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Heat capacity
Heat capacity (usually denoted by a capital C , often with subscripts), or thermal capacity , is the measurable physical quantity that characterizes the amount of heat required to change a body's temperature by a given amount. In the International System of Units (SI), heat capacity is expressed in units of joules per kelvin.
Derived quantities that specify heat capacity as an intensive property, independent of the size of a sample, are the molar heat capacity , which is the heat capacity per mole of a pure substance, and the specific heat capacity , often simply called specific heat , which is the heat capacity per unit mass of a material.
Temperature reflects the average total kinetic energy of particles in matter. Heat is transfer of thermal energy; it flows from regions of high temperature to regions of low temperature. Thermal energy is stored as kinetic energy in the random modes of translation in monatomic substances, and translations and rotations of polyatomic molecules in gases. Additionally, some thermal energy may be stored as the potential energy associated with higher-energy-modes of vibration, whenever they occur in interatomic bonds in any substance. Translation, rotation, and the two types of energy in vibration (kinetic and potential) represent the degrees of freedom of motion which classically contribute to the heat capacity of a thermodynamic system.
For quantum mechanical reasons, some of these degrees of freedom may not be available, or only partially available to store thermal energy, at a given temperature. As the temperature approaches absolute zero, the specific heat capacity of a system also approaches zero, due to loss of available degrees of freedom due to the quantum mechanical effect. Quantum theory can be used to quantitatively predict specific heat capacities in simple systems.
Background
Before the development of modern thermodynamics, it was thought that heat was a fluid, the so-called caloric. Bodies were capable of holding a certain amount of this fluid, hence the term heat capacity , named and first investigated by Joseph Black in the 1750s.[1]^ Today one instead discusses the internal energy of a system. This is made up of its microscopic kinetic and potential energy. Heat is no longer considered a fluid. Rather, it is a transfer of disordered energy at the microscopic level. Nevertheless, at least in English, the term "heat capacity" survives. Some other languages prefer the term thermal capacity , which is also sometimes used in English.
Extensive and intensive quantities
An object's heat capacity (symbol C ) is defined as the ratio between the amount of heat energy transferred to the object and the resulting increase in temperature of the object,
In the International System of Units, heat capacity has the unit joules per kelvin.
Heat capacity is an extensive property, meaning it is a physical property that scales with the size of a physical system. A sample containing twice the amount of substance of another requires twice the amount of heat transfer ( ) to achieve the same change in temperature ( ).
For many experimental and theoretical purposes it is more convenient to report heat capacity as an intensive property, as an intrinsically characteristic property of a particular substance. This is most often accomplished by the specification of the property per a unit of mass. In science and engineering, such properties are often prefixed with the term specific. .[2]^ International standards now recommend that specific heat capacity always refer to division by mass.[3]^ The units for the specific heat capacity are.
In chemistry, the heat capacity is also often specified relative one mole, the unit for amount of substance, and is called the molar heat capacity , having the unit.
For some considerations it is useful to specify the volume-specific heat capacity, commonly called volumetric heat capacity, which is the heat capacity per unit volume and has SI units. This is used almost exclusively
for liquids and solids, since for gasses it may be confused with specific heat capacity at constant volume.
Metrology
The heat capacity of most systems is not a constant. Rather, it depends on the state variables of the thermodynamic system under study. In particular it is dependent on temperature itself, as well as on the pressure and the volume of the system.
Different measurements of heat capacity can therefore be performed, most commonly at constant pressure and constant volume. The values thus measured are usually subscripted (by p and V , respectively) to indicate the definition. Gases and liquids are typically also measured at constant volume. Measurements under constant pressure produce larger values than those at constant volume because the constant pressure values also include heat energy that is used to do work to expand the substance against the constant pressure as its temperature increases. This difference is particularly notable in gases where values under constant pressure are typically 30% to 66.7% greater than those at constant volume.
The specific heat capacities of substances comprising molecules (as distinct from monatomic gases) are not fixed constants and vary somewhat depending on temperature. Accordingly, the temperature at which the measurement is made is usually also specified. Examples of two common ways to cite the specific heat of a substance are as follows:
- Water (liquid): cp = 4.1855 [J/(g·K)] (15 °C, 101.325 kPa)
- Water (liquid): CvH = 74.539 J/(mol·K) (25 °C)
For liquids and gases, it is important to know the pressure for which given heat-capacity data refer. Most published data are given for standard pressure. However, quite different standard conditions for temperature and pressure have been defined by different organizations. The International Union of Pure and Applied Chemistry (IUPAC) changed its recommendation from one Atmosphere to the round value 100 kPa (≈750.062 Torr).[4]
Calculation
The path integral Monte Carlo method is a numerical approach for determining the values of heat capacity, based on quantum dynamical principles.
