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Thus far we've talked about molecules, covalent bonds and other aspects of chemistry that involves physics. However we left out probably the most important physics-related topic in chemistry: energy.
This article will discuss, in general, on how the theories of energy physics are applied in chemistry. |
"The Second Law of Thermodynamics states that all energy systems run down like a clock and never rewind themselves. But life not only 'runs up,' converting low energy sea-water, sunlight and air into high-energy chemicals, it keeps multiplying itself into more and better clocks that keep 'running up' faster and faster. Why, for example, should a group of simple, stable compounds of carbon, hydrogen, oxygen and nitrogen struggle for billions of years to organize themselves into a professor of chemistry?"
Robert M. Pirsig, 'Lila', 1991
Energy is notoriously difficult to define. It is often defined as the ability to do work, but this is an incomplete definition. It is easy to recognize in most of its several forms: mechanical (including kinetic and gravitational potential), chemical, nuclear, electromagnetic (including light and other non-nuclear radiations), elastic, and thermal. The SI unit of energy is the joule, named in honor of James Prescott Joule, who discovered the equivalence of heat and mechanical energy. The joule is a derived unit, equivalent to kg m²/s² . It can also be expressed as newton-meters (Nm), but is not usually done to avoid confusion with the units of Torque (N x m) The word energy comes from the Greek words “en” and “ergon” which means at work.
Work is also measured in joules, which shows their close relationship. Energy can be used to perform work; work can be made to increase the energy of a system. This is one way of stating an important principle, the work-energy theorem. The two quantities are not the same, any more than mass and energy are the same thing, but one can be converted into the other. Unlike the situation with mass and energy however, there is not 100% conversion. The system will always lose some energy as heat, which will increase the entropy, or disorder, of the system. Note that this does not mean that some of the energy somehow disappears. It is merely lost from the human point of view, but not in terms of the Universe. It is no longer in a form that is useful to us. Heat is a form of energy and "counts" in the conservation law, and entropy is a measure of the amount of disorder that is created by the dissipation of heat.
The second law of thermodynamics says that when energy changes from one form to another form, or matter moves freely, entropy (disorder) increases.
Differences in temperature, pressure, and density tend to even out horizontally after a while. Due to the force of gravity, density and pressure do not even out vertically. Density and pressure on the bottom will be more than at the top. Entropy is therefore a measure of spread of matter and energy to everywhere they have access.
The most common wording for the second law of thermodynamics is essentially due to Rudolf Clausius: “It is impossible to construct a device which produces no other effect than transfer of heat from lower temperature body to higher temperature body.”
Energy in Chemistry
Chemical substances are made of atoms, or more generally, of positively charged nuclei surrounded by negatively charged electrons. A molecule such as dihydrogen, H2, is held together by electrostatic attractions mediated by the electrons shared between the two nuclei. The total potential energy of the molecule is the sum of the repulsions between like charges and the attractions between electrons and nuclei.
In other words, the potential energy of a molecule depends on the time-averaged relative locations of its constituent nuclei and electrons. This dependence is expressed by the familiar potential energy curve which serves as an important description of the chemical bond between two atoms.
In addition to translation, molecules composed of two or more atoms can possess other kinds of motion. Because a chemical bond acts as a kind of spring, the two atoms in H2 will have a natural vibrational frequency. In more complicated molecules, many different modes of vibration become possible, and these all contribute a vibrational term KE-vib to the total kinetic energy. Finally, a molecule can undergo rotational motions which give rise to a third term KE-rot.
Although this formula is simple and straightforward, it cannot take us very far in understanding and predicting the behavior of even one molecule, let alone a large number of them. The reason, of course, is the chaotic and unpredictable nature of molecular motion. Fortunately, the behavior of a large collection of molecules, like that of a large population of people, can be described by statistical methods. The study of thermochemistry attempts to understand and apply this phenomenon.
An example would be the breaking and formation of bonds, as below:
Work is also measured in joules, which shows their close relationship. Energy can be used to perform work; work can be made to increase the energy of a system. This is one way of stating an important principle, the work-energy theorem. The two quantities are not the same, any more than mass and energy are the same thing, but one can be converted into the other. Unlike the situation with mass and energy however, there is not 100% conversion. The system will always lose some energy as heat, which will increase the entropy, or disorder, of the system. Note that this does not mean that some of the energy somehow disappears. It is merely lost from the human point of view, but not in terms of the Universe. It is no longer in a form that is useful to us. Heat is a form of energy and "counts" in the conservation law, and entropy is a measure of the amount of disorder that is created by the dissipation of heat.
The second law of thermodynamics says that when energy changes from one form to another form, or matter moves freely, entropy (disorder) increases.
Differences in temperature, pressure, and density tend to even out horizontally after a while. Due to the force of gravity, density and pressure do not even out vertically. Density and pressure on the bottom will be more than at the top. Entropy is therefore a measure of spread of matter and energy to everywhere they have access.
The most common wording for the second law of thermodynamics is essentially due to Rudolf Clausius: “It is impossible to construct a device which produces no other effect than transfer of heat from lower temperature body to higher temperature body.”
