta particles are fast moving electrons, which are relatively light
- does moderate damage, but more penetrating than alpha particles (stopped by 10 cm of air)
- found in two forms electrons (𝛃- = 0-1e) or positrons ((𝛃+ = 0+1e); matter and anti-matter
Here is a video showing how to arrive at the answers for the above three questions. (I misspoke in the first part "zero plus ninety equals zero". Oops, it should be "zero plus ninety equals 90").
(c) Gamma Rays (𝝲)
- gamma rays are high energy rays or light, which have no mass
- does little damage, but very penetrating (stopped by 1 m of lead)
Half Life
Half-life is the amount of time required for a radioactive sample to decay to one-half its original mass, which is given by the equation:
Nuclear Reactions
(I) Fusion
Fusion is a nuclear process in which two light nuclei combine to form a single heavier nucleus. An example of a fusion reaction, important in thermonuclear weapons and in future nuclear reactors, is the reaction between two different hydrogen isotopes to form an isotope of helium:
This reaction liberates an amount of energy more than a million times greater than one gets from a typical chemical reaction. Even though fusion is an energetically favourable reaction for light nuclei, it does not occur under standard conditions here on Earth because of the large energy investment that is required (temperature of approximately 106 K).
Fusion reactions have been going on for billions of years in our universe. In fact, nuclear fusion reactions are responsible for the energy output of most stars, including our own Sun. Scientists on Earth have been able to produce fusion reactions for only about the last eighty years. At first, there were small-scale studies in which only a few fusion reactions actually occurred. However, these first experiments later lead to the development of thermonuclear fusion weapons (hydrogen bombs).
Fusion is the process that takes place in stars like our Sun. Whenever we feel the warmth of the Sun and see by its light, we are observing the products of fusion. We know that all life on Earth exists because the light generated by the Sun produces food and warms our planet. Therefore, we can say that fusion is the basis for our life.
(II) Fission
Fission is a nuclear process in which a heavy nucleus splits into two smaller nuclei. An example of a fission reaction that was used in the first atomic bomb and is still used in nuclear reactors is:
The products in the above equation are only one set of many possible product nuclei. Fission reactions can produce any combination of lighter nuclei so long as the number of protons and neutrons in the products sum up to those in the initial fissioning nucleus. In this case, the products, 134Xe and 100Sr are highly radioactive and will emit alpha, beta and gamma radiation. This is a major concern since these materials will be left in the atmosphere after a nuclear bomb detonation.
As with fusion, a great amount of energy can be released in fission because for heavy nuclei, the summed masses of the lighter product nuclei is less than the mass of the fissioning nucleus.
Fission occurs because of the electrostatic repulsion created by the large number of positively charged protons contained in a heavy nucleus. Two smaller nuclei have less internal electrostatic repulsion than one larger nucleus. So, once the larger nucleus can overcome the strong nuclear force which holds it together, it can fission. Fission can be seen as a "tug-of-war" between the strong attractive nuclear force and the repulsive electrostatic force. In fission reactions, electrostatic repulsion wins.
Fission is a process that has been occurring in the universe for billions of years. As mentioned above, we have not only used fission to produce energy for nuclear bombs, but we also use fission peacefully everyday to produce energy in nuclear power plants.
(III) Chain Reactions
A nuclear chain reaction is a self-sustaining fission reaction in which products from one event cause one or more subsequent events. For a chain reaction to occur, the sample of fissionable material must have a certain minimum mass called the critical mass. Otherwise, neutrons escape from the sample before they have the opportunity to strike another nucleus and cause additional fission. The chain stops if enough neutrons are lost.
Both nuclear bombs and nuclear power reactors employ chain reactions. The major difference between the two uses of fission is in how quickly the energy is released. A power plant produces controlled nuclear chain reactions, in which the energy produced is released slowly and manageably. In contrast, an atomic bomb releases its energy very quickly and violently, in an uncontrolled nuclear reaction.
Fill in the blank answers: ∞, 22286Rn, 9039Y, 0+1e, 73Li, 00𝝲
Homework:
Answers:
Enthalpy Review - Do all the T/F and multiple choice questions. Do at least 6 written questions.
Enthalpy Review Answers:
- theory:
- definitions (thermochemistry, enthalpy, system, surroundings, exothermic, endothermic, calorimetry, specific heat capacity, Hess' Law, state function, fission, fusion, critical mass, etc)
- properties of enthalpy
- calorimetry (assumptions)
- representing enthalpy changes
- standard enthalpy of formation
- present & future energy sources
- nuclear chemistry (comparison of PC/CC/NC, nuclear reactions, natural radioactivity, balancing nuclear equations, fission, fusion)
- calculations:
- calorimetry (q=mΔTc, ΔH=q/n)
- Hess' Law
- Heat of Formation (ΔHrxn=∑ΔHf(P) - ∑ΔHf(R))
- multi-step calculations
- half-life