Alternative units
An older unit of heat is the kilogram-calorie (Cal), originally defined as the energy required to raise the temperature of one kilogram of water by one degree centigrade, typically from 15°C to 16°C. The specific heat capacity of water on this scale would therefore be exactly 1 Cal/(K·kg). However, due to the temperature-dependence of the specific heat, a large number of different definitions of the calorie came into being. Whilst once it was very prevalent, especially its smaller cgs variant the gram-calorie (cal), defined so that the specific heat of water would be 1 cal/(K·g), in most fields the use of the calorie is now archaic.
In the United States other units of measure for heat capacity may be quoted in disciplines such as construction, civil engineering, and chemical engineering. A still common system is the English Engineering Units in which the mass reference is pound mass and the temperature is specified in degrees Fahrenheit or Rankine. One (rare) unit of heat is the pound calorie (lb-cal), defined as the amount of heat required to raise the temperature of one pound of water by one degree centigrade. On this scale the specific heat of water would be 1 lb-cal/(K·lb). More common is the British thermal unit, the standard unit of heat in the U.S. construction industry. This is defined such that the specific heat of water is 1 BTU/(°F·lb).
Ideal gas
For an ideal gas, evaluating the partial derivatives above according to the equation of state
the relation can be found to reduce to
where n is number of moles of gas in the thermodynamic system under consideration, and R is the universal gas constant. Dividing through by n , this equation reduces simply to Mayer's relation,
where and are intensive property heat capacities expressed on a per mole basis at constant pressure and
constant volume, respectively.
Specific heat capacity
The specific heat capacity of a material on a per mass basis is
which in the absence of phase transitions is equivalent to
where
is the heat capacity of a body made of the material in question, is the mass of the body, is the volume of the body, and
is the density of the material.
For gases, and also for other materials under high pressures, there is need to distinguish between different boundary conditions for the processes under consideration (since values differ significantly between different conditions). Typical processes for which a heat capacity may be defined include isobaric (constant pressure, ) or
isochoric (constant volume, ) processes. The corresponding specific heat capacities are expressed as
From the results of the previous section, dividing through by the mass gives the relation
A related parameter to is , the volumetric heat capacity. In engineering practice, for solids or liquids
often signifies a volumetric heat capacity, rather than a constant-volume one. In such cases, the mass-specific heat capacity (specific heat) is often explicitly written with the subscript , as. Of course, from the above relationships, for solids one writes
For pure homogeneous chemical compounds with established molecular or molar mass or a molar quantity is established, heat capacity as an intensive property can be expressed on a per mole basis instead of a per mass basis
by the following equations analogous to the per mass equations:
= molar heat capacity at constant pressure
= molar heat capacity at constant volume
where n = number of moles in the body or thermodynamic system. One may refer to such a per mole quantity as molar heat capacity to distinguish it from specific heat capacity on a per mass basis.
Polytropic heat capacity
The polytropic heat capacity is calculated at processes if all the thermodynamic properties (pressure, volume, temperature) change
= molar heat capacity at polytropic process
The most important polytropic processes run between the adiabatic and the isotherm functions, the polytropic index is between 1 and the adiabatic exponent (γ or κ)
Dimensionless heat capacity
The dimensionless heat capacity of a material is
where
C is the heat capacity of a body made of the material in question (J/K) n is the amount of substance in the body (mol) R is the gas constant (J/(K·mol) N is the number of molecules in the body. (dimensionless) k is Boltzmann’s constant (J/(K·molecule)
In the ideal gas article, dimensionless heat capacity is expressed as , and is related there directly to half the number of degrees of freedom per particle. This holds true for quadratic degrees of freedom, a consequence of the equipartition theorem.
More generally, the dimensionless heat capacity relates the logarithmic increase in temperature to the increase in the dimensionless entropy per particle , measured in nats.
Alternatively, using base 2 logarithms, C *^ relates the base-2 logarithmic increase in temperature to the increase in the dimensionless entropy measured in bits.[5]
Theory of heat capacity
Factors that affect specific heat capacity
Molecules undergo many characteristic internal vibrations. Potential energy stored in these internal degrees of freedom contributes to a sample’s energy content, [10] [11] but not to its temperature. More internal degrees of freedom tend to increase a substance's specific heat capacity, so long as temperatures are high enough to overcome quantum effects.
For any given substance, the heat capacity of a body is directly proportional to the amount of substance it contains (measured in terms of mass or moles or volume). Doubling the amount of substance in a body doubles its heat capacity, etc.
