CHEMISTRY GLOSSARY

siemens    →   simens

Siemens (S) is the SI derived unit of of electrical conductance of a conductor in which a current of one ampere is produced by an electrical potential difference of one volt (S = A·V^-1 = Ω^-1). The unit was named after the German scientist Ernst Werner von Siemens (1816-1892).

epoxy resin    →   epoksi smola

Epoxy resins are thermosetting resins produced by copolymerising epoxide compounds with phenols (e.g. epichlorohydrin and bisphenol A). They contain ether linkages (-O-) and form a tight, cross-linked polymer network. Toughness, good adhesion, corrosive-chemical resistance, and good dielectric properties characterise epoxy resins. Most epoxy resins are two-part types which harden when blended.

ferrite    →   ferit

Ferrites are ceramic materials of the nominal formula MO·Fe₂O₃, where M is a divalent metal (Co, Mn, NI, or Zn). The ferrites show either ferrimagnetism or ferromagnetism, but are not electrical conductors, and they are used in high-frequency circuits as magnetic cores, in rectifiers on memory and record tapes, and various related uses in radio, television, radar, computers, and automatic control systems.

flammable limit    →   granica zapaljivosti

Flammable limits refer to the conditions under which a mixture of a flammable material and air may catch fire or explode. When vapour s of a flammable or combustible liquid are mixed with air in the proper proportions in the presence of a source of ignition, rapid combustion or an explosion can occur. The proper proportion is called the flammable range and is also often referred to as the explosive range. The flammable range includes all concentrations of flammable vapour or gas in air, in which a flash will occur or a flame will travel if the mixture is ignited.

The lower flammable limit (LEL) or the lower explosive limit is the lowest concentration of a flammable vapour or gas in air that will propagate a flame from an ignition source. The upper flammable limit (UEL) or the upper explosive limit is the highest concentration of a flammable vapour or gas in air that will propagate a flame from an ignition source. Any concentration between these limits can ignite or explode.

fuel cell    →   gorivi članak

Fuel cell is a device that converts chemical energy into electrical energy. It is different from a battery in that the energy conversion continues as long as fuel and oxidising agent are fed to the fuel cell; that is, in principle indefinitely. (A battery is manufactured with a limited amount of chemicals, and it is exhausted when all the chemicals have reacted.) It is a galvanic cell where spontaneous chemical reactions occur at the electrodes. The fuel is oxidised at the anode, and the oxidising agent (almost always oxygen or air) is reduced at the cathode. Presently, the most commonly used fuel is hydrogen. More conventional fuels (e.g., petrol or natural gas) must be converted (reformed) into hydrogen before they can be utilised in a fuel cell.

Some fuel cells employ an aqueous solution as electrolyte, that can be either acidic or basic (alkaline), or an ion-exchange membrane soaked in aqueous solution can act as the electrolyte. These fuel cells operate at relatively low temperatures (from room temperature to not much above the boiling point of water). Some fuel cells employ molten salts (especially carbonates) as electrolytes and have to operate at temperatures of several hundred degrees centigrade (Celsius). Others employ ionically conductive solids as electrolyte and must operate close to 1 000 °C.

strength    →   čvrstoća

Strength is a mechanical characteristic of a solid body, that gives the resistance to cutting. The solid body comprises from one to more crystals.

strong electrolyte    →   jaki elektrolit

Strong electrolytes are those electrolytes which in water solutions completely dissociate into their ions. They conduct electric current very well.

galvanic celll    →   galvanski članak

Galvanic cell (voltaic cell) is a simple device with which chemical energy is converted into electrical energy. Galvanic cells consist of two separate compartments called half cells containing electrolyte solutions and electrodes that can be connected in a circuit. Two dissimilar metals (e.g., copper and zinc) are immersed in an electrolyte. If the metals are connected by an external circuit, one metal is reduced (i.e., gains electrons) while the other metal is oxidized (i.e., loses electrons).

In the example above, copper is reduced and zinc is oxidized. The difference in the oxidation potentials of the two metals provides the electric power of the cell.

A voltaic cell can be diagrammed using some simple symbols. In the diagram the electrodes are on the outer side of the diagram and a vertical line (|) is used to separate the electrode from the electrolyte solution found in the compartment. A double vertical line (||) is used to separate the cell compartments and is symbolic of the salt bridge. Usually in a diagram the species oxidized is written to the left of the double slash. Here is an example of the Daniell cell:

The names refer to the 18th-century Italian scientists Alessandro Volta (1745-1827) and Luigi Galvani (1737-1798).

gasoline    →   motorni benzin

Gasoline is a complex mixture of volatile hydrocarbons that may have between 5 to 12 carbons. The major components are branched-chain paraffins, cycloparaffins, and aromatics. Gasoline is most often produced by the fractional distillation of crude oil as the fraction of hydrocarbons in petroleum boiling between 30 °C and 200 °C. The quality of a fuel is measured with its octane number. Octane number is the measure of the resistance of gasoline against detonation or preignition of the fuel in the engine. The higher the octane number, the more compression the fuel can withstand before detonating. The octane number is determined by comparing the characteristics of a gasoline to isooctane with good knocking properties (octane number of 100) and heptane with bad (octane number of 0).

temperature rating    →   temperaturno područje

Temperature rating is the maximum and minimum temperature at which the material may be used in continuous operation without loss of its basic properties. For example, temperature ratings are often quoted for electrical insulators, specifying the maximum temperature at which they provide adequate protection against electrical breakdown.