Specific heat capacity

From Wikipedia, the free encyclopedia

Material Properties
Specific heat
Compressibility
Thermal expansion
edit

Specific heat capacity, also known simply as specific heat (Symbol: C or c) is the measure of the heat energy required to raise the temperature of one gram of a substance (thus, the name “specific” heat) by one kelvin. A kelvin is a unit increment of thermodynamic temperature and is precisely equal to an increment of one degree Celsius. Virtually any substance may have its specific heat capacity measured, including pure chemical elements, compounds, alloys, solutions, and composites.

[edit] Symbols and standards

Specific heat capacity is usually expressed in terms of either mass or moles. For liquids and especially solids in engineering thermal applications, sometimes the specific unit-quantity is chosen as volume, and in this case the term volume-specific heat capacity or volumetric heat capacity is then used, and a subscript v is added. Unless volume-specific heat capacities are used elsewhere in the text, when mass is the unit quantity, the subscript m is often omitted , and in this case, the symbol for specific heat capacity (meaning mass-specific heat capacity) is usually a simple lowercase c with no subscript. When the mole is the unit quantity, the symbol often used is uppercase C and is known as molar heat capacity, but occassionally a subscript italic m is used for these cases. Alternatively—especially in chemistry as opposed to engineering—the uppercase version for specific heat, C, may be used in combination with the symbol for enthalpy (H or h) as a suffix. When the mole is the unit quantity, the enthalpy symbol is uppercase H and when mass is the unit quantity, the symbol is lowercase h.

In engineering, as noted, when mass-specific and volume-specific heat capacities are both used in calculations (usually for solids or liquids), the subscripts c_m and c_v are often added for clarity. Care should be taken not to confuse the engineering c_v for the constant-volume gas heat capacity often used by chemists (see discussion below), nor the subscript m as used by engineers, for molar heat capacity (in this case, it simply means ordinary mass-specific heat capacity). There are cases (see heat capacity for more) where it is necessary to use double subscripts to indicate intensive quantities in engineering and physics, such as the C_V,m used for molar constant-volume gas heat capacity.

The modern SI units for measuring specific heat capacity are either the joule per gram per kelvin (J g^–1 K^–1) or the joule per mole per kelvin (J mol^–1 K^–1). The various SI prefixes can create variations of these units (such as kJ kg^–1 K^–1 and kJ mol^–1 K^–1). Other units of measure are often employed in the measure of specific heat capacity. These include calories and BTUs for energy, pounds-mass for quantity, and degree Fahrenheit (°F) for the increment of temperature.

There are two distinctly different experimental conditions under which specific heat capacity is measured and these are denoted with a subscripted suffix modifying the symbols C or c. The specific heat of substances are typically measured under constant pressure (Symbols: C_p or c_p). However, fluids (gases and liquids) are typically also measured at constant volume (Symbols: C_v or c_v). Measurements under constant pressure produces greater values than those at constant volume because work must be performed in the former. This difference is particularly great in gases, where values under constant pressure are typically 30% to 66.7% greater than those at constant volume

Thus, the symbols for specific heat capacity are as follows:

The specific heat capacities of substances comprising molecules (distinct from the 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): c_p = 4.1855 J g^–1 K^–1 (15 °C), and…
Water (liquid): C_vH = 74.539 J mol^–1 K^–1 (25 °C)

The pressure at which specific heat capacity is measured is especially important for gases and liquids. The standard pressure was once virtually always “one standard atmosphere” which is defined as the sea level–equivalent value of precisely 101.325 kPa (760 Torr). In the case of water, 101.325 kPa is still typically used due to water’s unique role in temperature and physical standards. However, in 1982, the [[International Union of Pure and Applied Chemistry]] (IUPAC) recommended that for the purposes of specifying the physical properties of substances, “the standard pressure” should be defined as precisely 100 kPa (≈750.062 Torr).^[1] 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 meters (which is closer to the 194–meter, world–wide median altitude of human habitation). Accordingly, the pressure at which specific heat capacity is measured should be specified since one can not assume its value. An example of how pressure is specified is as follows:

Water (gas): C_vH = 28.03 J mol^–1 K^–1 (100 °C, 101.325 kPa)

Note in the above specification that the experimental condition is at constant volume. Still, the pressure within this fixed volume is controlled and specified.