However, when this effect has been corrected for, by dividing the heat capacity by the quantity of substance in a body, the resulting specific heat capacity is a function of the structure of the substance itself. In particular, it depends on the number of degrees of freedom that are available to the particles in the substance, each of which type of freedom allows substance particles to store thermal energy. The kinetic energy of substance particles is the only one of the many possible degrees of freedom which manifests as temperature change , and thus the larger the number of degrees of freedom available to the particles of a substance other than kinetic energy, the larger will be the specific heat capacity for the substance.
In addition, quantum effects require that whenever energy be stored in any mechanism associated with a bound system which confers a degree of freedom, it must be stored in certain minimal-sized deposits (quanta) of energy, or else not stored at all. Such effects limit the full ability of some degrees of freedom to store energy when their lowest energy storage quantum amount is not easily supplied at the average energy of particles at a given temperature. In general, for this reason, specific heat capacities tend to fall at lower temperatures where the average thermal energy available to each particle degree of freedom is smaller, and thermal energy storage begins to be limited by these quantum effects. Due to this process, as temperature falls toward absolute zero, so also does heat capacity.
Degrees of freedom
Molecules are quite different from the monatomic gases like helium and argon. With monatomic gases, thermal energy comprises only translational motions. Translational motions are ordinary, whole-body movements in 3D space whereby particles move about and exchange energy in collisions—like rubber balls in a vigorously shaken container (see animation here [12]). These simple movements in the three X, Y, and Z–axis dimensions of space means individual atoms have three translational degrees of freedom. A degree of freedom is any form of energy in which heat transferred into an object can be stored. This can be in translational kinetic energy, rotational kinetic energy, or other forms such as potential energy in vibrational modes. Only three translational degrees of freedom (corresponding to the three independent directions in space) are available for any individual atom, whether it is free, as a monatomic molecule, or bound into a polyatomic molecule.
As to rotation about an atom's axis (again, whether the atom is bound or free), its energy of rotation is proportional to the moment of inertia for the atom, which is extremely small compared to moments of inertia of collections of atoms. This is because almost all of the mass of a single atom is concentrated in its nucleus, which has a radius too small to
give a significant moment of inertia. In contrast, the spacing of quantum energy levels for a rotating object is inversely proportional to its moment of inertia, and so this spacing becomes very large for objects with very small moments of inertia. For these reasons, the contribution from rotation of atoms on their axes is essentially zero in monatomic gases, because the energy-spacing of the associated quantum levels is too large for significant thermal energy to be stored in rotation of systems such small moments of inertia. For similar reasons, axial rotation around bonds joining atoms in diatomic gases (or along the linear axis in a linear molecule of any length) can also be neglected as a possible "degree of freedom" as well, since such rotation is similar to rotation of monatomic atoms, and so occurs about an axis with a moment of inertia too small to be able to store significant heat energy.
In polyatomic molecules, other rotational modes may become active, due to the much higher moments of inertia about certain axes which do not coincide with the linear axis of a linear molecule. These modes take the place of some translational degrees of freedom for individual atoms, since the atoms are moving in 3-D space, as the molecule rotates. The narrowing of quantum mechanically-determined energy spacing between rotational states results from situations where atoms are rotating around an axis that does not connect them, and thus form an assembly that has a large moment of inertia. This small difference between energy states allows the kinetic energy of this type of rotational motion to store heat energy at ambient temperatures. Furthermore (although usually at higher temperatures than are able to store heat in rotational motion) internal vibrational degrees of freedom also may become active (these are also a type of translation, as seen from the view of each atom). In summary, molecules are complex objects with a population of atoms that may move about within the molecule in a number of different ways (see animation at right), and each of these ways of moving is capable of storing energy if the temperature is sufficient.
The heat capacity of molecules ( on a per-atom, or atom-molar, basis ) does not exceed the heat capacity of monatomic gases, unless vibrational modes are brought into play. The reason for this is that vibrational modes allow energy to be stored as potential energy in intra-atomic bonds in a molecule, which are not available to atoms in monatomic gases. Up to about twice as much energy (on a per-atom basis) per unit of temperature increase can be stored in a solid as in a monatomic gas, by this mechanism of storing energy in the potentials of interatomic bonds. This gives many solids about twice the atom-molar heat capacity of monatomic gases, at the highest temperatures their structure can withstand.
However, quantum effects heavily affect the actual ratio at lower temperatures—especially in solids with light and tightly-bound atoms (e.g., beryllium metal). Smaller polyatomic gases store intermediate amounts of energy, giving them a per-atom heat capacity that is between that of monatomic gases (^3 ⁄ 2 R per mole, where R is the ideal gas constant), and the maximum of fully-excited warmer solids (3 R per mole). For gases, heat capacity never falls below the minumum of 3 ⁄ 2 R per mole, since the kinetic energy of gas molecules is always available to store this much heat energy. However, for cryogenic temperatures in solids, heat capacity falls toward zero, as temperature approaches absolute zero.
