CHEMISTRY GLOSSARY

Frasch proces    →   Fraschov postupak

Frasch proces is a method of obtaining sulphur from underground deposits using a tube consisting of three concentric pipes. Superheated steam is passed down the outer pipe to melt the sulphur, which is forced up through the middle pipe by compressed air fed through the inner tube. The steam in the outer casing keeps the sulphur molten in the pipe. It was named after the German-born American chemist Herman Frasch (1851-1914).

fugacity    →   fugacitet

Fugacity (f) is a thermodynamic function used in place of partial pressure in reactions involving real gases and mixtures. For a component of a mixture, it is defined by

where μ is the chemical potential.

The fugacity of a gas is equal to the pressure if the gas is ideal. The fugacity of a liquid or solid is the fugacity of the vapour with which it is in equilibrium. The ratio of the fugacity to the fugacity in some standard state is the activity.

Geiger counter    →   Geigerov brojač

Geiger counter (Geiger-Muller counter) is a device used to detect and measure ionising radiation. It consists of a tube containing a low-pressure gas (usually argon or neon with methane) and a cylindrical hollow cathode through the centre of which runs a fine-wire anode. A potential difference of about 1 000 V is maintained between the electrodes. An ionising particle or photon passing through a window into the tube will cause an ion to be produced and the high potential will accelerate it towards its appropriate electrode, causing an avalanche of further ionisations by collision. The consequent current pulses can be counted in electronic circuits or simply amplified to work a small loudspeaker in the instrument. It was first devised in 1908 by the German physicist Hans Geiger (1882-1945). Geiger and W. Muller produced an improved design in 1928.

recrystallisation    →   rekristalizacija

Recrystallisation is a formation of a new, strain free grain structure from the existing one in cold worked metal, usually accomplished by heating. The change from one crystal structure to another, as occurs on heating or cooling through a critical temperature.

relative humidity    →   relativna vlažnost

Relative humidity is the ratio of the partial pressure of water vapour in air to the saturation vapour pressure of water at the same temperature, expressed as a percentage.

reverse osmosis    →   reverzna osmoza

Reverse osmosis is the method used for obtaining freshwater from saltwater. The process uses a semi-permeable membrane through which pure water and not the salts will pass. The saltwater must be pressurised to approximately 25 bar, which makes it operationally expensive to produce large quantities of fresh water by this method.

stp    →   stp

STP (or NTP) is abbreviation for standard (normal) temperature (273.15 K or 0 °C) and pressure (10⁵ Pa). These conditions are used when comparing the properties of gases.

superconductivity    →   supravodljivost

Superconductivity is the phenomenon in which certain metals, alloys, and compounds below a certain temperature, the transition point (T_c), lose electrical resistance and magnetic permeability, i.e. have infinite electrical conductivity (Meissner effect and Josephson effect).

Gibbs free energy    →   Gibbsova slobodna energija

Gibbs free energy (G) is an important function in chemical thermodynamics, defined by

where H is the enthalpy, S the entropy, and T the thermodynamic temperature. Gibbs free energy is the energy liberated or absorbed in a reversible process at constant pressure and constant temperature. Sometimes called Gibbs energy and, in older literature, simply free energy.

Changes in Gibbs free energy, ΔG, are useful in indicating the conditions under which a chemical reaction will occur. If ΔG is negative the reaction will proceed spontaneously to equilibrium. In equilibrium position ΔG = 0.

Haber process    →   Haberov proces

Haber process is an industrial process for producing ammonia by reaction of nitrogen with hydrogen:

The reaction is reversible and exothermic, so that a high yield of ammonia is favoured by low temperature. However, the rate of reaction would be too slow for equilibrium to be reached at normal temperatures, so an optimum temperature of about 450 °C is used, with a catalyst of iron containing potassium aluminium oxide promoters. The higher the pressure the greater the yield, although there are technical difficulties in using very high pressures. A pressure of about 250 atmospheres is commonly employed. The removal of ammonia from the batch as soon as it is formed ensures that an equilibrium favouring product formation is maintained. The nitrogen is obtained from air. Formerly, the hydrogen was from water gas and the water-gas shift reaction (the Bosch process) but now the raw material (called synthesis gas) is obtained by steam reforming natural gas.

The process is of immense importance for the fixation of nitrogen for fertilisers and explosives. It was developed in 1908 by German chemist Fritz Haber (1868-1934) and was developed for industrial use by Carl Bosch (1874-1940), hence the alternative name Haber-Bosch process.