Energy Changes in Chemical and Nuclear Reactions
What happens when matter undergoes change? Clearly, new substances or states are produced, but energy changes also occur. If chemistry is the study of matter and its transformations, then thermochemistry is the study of the energy changes that accompany these transformations.
Changes that occur in matter may be classified as physical, chemical, or nuclear, depending on whether a change has occurred in the arrangements of the molecules, their electronic structure, or the nuclei of the atoms involved. Whether ice melts, iron rusts, or an isotope used in medical therapy undergoes radioactive decay, changes occur in the energy of chemical substances.
The study of energy changes during physical or chemical changes in matter is called thermochemistry. The study of thermochemistry uses some familiar terms in very precise ways. Scientists define energy as the ability to do work. Work is the energy transferred to an object by a force that causes the object to move.
All forms of energy can be classified as either kinetic energy or potential energy. Potential energy is energy due to the position or composition of an object. Kinetic energy is the energy of motion.
The energy associated with chemical bonds is also potential energy. The amount of energy released or absorbed in a chemical reaction equals the difference between the potential energy of the bonds in the reactants and the potential energy of the bonds in the products.
The total quantity of potential energy and kinetic energy of a substance is called thermal energy. In general, the quantity of thermal energy of a substance depends on how fast its entities atoms, ions, molecules, or polyatomic ions are moving.
Energy cannot be created or destroyed.
When you are studying such transfers of energy, it is important to distinguish between the substances undergoing a change, called the chemical system, and the system s environment, called the surroundings. A system is often represented by a chemical equation. For the burning of ethyne, the equation is:
The surroundings in this reaction would include anything that could absorb the thermal energy that has been released, such as metal parts, the air, and the welder s protective clothing. When the reaction occurs, heat, q, is transferred between substances. (An object possesses thermal energy but cannot possess heat.)
When heat transfers between a system and its surroundings, measurements of the temperature of the surroundings are used to classify the change as exothermic or endothermic. Exothermic: releasing energy to the surroundings Endothermic: absorbing energy from the surroundings
The acetylene torch reaction is clearly an exothermic reaction because heat flows into the surroundings. Chemical potential energy in the system is converted to heat energy, which is transferred to the surroundings and used to increase the thermal energy of the molecules of metal and air. Since the molecules in the surroundings have greater kinetic energy, the temperature of the surroundings increases measurably.
Chemical systems may be further classified. A chemical reaction that produces a gas in a solution in a beaker is described as an open system, since both energy and matter can flow into or out of the system. The surroundings include the beaker itself, the surface on which the beaker sits, and the air around the beaker. In the same way, most explosive reactions are considered to be open systems because it is so difficult to contain the energy and matter produced.
Most calculations of energy changes involve systems in which careful measurements of mass and temperature changes are made. These are considered to be isolated systems for the purpose of calculation. However, it is impossible to completely prevent energy from entering or leaving any system. In reality, the contents of a calorimeter, or of any container that prevents movement of matter, form a closed system.
Unlike chemical reactions, all nuclear reactions are exothermic. Per unit of mass, nuclear reactions release much more energy than exothermic chemical reactions. Two nuclear reactions involving large quantities of energy are fission and fusion.
A fusion reaction occurs when nuclei of small atomic mass combine to form larger, heavier nuclei. Fusion reactions are responsible for the release of vast amounts of energy in stars, including the Sun. The Sun consists of 73 % hydrogen, 26 % helium, and 1 % other elements. Hydrogen nuclei under immensely high pressure in the Sun undergo fusion and form helium nuclei.
The fusion process releases energy, some of which reaches Earth. The fusion of hydrogen atoms produces 1.7 x 10 9 kj of energy for each mole of helium produced.
During fission, large nuclei with high atomic mass are split into smaller, lighter nuclei by collision with a neutron. The nuclei of all elements above atomic number 83 are unstable and can undergo fission. Fission does not normally occur in nature. Fission reactions produce vast quantities of energy millions of times more than is released in chemical reactions.
Nuclear power plants use the fission of uranium 235 to produce electricity. When a neutron collides with a nucleus, the uranium nucleus splits into smaller nuclei and releases energy and additional neutrons. These neutrons collide with more uranium nuclei, causing these nuclei to split and release more energy and even more neutrons.
About 15 % of all electrical energy in Canada is produced by the fission of uranium in nuclear power plants. Nuclear power plants can generate much more electricity from a small amount of fuel than can power plants that use fossil fuels. For example, uranium fission can produce about 26 million times more energy than the combustion of an equal mass of methane.