Example of temperature-dependent specific heat capacity, in a diatomic gas
To illustrate the role of various degrees of freedom in storing heat, we may consider nitrogen, a diatomic molecule that has five active degrees of freedom at room temperature: the three comprising translational motion plus two rotational degrees of freedom internally. Although the constant-volume molar heat capacity of nitrogen at this temperature is five-thirds that of monatomic gases, on a per-mole of atoms basis, it is five-sixths that of a monatomic gas. The reason for this is the loss of a degree of freedom due to the bond when it does not allow storage of thermal energy. Two separate nitrogen atoms would have a total of six degrees of freedom—the three translational degrees of freedom of each atom. When the atoms are bonded the molecule will still only have three translational degrees of freedom, as the two atoms in the molecule move as one. However, the molecule cannot be treated as a point object, and the moment of inertia has increased sufficiently about two axes to allow two rotational degrees of freedom to be active at room temperature to give five degrees of freedom. The moment of inertia about the third axis remains small, as this is the axis passing through the centres of the two atoms, and so is similar to the small moment of inertia
available to each atom. Each of these six contributes 1 ⁄ 2 R specific heat capacity per mole of atoms.[16]^ This limit of 3 R per mole specific heat capacity is approached at room temperature for most solids, with significant departures at this temperature only for solids composed of the lightest atoms which are bound very strongly, such as beryllium (where the value is only of 66% of 3 R ), or diamond (where it is only 24% of 3 R ). These large departures are due to quantum effects which prevent full distribution of heat into all vibrational modes, when the energy difference between vibrational quantum states is very large compared to the average energy available to each atom from the ambient temperature.
For monatomic gases, the specific heat is only half of 3 R per mole, i.e. (^3 ⁄ 2 R per mole) due to loss of all potential energy degrees of freedom in these gases. For polyatomic gases, the heat capacity will be intermediate between these values on a per-mole-of-atoms basis, and (for heat-stable molecules) would approach the limit of 3 R per mole of atoms, for gases composed of complex molecules, and at higher temperatures at which all vibrational modes accept excitational energy. This is because very large and complex gas molecules may be thought of as relatively large blocks of solid matter which have lost only a relatively small fraction of degrees of freedom, as compared to a fully-integrated solid.
Corollaries of these considerations for solids (volume-specific heat capacity)
Since the bulk density of a solid chemical element is strongly related to its molar mass (usually about 3 R per mole, as noted above), there exists noticeable inverse correlation between a solid’s density and its specific heat capacity on a per-mass basis. This is due to a very approximate tendency of atoms of most elements to be about the same size, despite much wider variations in density and atomic weight. These two factors (constancy of atomic volume and constancy of mole-specific heat capacity) result in a good correlation between the volume of any given solid chemical element and its total heat capacity. Another way of stating this, is that the volume-specific heat capacity (volumetric heat capacity) of solid elements is roughly a constant. The molar volume of solid elements is very roughly constant, and (even more reliably) so also is the molar heat capacity for most solid substances. These two factors determine the volumetric heat capacity, which as a bulk property may be striking in consistency. For example, the element uranium is a metal which has a density almost 36 times that of the metal lithium, but uranium's specific heat capacity on a volumetric basis (i.e. per given volume of metal) is only 18% larger than lithium's.
Since the volume-specific corollary of the Dulong-Petit specific heat capacity relationship requires that atoms of all elements take up (on average) the same volume in solids, there are many departures from it, with most of these due to variations in atomic size. For instance, arsenic, which is only 14.5% less dense than antimony, has nearly 59% more specific heat capacity on a mass basis. In other words; even though an ingot of arsenic is only about 17% larger than an antimony one of the same mass, it absorbs about 59% more heat for a given temperature rise. The heat capacity ratios of the two substances closely follows the ratios of their molar volumes (the ratios of numbers of atoms in the same volume of each substance); the departure from the correlation to simple volumes in this case is due to lighter arsenic atoms being significantly more closely-packed than antimony atoms, instead of similar size. In other words, similar-sized atoms would cause a mole of arsenic to be 63% larger than a mole of antimony, with a correspondingly lower density, allowing its volume to more closely mirror its heat capacity behavior.
Other factors
Hydrogen bonds
Hydrogen-containing polar molecules like ethanol, ammonia, and water have powerful, intermolecular hydrogen bonds when in their liquid phase. These bonds provide another place where heat may be stored as potential energy of vibration, even at comparatively low temperatures. Hydrogen bonds account for the fact that liquid water stores nearly the theoretical limit of 3 R per mole of atoms, even at relatively low temperatures (i.e. near the freezing point of water).
