CHEMISTRY GLOSSARY

latent heat    →   latentna toplina

Latent heat (L) is the quantity of heat absorbed or released when a substance changes its physical phase at constant temperature (e.g. from solid to liquid at the melting point or from liquid to gas at the boiling point).

heat transfer    →   prijenos topline

From observations and experiments it has been found that heat energy can be transferred from one position to another through three different modes: conduction, convection and radiation.

heat capacity    →   toplinski kapacitet

Heat capacity is defined in general as dQ/dT, where dQ is the amount of heat that must be added to a system to increase its temperature by a small amount dT. The heat capacity at a constant pressure is C_p = (∂H/∂T)_p; that at a constant volume is C_V = (∂E/∂T)_V, where H is enthalpy, E is internal energy, p is pressure, V is volume, and T is temperature. An upper case C normally indicates the molar heat capacity, while a lower case c is used for the specific (per unit mass) heat capacity.

specific heat    →   specifični toplinski kapacitet

Specific heat is the quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius.

thermal conductivity    →   toplinska vodljivost

Thermal conductivity (Λ) is rate of heat flow divided by the area and by the temperature gradient.

thermal pollution    →   toplinsko zagađenje

Thermal pollution is the increase in temperature of natural waters resulting from the discharge to these waters of hot effluents from industrial and power plants. The higher temperatures reduce the concentration of dissolved oxygen.

thermal expansion    →   toplinsko rastezanje

Thermal expansion is a change in dimensions of a material resulting from a change in temperature. All objects change size with changes in temperature. The change ΔL in any linear dimension L is given by

in which α is the thermal coefficient of linear expansion, L_o is the initial or reference dimension at temperature T_o (reference temperature) and ΔT is change in temperature which causes the change in dimension.

The change ΔV in the volume of a sample of solid or liquid is

Here γ is coefficient of volume expansion, V_o is the volume of the sample at temperature T_o and ΔV is the change in volume over the temperature range ΔT. With isotropic substances, the coefficient of volume expansion can be calculated from the coefficient of linear expansion: γ = 3α.

thermal resistance    →   toplinski otpor

Heat always flows from a higher to a lower temperature level. The driving force for the heat flux lies in the temperature difference ΔT between two temperature levels. Analogous to Ohm’s law, the following holds:

where H = dQ/dt is heat flux, measured in watts, ΔT is temperature difference across the thermal resistance, measured in kelvin, and R_th is thermal resistance, measured in K/W.

For example, suppose there were two houses with walls of equal thickness; one is made of glass and the other of asbestos. On a cold day, heat would pass through the glass house much faster. The thermal restistance of asbestos is then higher than of glass.

If the thermal Ohm’s law is divided by the heat capacity C, Newton’s law of cooling is obtained:

where dT/dt is rate of cooling or heating, measured in K s^-1, and C is heat capacity, measured in J K^-1.

Born-Haber cycle    →   Born-Haberov kružni proces

Born-Haber cycle is a cycle of reactions used for calculating the lattice energies of ionic crystalline solids. For a compound MX, the lattice energy is the enthalpy of the reaction

The standard enthalpy of formation of the ionic solid is the enthalpy of the reaction

The cycle involves equating this enthalpy (which can be measured) to the sum of the enthalpies of a number of steps proceeding from the elements to the ionic solid. The steps are:

1) Atomization of the metal

2) Atomization of the nonmetal

3) Ionisation of the metal

This is obtained from the ionisation potential.

4) Ionisation of the nonmetal

This is electron affinity.

5) Formation of the ionic solids

Equation of the enthalpies gives

from which ΔH_L can be found.

absolute zero    →   apsolutna nula temperature

Absolute zero is theoretically, the lowest attainable temperature. It is the energy at which the kinetic energy of atom and molecules is minimal and is equivalent to -273.15 °C.