Thermodynamics
Thermodynamics is the study of the transformation of energy. In chemistry, thermodynamics refers to the transformations of energy associated with chemical reactions.
Thermodynamics deals with quantities (e.g., energy, entropy) known as state functions. A state function is a property of a system that does not depend on pathways, only on the initial and final states.
The First Law of Thermodynamics states that energy is conserved; it can neither be created nor destroyed. The Second Law of Thermodynamics states that, in a isolated system, entropy--a measure of amount of energy in a system unavailable to do work--must increase as time passes. The Third Law of Thermodynamics states that the entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero (0 K). There is a fourth law called the "Zeroth" law that states if two objects are each in thermal equilibrium with a third object, then all three bodies are in thermal equilibrium with each other.
Although energy is one of the most fundamental concepts in physics or chemistry-- and is used in the fundamental definitions relating to thermodynamics-- the term energy is difficult to define. Energy is usually defined as "the ability to do work." The unit of energy in the International System of Units (SI), the joule (J), is replacing many older units (e.g., calorie).
Energy is a property of a system—not something that is exchanged between systems. Of course, the energy of a system can increase or decrease, but various mechanisms are required to accomplish these changes.. The mechanisms that accomplish changes in energy are forms of work. Heating is one such form of work.
Nobel prize winning physicist and renowned teacher of physics, Richard Feynman (1918-1988), once constructed an analogy between the law of conservation of energy and a mother's accounting for a child's indestructible blocks. Feynman, with his superb wit and intellect, ended his analogy by stating that accounting for energy was like accounting for the child's blocks except, "there are no blocks." This was Feynman's way of saying that we do not have a precise definition for energy. We can describe what it does and we can account for it (as we do in the laws of thermodynamics) but precisely defining exactly what energy is remains difficult and elusive.
Whatever energy is described to be, the mechanisms of work (e.g., heat) are the ways that systems change their state of energy. Energy itself does not flow or move between objects, or between systems and surroundings.
The total internal energy in a system is best described as the sum of the forms of energy of its components. Chemistry is usually concerned with the internal energy (E) of a system where the internal energy is the sum of energy (e.g., potential, kinetic, etc.) of all the particles (atoms, molecules, compounds, or complexes) in the system.
Most chemical reactions are performed under constant pressure conditions (at atmospheric pressure). As a result, except for reactions involving gases most of the changes of energy in a system are accomplished through the transfer of heat. Enthalpy (H) is the thermodynamic property that measures of the heat or heat changes in a system. Enthalpy of formation is defined as the enthalpy change in a reaction that forms a compound from its constituent elements in their naturally-occurring forms.
When a chemical reaction requires heat in order for it to proceed spontaneously (i.e., proceed without further outside assistance), the reaction is said to be endothermic. When reactions yield heat, they are considered to be exothermic.
Most thermodynamic calculations are carried out at what are termed standard thermodynamic conditions. Standard thermodynamic conditions exist when all gases are at a pressure of one atmosphere, all solids and liquids are pure and aqueous solutions are concentrations of one mole solute per liter (1 M). Standard enthalpies of formation are almost always reported at a temperature of 298 K.
Just as matter is conserved, energy is always conserved in chemical reactions. That is, if the masses of reactants used in a reaction are constant then the masses of the products formed and the changes in energy between the reactants and products is also constant.
It is easy to recognize that the First Law of Thermodynamics is a law of conservation similar to the law of conservation of matter. Considered together, the two laws demonstrate the equivalence of mass and energy. Like matter, energy is conserved in all chemical reactions. Further, just as the masses of reactants and products are constant, the amount of energy emitted or absorbed in a specific chemical reaction is a constant. The First Law of Thermodynamics also demands conservation of energy. As a result, the sum of the energy or a system and its surroundings must remain constant. As the energy level of a system drops, the energy level of the system's surroundings must increase by the same amount.
Systems and surroundings are described by variables such as pressure, volume, temperature, specific heat, density, compressibility, and the thermal expansion coefficient. Regardless, the first law of thermodynamics demands that all of the energy must be accounted for because none is lost or destroyed.
Although, according to the First Law of Thermodynamics, energy can neither be created or destroyed, it can change its form. There are not, however, an unlimited mechanism to accomplish these transformations. The forms include nuclear energy, kinetic energy (i.e., the energy possessed by moving bodies), potential energy, heat energy, electrical energy, mechanical energy, light energy and chemical energy (i.e., energy stored in chemical bonds).
The Second law of Thermodynamics implies that the entropy of the universe is increasing over time. With regard to isolated system-- those that can not do work upon their surroundings nor have work done upon them by the surroundings --entropy must also always increase over time.
