CHEMISTRY GLOSSARY

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).

accelerator    →   akcelerator

Accelerator is a device (machine) used for acceleration of charged particles (protons, deuterons, α-particles). Particles are accelerated under the influence of an electric field and with the help of a magnetic field are kept inside a certain space. When the particles reach enough acceleration (that is sufficient energy), they are directed on a target we wish to bomb. Best known types cyclotron, synchrotron, betatron.

Accelerator is a substance that increases the rate of chemical reaction, i.e. a catalyst.

accumulator    →   akumulator

Accumulator (secondary cell, storage battery) is a type of voltaic cell or battery that can be recharged by passing current through it from an external D.C. supply. The charging current reverses the chemical reactions in the cell. The common types are the lead-acid accumulator and the nickel-cadmium cell.

activated charcoal    →   aktivni ugljen

Activated charcoal or activated carbon is charcoal that has been activated for adsorption by steaming or by heating in a vacuum. Charcoal is obtained by burning wood, nutshells, coconut husks or other materials. Charcoal becomes activated by heating it with steam to approximately 1000 °C in the absence of oxygen.

The chemical nature of amorphous carbon, combined with a high surface area makes it an ideal medium for the adsorption of organic chemicals. A single gram of such material can have 400 m² to 1 200 m² square meters of surface area. Activated charcoal is widely used to decolorize liquids, recover solvents, and remove toxins from water and air.

additive    →   aditivi

Additive is a substance added to a material to improve its properties in some way. Additives are often present in small amounts and are used for a variety of purposes, as in preventing corrosion, stabilizing polymers, and preserving and improving food.

ageing    →   starenje

Ageing is the physical and/or chemical change of a material with respect to time, under defined environmental conditions, leading to improvement or deterioration of properties.

allomerism    →   alomerija

Allomerism is the appearance of substances with different chemical composition but the same crystalline form.

amperometry    →   amperometrija

Amperometry is determining the concentration of a material in a sample by measuring electric current passing through a cell containing the solution.

acid    →   kiselina

Acid is a type of compound that contains hydrogen and dissociates in water to produce positive hydrogen ions. The reaction for an acid HA is commonly written:

In fact, the hydrogen ion (the proton) is solvated, and the complete reaction is:

This definition of acids comes from the Arrhenius theory. Such acids tend to be corrosive substances with a sharp taste, which turn litmus red and produce colour changes with other indicators. They are referred to as protonic acids and are classified into strong acids, which are almost completely dissociated in water, (e.g. sulphuric acid and hydrochloric acid), and weak acids, which are only partially dissociated (e.g. acetic acid and hydrogen sulphide). The strength of an acid depends on the extent to which it dissociates, and is measured by its dissociation constant.

In the Lowry-Brønsted theory of acids and bases (1923), the definition was extended to one in which an acid is a proton donor (a Brønsted acid), and a base is a proton acceptor (a Brønsted base). An important feature of the Lowry-Brønsted concept is that when an acid gives up a proton, a conjugate base is formed that is capable of accepting a proton.

Similarly, every base produces its conjugate acid as a result of accepting a proton.

For example, acetate ion is the conjugate base of acetic acid, and ammonium ion is the conjugate acid of ammonia.

As the acid of a conjugate acid/base pair becomes weaker, its conjugate base becomes stronger and vice versa.

A further extension of the idea of acids and bases was made in the Lewis theory. In this, a G. N. Lewis acid is a compound or atom that can accept a pair of electrons and a Lewis base is one that can donate an electron pair. This definition encompasses "traditional" acid-base reactions, but it also includes reactions that do not involve ions, e.g.

in which NH₃ is the base (donor) and BCl₃ the acid (acceptor).

analysis    →   analiza

Analysis. Chemical analysis is determination of the composition of a sample.