Impurities
In the case of alloys, there are several conditions in which small impurity concentrations can greatly affect the specific heat. Alloys may exhibit marked difference in behaviour even in the case of small amounts of impurities being one element of the alloy; for example impurities in semiconducting ferromagnetic alloys may lead to quite different specific heat properties.[17]
The simple case of the monatomic gas
In the case of a monatomic gas such as helium under constant volume, if it assumed that no electronic or nuclear quantum excitations occur, each atom in the gas has only 3 degrees of freedom, all of a translational type. No energy dependence is associated with the degrees of freedom which define the position of the atoms. While, in fact, the degrees of freedom corresponding to the momenta of the atoms are quadratic, and thus contribute to the heat capacity. There are N atoms, each of which has 3 components of momentum, which leads to 3 N total degrees of freedom. This gives:
where
is the heat capacity at constant volume of the gas is the molar heat capacity at constant volume of the gas N is the total number of atoms present in the container n is the number of moles of atoms present in the container ( n is the ratio of N and Avogadro’s number) R is the ideal gas constant, (8.314570[70] J/(mol·K). R is equal to the product of Boltzmann’s constant and Avogadro’s number The following table shows experimental molar constant volume heat capacity measurements taken for each noble monatomic gas (at 1 atm and 25 °C):
Diatomic gas C V, m (J/(mol·K)) C V, m / R H 2 20.18 2. CO 20.2 2. N 2 19.9 2. Cl 2 24.1 3. Br 2 (vapour) 28.2 3.
From the above table, clearly there is a problem with the above theory. All of the diatomics examined have heat capacities that are lower than those predicted by the equipartition theorem, except Br 2. However, as the atoms composing the molecules become heavier, the heat capacities move closer to their expected values. One of the reasons for this phenomenon is the quantization of vibrational, and to a lesser extent, rotational states. In fact, if it is assumed that the molecules remain in their lowest energy vibrational state because the inter-level energy spacings for vibration-energies are large, the predicted molar constant volume heat capacity for a diatomic molecule becomes just that from the contributions of translation and rotation:
which is a fairly close approximation of the heat capacities of the lighter molecules in the above table. If the quantum harmonic oscillator approximation is made, it turns out that the quantum vibrational energy level spacings are actually inversely proportional to the square root of the reduced mass of the atoms composing the diatomic molecule. Therefore, in the case of the heavier diatomic molecules such as chlorine or bromine, the quantum vibrational energy level spacings become finer, which allows more excitations into higher vibrational levels at lower temperatures. This limit for storing heat capacity in vibrational modes, as discussed above, becomes 7"R" /2 = 3.5 R per mole, which is fairly consistent with the measured value for Br 2 at room temperature. As temperatures rise, all diatomic gases approach this value.
General gas phase
The specific heat of the gas is best conceptualized in terms of the degrees of freedom of an individual molecule. The different degrees of freedom correspond to the different ways in which the molecule may store energy. The molecule may store energy in its translational motion according to the formula:
where m is the mass of the molecule and is velocity of the center of mass of the molecule. Each
direction of motion constitutes a degree of freedom, so that there are three translational degrees of freedom. In addition, a molecule may have rotational motion. The kinetic energy of rotational motion is generally expressed as
where I is the moment of inertia tensor of the molecule, and is the angular velocity pseudo-vector (in
a coordinate system aligned with the principle axes of the molecule). In general, then, there will be three additional degrees of freedom corresponding to the rotational motion of the molecule, (For linear molecules one of the inertia tensor terms vanishes and there are only two rotational degrees of freedom). The degrees of freedom corresponding to translations and rotations are called the rigid degrees of freedom, since they do not involve any deformation of the molecule. The motions of the atoms in a molecule which are not part of its gross translational motion or rotation may be classified as vibrational motions. It can be shown that if there are n atoms in the molecule, there will be as many as vibrational degrees of freedom, where is the number of rotational degrees of freedom. A
vibrational degree of freedom corresponds to a specific way in which all the atoms of a molecule can vibrate. The actual number of possible vibrations may be less than this maximal one, due to various symmetries.
For example, triatomic nitrous oxide N 2 O will have only 2 degrees of rotational freedom (since it is a linear molecule) and contains n=3 atoms: thus the number of possible vibrational degrees of freedom will be v = (3*3)-3- = 4. There are four ways or "modes" in which the three atoms can vibrate, corresponding to 1) A mode in which an atom at each end of the molecule moves away from, or towards, the center atom at the same time, 2) a mode in which either end atom moves asynchronously with regard to the other two, and 3) and 4) two modes in which the molecule bends out of line, from the center, in the two possible planar directions that are orthogonal to its axis. Each vibrational degree of freedom confers TWO total degrees of freedom, since vibrational energy mode partitions into 1 kinetic and 1 potential mode. This would give nitrous oxide 3 translational, 2 rotational, and 4 vibrational modes (but these last giving 8 vibrational degrees of freedom), for storing energy. This is a total of f = 3+2+8 = 13 total energy-storing degrees of freedom, for N 2 O.