[edit] Factors that affect heat capacity

Molecules have internal structure because they comprise atoms. Atoms have up to three different vibrational and rotational ways of moving within molecules. Heat energy is stored in these motions.

• Molar mass: When the specific heat capacity, c, of a material is measured (lowercase c means the unit quantity is in terms of mass), different values arise because different substances have different molar masses (essentially, the weight of the individual atoms or molecules). Heat energy arises, in part, due to the number of atoms or molecules that are vibrating. If a substance has a lighter molar mass, then each gram of it has more atoms or molecules available to store heat energy. This is why hydrogen—the lightest substance there is—has such a high specific heat capacity on a gram basis; one gram of it contains a relatively great many molecules. If specific heat capacity is measured on a molar basis (uppercase C), the differences between substances is less pronounced and hydrogen’s molar heat capacity is quite unremarkable.

• Degrees of freedom: Molecules are quite different from the monatomic gases like helium and argon. With monatomic gases, heat 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). These simple movements in the three X, Y, and Z–axis dimensions of space means monatomic atoms have three translational degrees of freedom.^[2] Molecules, however, have various internal vibrational and rotational degrees of freedom because they are complex objects; they are a population of atoms that can move about within a molecule in different ways (see animation at right). Heat energy is stored in these internal motions. Water for instance, can absorb a large amount of heat energy per mole with only a modest temperature change because it has six active degrees of freedom, the maximum available. Not surprisingly, water gas molecules (steam molecules) have twice^[3] the specific heat capacity per mole as do the monatomic gases which move only within the three degrees of freedom comprising translational motion. See Thermodynamic temperature for more information on translational motions, kinetic (heat) energy, and their relationship to temperature.

• Hydrogen bonds: Hydrogen-containing molecules like ethanol, ammonia, and water have powerful, intermolecular hydrogen bonds when in their liquid phase. These bonds provide yet another place where kinetic (heat) energy is stored.

[edit] Equations

• The equation relating heat energy to specific heat capacity, where the unit quantity is in terms of mass is:

Q = m c ΔT

where Q is the heat energy put into or taken out of the substance, m is the mass of the substance, c is the specific heat capacity, and ΔT is the temperature differential.

• Where the unit quantity is in terms of moles, the equation relating heat energy to specific heat capacity (also known as molar heat capacity) is

Q = n C ΔT

where Q is the heat energy put into or taken out of the substance, n is the number moles, C is the specific heat capacity, and ΔT is the temperature differential.

[edit] Table of specific heat capacities

^A Assuming an altitude of 194 meters above mean sea level (the world–wide median altitude of human habitation), an indoor temperature of 23 °C, a dewpoint of 9 °C (40.85% relative humidity), and 760 mm–Hg sea level–corrected barometric pressure (molar water vapor content = 1.16%).

[edit] Specific heat of building materials

Usually of interest to builders and solar designers

[edit] See also

[edit] References

1. ^ IUPAC.org, Gold Book, Standard Pressure
2. ^ Two conventions are used in science when counting degrees of freedom. The one used here counts each dimension or mode of movement as a single degree of freedom. The other convention counts each of these as two degrees (back and forth, up and down, etc.). Thus, water has six degrees of freedom according to the convention used here and twelve degrees in the other.
3. ^ Under constant pressure (C_p) steam has 1.803 times: 37.47 J mol^–1 K^–1 (100 °C, 101.325 kPa) v.s. 20.7862 J mol^–1 K^–1 for the monatomic gases. Under constant volume (C_v) steam has 2.247 times: 28.03 J mol^–1 K^–1 (100 °C, 101.325 kPa) v.s. 12.4717 J mol^–1 K^–1 for the monatomic gases. This is a 2.025 average for C_p and C_v. Citation: Water Structure and Behavior, Specific heat capacity (by London South Bank University). Link to Web site.