Energy in Chemistry
Chemical substances are made of atoms, or more generally, of positively charged nuclei surrounded by negatively charged electrons. A molecule such as dihydrogen, H2, is held together by electrostatic attractions mediated by the electrons shared between the two nuclei. The total potential energy of the molecule is the sum of the repulsions between like charges and the attractions between electrons and nuclei.
In other words, the potential energy of a molecule depends on the time-averaged relative locations of its constituent nuclei and electrons. This dependence is expressed by the familiar potential energy curve which serves as an important description of the chemical bond between two atoms.
In addition to translation, molecules composed of two or more atoms can possess other kinds of motion. Because a chemical bond acts as a kind of spring, the two atoms in H2 will have a natural vibrational frequency. In more complicated molecules, many different modes of vibration become possible, and these all contribute a vibrational term KE-vib to the total kinetic energy. Finally, a molecule can undergo rotational motions which give rise to a third term KE-rot.
Although this formula is simple and straightforward, it cannot take us very far in understanding and predicting the behavior of even one molecule, let alone a large number of them. The reason, of course, is the chaotic and unpredictable nature of molecular motion. Fortunately, the behavior of a large collection of molecules, like that of a large population of people, can be described by statistical methods. The study of thermochemistry attempts to understand and apply this phenomenon.
An example would be the breaking and formation of bonds, as below:
Thermochemistry
Thermochemistry is the study of the energy and heat to do with chemical reactions and physical transformations (physical changes). Physical transformations are when a state of matter (a solid or liquid, for example) changes to another state. Examples of transformations include melting (when a solid becomes a liquid) and boiling (when a liquid becomes a gas).
A reaction gives out or takes in energy. A physical transformation also gives out or takes in energy. Thermochemistry looks at these energy changes, particularly on a system's energy exchange with its surroundings. Thermochemistry is useful in predicting reactant and product quantities at all times during a given reaction. Thermochemists do this by using data, including entropy determinations. Thermochemists will tell if a reaction is spontaneous or non-spontaneous, favorable or unfavorable.
Endothermic reactions take in heat. Exothermic reactions give out heat. Thermochemistry combines the concepts of thermodynamics with the idea of energy in the form of chemical bonds. It includes calculations of such quantities as heat capacity, heat of combustion, heat of formation, enthalpy, entropy, free energy, and calories.
Summary
Energy plays a huge role in chemistry and forms a basis of our understanding of chemical reactions. Although in its most fundamental form, essentially a subject of physics, understanding energy within the context of chemistry allows us to apply the theory of thermodynamics (a branch of physics) into the practical use of solving everyday problems.
Without this understanding, we would not be able to provide power for our homes and cities, economically smelt minerals, or even made the production and transport of goods (including food) possible. A thousand words is not enough to explain the role of energy in chemistry, but we hope this article will get you to ponder more on the subject, while we prepare to revisit it in the future.
Thermochemistry is the study of the energy and heat to do with chemical reactions and physical transformations (physical changes). Physical transformations are when a state of matter (a solid or liquid, for example) changes to another state. Examples of transformations include melting (when a solid becomes a liquid) and boiling (when a liquid becomes a gas).
A reaction gives out or takes in energy. A physical transformation also gives out or takes in energy. Thermochemistry looks at these energy changes, particularly on a system's energy exchange with its surroundings. Thermochemistry is useful in predicting reactant and product quantities at all times during a given reaction. Thermochemists do this by using data, including entropy determinations. Thermochemists will tell if a reaction is spontaneous or non-spontaneous, favorable or unfavorable.
Endothermic reactions take in heat. Exothermic reactions give out heat. Thermochemistry combines the concepts of thermodynamics with the idea of energy in the form of chemical bonds. It includes calculations of such quantities as heat capacity, heat of combustion, heat of formation, enthalpy, entropy, free energy, and calories.
Summary
Energy plays a huge role in chemistry and forms a basis of our understanding of chemical reactions. Although in its most fundamental form, essentially a subject of physics, understanding energy within the context of chemistry allows us to apply the theory of thermodynamics (a branch of physics) into the practical use of solving everyday problems.
Without this understanding, we would not be able to provide power for our homes and cities, economically smelt minerals, or even made the production and transport of goods (including food) possible. A thousand words is not enough to explain the role of energy in chemistry, but we hope this article will get you to ponder more on the subject, while we prepare to revisit it in the future.
Ponder this
Why would an endothermic reaction not considered as a violation of the 2nd Law of Thermodynamics?
Are nonthermal photoluminescent reactions considered exothermic?
Discuss
List out all forms of energy in physics, and relate them to how they apply in chemistry. Discuss the implications, and applications of each of these approach of energy in chemistry.
Further readings
Energy, Heat and Work in Chemistry, a rather conprehensive coverage of the topic.
Thermochemistry, from the Chemistry Encyclopedia.
Chemical thermodynamics, from the UNC-Chapel Hill's Department of Chemistry.