For a bent molecule like water H 2 O, a similar calculation gives 9-3-3 = 3 modes of vibration, and 3 (translational) + 3 (rotational) + 6(vibratonal) = 12 degrees of freedom.
The storage of energy into degrees of freedom
If the molecule could be entirely described using classical mechanics, then the theorem of equipartition of energy could be used to predict that each degree of freedom would have an average energy in the amount of (1/2) kT where k is Boltzmann’s constant and T is the temperature. Our calculation of the constant-volume heat capacity would be straightforward. Each molecule would be holding, on average, an energy of ( f /2) kT where f is the total number of degrees of freedom in the molecule. Note that Nk = R if N is Avogadro's number, which is the case in considering the heat capacity of a mole of molecules. Thus, the total internal energy of the gas would be ( f /2) NkT where N is the total number of molecules. The heat capacity (at constant volume) would then be a constant ( f/ 2) Nk the mole-specific heat capacity would be ( f/ 2) R the molecule-specific heat capacity would be ( f /2) k and the dimensionless heat capacity would be just f /2. Here again, each vibrational degree of freedom contributes 2f. Thus, a mole of nitrous oxide would have a total constant-volume heat capacity (including vibration) of (13/2) R by this calculation.
In summary, the molar heat capacity (mole-specific heat capacity) of an ideal gas with f degrees of freedom is given by
This equation applies to all polyatomic gases, if the degrees of freedom are known.[18]
The constant-pressure heat capacity for any gas would exceed this by an extra factor of R (see Mayer's relation, above). As example Cp would be a total of (15/2)R/mole for nitrous oxide.
The effect of quantum energy levels in storing energy in degrees of freedom
The various degrees of freedom cannot generally be considered to obey classical mechanics, however. Classically, the energy residing in each degree of freedom is assumed to be continuous—it can take on any positive value, depending on the temperature. In reality, the amount of energy that may reside in a particular degree of freedom is quantized: It may only be increased and decreased in finite amounts. A good estimate of the size of this minimum amount is the energy of the first excited state of that degree of freedom above its ground state. For example, the first vibrational state of the hydrogen chloride (HCl) molecule has an energy of about 5.74 × 10−^20 joule. If this amount of energy were deposited in a classical degree of freedom, it would correspond to a temperature of about 4156 K.
If the temperature of the substance is so low that the equipartition energy of (1/2) kT is much smaller than this excitation energy, then there will be little or no energy in this degree of freedom. This degree of freedom is then said to be “frozen out". As mentioned above, the temperature corresponding to the first excited vibrational state of HCl is
breaking the bonds of the potential energy interactions between molecules of a substance. As in the case of hydrogen, it is also possible for phase changes to be hindered as the temperature drops, so that they do not catch up and become apparent, without a catalyst. For example, it is possible to supercool liquid water to below the freezing point, and not observe the heat evolved when the water changes to ice, so long as the water remains liquid. This heat appears instantly when the water freezes.
Solid phase
The dimensionless heat capacity divided by three, as a function of temperature as predicted by the Debye model and by Einstein’s earlier model. The horizontal axis is the temperature divided by the Debye temperature. Note that, as expected, the dimensionless heat capacity is zero at absolute zero, and rises to a value of three as the temperature becomes much larger than the Debye temperature. The red line corresponds to the classical limit of the Dulong-Petit law
For matter in a crystalline solid phase, the Dulong-Petit law, which was discovered empirically, states that the mole-specific heat capacity assumes the value 3 R. Indeed, for solid metallic chemical elements at room temperature, molar heat capacities range from about 2.8 R to 3.4 R. Large exceptions involve solids composed of light, tightly-bonded atoms such as beryllium at 2.0 R , and diamond at only 0.735 R. The latter conditions create large quantum vibrational energy spacing, so that many vibrational modes are not available (are frozen out) at room temperature.
The theoretical maximum heat capacity for larger and larger multi-atomic gases at higher temperatures, also approaches the Dulong-Petit limit of 3 R , so long as this is calculated per mole of atoms, not molecules. The reason is that gases with very large molecules, in theory have almost the same high-temperature heat capacity as solids, lacking only the (small) heat capacity contribution that comes from potential energy that cannot be stored between separate molecules in a gas.