When dealing with systems, entropy is a measure of the organization of a system. With regard to chemistry, a change in entropy is reflected in a change in the order or disorder of molecules. Because entropy is actually a measurement of disorder, an increase in entropy means an increase in the disorder of any system. Changes in entropy are measured in joules/Kelvin and is always related to absolute temperature (Kelvin scale).
As a consequence of the Second Law of Thermodynamics, natural process always move toward increasingly disorder or the lowest energy state. The increase in entropy within a system is spontaneous. During many processes-- the organization of molecules in living things‐ entropy in the system may decreases, that is, the system becomes more ordered. It is important to note, however, that this decrease in entropy is never accomplished without work being done on a system. The earth is not an isolated system-- liberalizing our definition of energy-- the sun is constantly supplying energy to Earth's systems.
According to the Second Law of Thermodynamics atoms seek the lowest energy level (i.e., ground state). Energy in an excited atom is often reduced by conversion into light energy as photons are emitted by the atom. As the energy levels in the excited hydrogen atom decrease, the electron returns to lower energy orbitals closer to the nucleus.
One of the earliest statements of the first two laws of thermodynamics was put forth by German physicist Rudolf Clausius (1822-1888) in a paper published in 1865. Clausius stated that the energy of the universe is constant. and that the entropy of the universe tends to a maximum. Much of Clausius's work was based on earlier writings of French mathematician Sadi Nicolas Léonard Carnot (1796-1832). Carnot had meticulously studied and articulated the physical and chemical principles dealing with the operation of heat engines (steam engines) designed to convert heat into mechanical work.
Carnot's observations laid the intellectual and mathematical groundwork for the formal statement of the first two laws of thermodynamics. An ideal cycle would be performed by a perfectly efficient heat engine, that is, all the heat would be converted to mechanical work. Carnot's calculations dealing with thermodynamic cycles in steam engines proved that an ideal engine-- one that was 100% efficient in the transformation of heat energy into mechanical energy could never exist. Because entropy increases some energy in every system is always made useless or unavailable.
There have been many attempts to build perpetual motion machines and energy contraptions that would have to violate the laws of thermodynamics if they were to operate as advertised. All have failed or been exposed as frauds either because they were purposefully designed to deceive (somewhat akin to magic tricks) or their proponents were careless in their analysis of the thermodynamics of the system and surroundings. Most of the time, wild claims to have discovered an exception to either of the first two laws of thermodynamics is simply sloppy "accounting for the blocks."
There are no known exceptions to the first two laws of thermodynamics.
The concept of entropy increasing, that is, of a isolated system becoming more random, with increasing amounts of energy unavailable to do work is what gives an arrow of time to reactions. We would find it most distressing to have water run uphill or shriveled fruit become ripe and then regress to an embryonic seed. The order of reactions in the natural world is based on the Second Law of Thermodynamics.
Third Law of Thermodynamics (the entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero (0 K)) implies that all molecular motion stops at absolute zero. The atoms in a perfectly pure crystal would be perfectly aligned and would not move. It is important to understand that this means that there is no movement between atoms. Atoms do not "freeze" and there is no cessation of movement of atomic particles within the atom.
The zeroth law of thermodynamics is commonly expressed as heat flowing from hot to cold objects. It would be unnatural to put a cube of ice in a warm drink and not have the ice melt (the increasing temperature allowing the constituent water molecule to move into the liquid phase) or for the drink not to get at least a bit cooler.
Much of the study of thermodynamics involves the use of sophisticated statistical analysis. Indeed, the application of statistical methods to the science is revolutionizing our views of natural processes in much the manner as did the concepts of relativity and quantum mechanics. In fact, an accurate depiction of the universe depends on the understanding and use of all three concepts.
It is a fundamental hypothesis of science, including chemistry, that natural phenomena are governed by laws that can be formulated in such a manner that they can be used to predict the outcome of events and reactions. Nonlinear thermodynamics is part of chaos theory and deals with efforts to find structure in systems that are so complex that they are usually termed "unpredictable."
In linear thermodynamics small changes produce small and predictable changes in systems. A certain measure of heat will always raise the energy level of a system by a certain amount. In nonlinear systems, the result of an action is highly dependent upon the conditions of the system. As a result, the change in energy in a nonlinear system would not only depend on the measure of heat but also on the state of the system.
Although nonlinear theory is more than half a century old, the application of high-speed computing rapidly advancing the application of the fundamental theories. A common feature of nonlinear systems is that their properties can be unexpected, counterintuitive and surprising. Non-linear thermodynamics deals with concepts such as self-organization.
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