CHEMISTRY GLOSSARY

osmometry    →   osmometrija

Osmometry is a determination of the average molecular weight of a dissolved substance from measurements of osmotic pressure.

osmosis    →   osmoza

Osmosis is the flow of a solvent in a system in which two solutions of different concentration are separated by a semipermeable membrane which cannot pass solute molecules. The solvent will flow from the side of lower concentration to that of higher concentration, thus tending to equalise the concentrations. The pressure that must be applied to the more concentrated side to stop the flow is called the osmotic pressure.

pascal    →   paskal

Pascal (Pa) is the SI derived unit of pressure. One pascal is the pressure generated by a force of one newton acting on an area of one square metre (Pa = N/m²). It was named after the French mathematician, physicist and philosopher Blaise Pascal (1623-1662).

equilibrium constant    →   konstanta ravnoteže

The equilibrium constant (K) was originally introduced in 1863 by Norwegian chemists C.M. Guldberg and P. Waage using the law of mass action. For a reversible chemical reaction represented by the equation

chemical equilibrium occurs when the rate of the forward reaction equals the rate of the back reaction, so that the concentrations of products and reactants reach steady-state values.

The equilibrium constant is the ratio of chemical activities of the species A, B, C, and D at equilibrium.

To a certain approximation, the activities can be replaced by concentrations.

For gas reactions, partial pressures are used rather than concentrations

The units of K_p and K_c depend on the numbers of molecules appearing in the stoichiometric equation (a, b, c, and d).

The value equilibrium constant depends on the temperature. If the forward reaction is exothermic, the equilibrium constant decreases as the temperature rises. The equilibrium constant shows the position of equilibrium. A low value of K indicates that [C] and [D] are small compared to [A] and [B]; i.e. that the back reaction predominates.

The equilibrium constant is related to Δ_rG°, the standard Gibbs free energy change in the reaction, by

polymorphic transition    →   polimorfni prijelaz

Polymorphic transition is a reversible transition of a solid crystalline phase at a certain temperature and pressure to another phase of the same chemical composition with a different crystal structure. For examples, the transitions of quartz (SiO₂) at 1 143 K to tridymite, and at 1 743 K to cristobalite.

Raoult’s law    →   Raoultov zakon

Raoult’s law is the expression for the vapour pressure p_A of component A in an ideal solution, viz.,

where x_A is the mole fraction of component A and p_A^o the vapour pressure of the pure substance A.

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.

gas liquefying    →   ukapljivanje plinova

In order to achieve transition of a gas into liquid state it is necessary to lower its temperature, or decrease its volume, or increase its pressure. Above the critical temperature it is impossible to liquefy a gas. When liquefying a gas by Linde’s procedure, dampening or Joule-Thomson’s effect is used. First, the compressed air from the compressor is cooled with cooling water, the cooled air expands at a lower pressure in the dampening valve at which it cooled. The cooled air now returns to the compressor, cooling down the expanding air. By repeating this process the air is cooled enough to transit to the liquid state.

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.