The Dulong-Petit limit results from the equipartition theorem, and as such is only valid in the classical limit of a microstate continuum, which is a high temperature limit. For light and non-metallic elements, as well as most of the common molecular solids based on carbon compounds at standard ambient temperature, quantum effects may also play an important role, as they do in multi-atomic gases. These effects usually combine to give heat capacities lower than 3 R per mole of atoms in the solid, although in molecular solids, heat capacities calculated per mole of molecules in molecular solids may be more than 3 R. For example, the heat capacity of water ice at the melting point is about 4.6 R per mole of molecules, but only 1.5 R per mole of atoms. The lower than 3 R number "per atom" (as is the case with diamond and beryllium) results from the “freezing out” of possible vibration modes for light atoms at suitably low temperatures, just as in many low-mass-atom gases at room temperatures. Because of high crystal binding energies, these effects are seen in solids more often than liquids: for example the heat capacity of liquid water is twice that of ice at near the same temperature, and is again close to the 3 R per mole of atoms of the Dulong-Petit theoretical maximum.
For a more modern and precise analysis of the heat capacities of solids, especially at low temperatures, it is useful to use the idea of phonons. See Debye model.
The specific heat of amorphous materials has characteristic discontinuities at the glass transition temperature due to rearrangements that occur in the distribution of atoms.[19]^ These discontinuities are frequently used to detect the glass transition temperature where a supercooled liquid transforms to a glass.[20]
Table of specific heat capacities
Note that especially high values, as for paraffin, water and ammonia, result from calculating specific heats in terms of moles of molecules. If specific heat is expressed per mole of atoms for these substances, few constant-volume values exceed the theoretical Dulong-Petit limit of 25 J/(mol·K) = 3 R per mole of atoms.
Table of specific heat capacities at 25 °C unless otherwise noted
Substance Phase (^) c p J·g−^1 ·K−^1
Cp,m J·mol−^1 ·K−^1
Cv,m J·mol−^1 ·K−^1
Volumetric heat capacity J·cm−^3 ·K−^1 Air (Sea level, dry, 0 °C) gas 1.0035 29.07 20.7643 0. Air (typical room conditionsA) gas^ 1.012^ 29.19^ 20. Aluminium solid 0.897 24.2 2. Ammonia liquid 4.700 80.08 3. Animal (and human) tissue[21]^ mixed^ 3.5^ —^ 3.7* Antimony solid 0.207 25.2 1. Argon gas 0.5203 20.7862 12. Arsenic solid 0.328 24.6 1. Beryllium solid 1.82 16.4 3. Bismuth[22]^ solid^ 0.123^ 25.7^ 1. Cadmium solid 0.231 — — Carbon dioxide CO 2 [18]^ gas^ 0.839^ 36.94^ 28. Chromium solid 0.449 — — Copper solid 0.385 24.47 3. Diamond solid 0.5091 6.115 1. Ethanol liquid 2.44 112 1. Gasoline liquid 2.22 228 1. Glass[22]^ solid^ 0. Gold solid 0.129 25.42 2. Granite[22]^ solid^ 0.790^ 2. Graphite solid 0.710 8.53 1. Helium gas 5.1932 20.7862 12. Hydrogen gas 14.30 28. Hydrogen sulfide H 2 S[18]^ gas^ 1.015^ 34. Iron solid 0.450 25.1 3.
Specific heat capacity of building materials
(Usually of interest to builders and solar designers)
Specific heat capacity of building materials
Substance Phase cp J/(g·K) Asphalt solid 0. Brick solid 0. Concrete solid 0. Glass, silica solid 0. Glass, crown solid 0. Glass, flint solid 0. Glass, pyrex solid 0. Granite solid 0. Gypsum solid 1. Marble, mica solid 0. Sand solid 0. Soil solid 0. Wood solid 1.7 (1.2 to 2.3) Substance Phase cp J/(g·K)
Notes
[1] Laider, Keith, J. (1993). The World of Physical Chemistry (http:/ / books. google. com/ ?id=01LRlPbH80cC& printsec=frontcover). Oxford University Press. ISBN 0-19-855919-4.. [2] International Union of Pure and Applied Chemistry, Physical Chemistry Division. "Quantities, Units and Symbols in Physical Chemistry" (http:/ / old. iupac. org/ publications/ books/ gbook/ green_book_2ed. pdf). Blackwell Sciences. p. 7.. "The adjective specific before the name of an extensive quantity is often used to mean divided by mass." [3] International Bureau of Weights and Measures (2006), The International System of Units (SI) (http:/ / www. bipm. org/ utils/ common/ pdf/ si_brochure_8_en. pdf) (8th ed.), ISBN 92-822-2213-6, [4] IUPAC.org, Gold Book, Standard Pressure (http:/ / goldbook. iupac. org/ S05921. html). Besides being a round number, this had a very practical effect: relatively few people live and work at precisely sea level; 100 kPa equates to the mean pressure at an altitude of about 112 metres (which is closer to the 194–metre, world–wide median altitude of human habitation). [5] Fraundorf, P. (2003). "Heat capacity in bits". American Journal of Physics 71 : 1142. doi:10.1119/1.1593658.( arXiv:cond-mat/ (http:/ / arxiv. org/ abs/ cond-mat/ 9711074)) [6] D. Lynden-Bell & R. M. Lynden-Bell (Nov. 1977). "On the negative specific heat paradox". Monthly Notices of the Royal Astronomical Society 181 : 405–419. Bibcode 1977MNRAS.181..405L. doi:10.1016/S0378-4371(98)00518-4. [7] Lynden-Bell, D. (Dec. 1998). Negative Specific Heat in Astronomy, Physics and Chemistry. arXiv:cond-mat/9812172v1. [8] Schmidt, Martin; Kusche, Robert; Hippler, Thomas; Donges, Jörn; Kronmüller, Werner; Von Issendorff, Bernd; Haberland, Hellmut (2001). "Negative Heat Capacity for a Cluster of 147 Sodium Atoms". Physical Review Letters 86 (7): 1191. doi:10.1103/PhysRevLett.86.1191. PMID 11178041. [9] See eg, Wallace, David. "Gravity, entropy, and cosmology: in search of clarity" (http:/ / philsci-archive. pitt. edu/ archive/ 00004744/ 01/ gravent_archive. pdf). British Journal for the Philosophy of Science.. Section 4 and onwards. [10] Reif, F. (1965). Fundamentals of statistical and thermal physics. McGraw-Hill. pp. 253–254. ISBN 07-051800-9. [11] Charles Kittel; Herbert Kroemer (2000). Thermal physics. Freeman. pp. 78. ISBN 0716710889. [12] http:/ / upload. wikimedia. org/ wikipedia/ commons/ 6/ 6d/ Translational_motion. gif [13] Smith, C. G. (2008). Quantum Physics and the Physics of large systems, Part 1A Physics. University of Cambridge.
[14] The comparison must be made under constant-volume conditions— CvH —so that no work is performed. Nitrogen’s CvH (100 kPa, 20 °C) = 20.8 J mol–^1 K–^1 vs. the monatomic gases which equal 12.4717 J mol–^1 K–^1. Citations: W.H. Freeman’s (http:/ / www. whfreeman. com/ ) Physical Chemistry , Part 3: Change ( 422 kB PDF, here (http:/ / www. whfreeman. com/ college/ pdfs/ pchem8e/ PC8eC21. pdf)), Exercise 21.20b, Pg. 787. Also Georgia State University’s (http:/ / www. gsu. edu/ ) Molar Specific Heats of Gases (http:/ / hyperphysics. phy-astr. gsu. edu/ hbase/ kinetic/ shegas. html). [15] Petit A.-T., Dulong P.-L. (1819). Translation "Recherches sur quelques points importants de la Théorie de la Chaleur" (http:/ / web. lemoyne. edu/ ~giunta/ PETIT. html). Annales de Chimie et de Physique 10 : 395–413. Translation. [16] "Discussion of heat capacity of solids in terms of degrees of freedom" (http:/ / ruelle. phys. unsw. edu. au/ ~gary/ Site/ PHYS3020_files/ SM3_6. pdf) (PDF).. [17] Hogan, C. (1969). "Density of States of an Insulating Ferromagnetic Alloy". Physical Review 188 : 870. doi:10.1103/PhysRev.188.870. [18] Young; Geller (2008). Young and Geller College Physics (8th ed.). Pearson Education. ISBN 0805392181. [19] Ojovan, M. I. (2008). "Configurons: thermodynamic parameters and symmetry changes at glass transition" (http:/ / www. mdpi. org/ entropy/ papers/ e10030334. pdf) (PDF). Entropy 10 : 334–364. doi:10.3390/e10030334.. [20] Ojovan, Michael I. (2008). "Viscosity and Glass Transition in Amorphous Oxides". Advances in Condensed Matter Physics 2008 : 1. doi:10.1155/2008/817829. [21] Page 183 in: Cornelius, Flemming (2008). Medical biophysics (6th ed.). ISBN 1402071108. (also giving a density of 1.06 kg/L) [22] "Table of Specific Heats" (http:/ / hyperphysics. phy-astr. gsu. edu/ hbase/ tables/ sphtt. html#c1).. [23] "Materials Properites Handbook" (http:/ / fusionnet. seas. ucla. edu/ input/ PDF/ 1997 - Iter Material Properties Handbook - volAR01-3108 - no1 - p1-4. pdf).. [24] Crawford, R. J.. Rotational molding of plastics. ISBN 1591241928. [25] Faber, P.; Garby, L. (1995). "Fat content affects heat capacity: a study in mice". Acta Physiologica Scandinavica 153 (2): 185. doi:10.1111/j.1748-1716.1995.tb09850.x. PMID 7778459.
References
