CHEMISTRY GLOSSARY

electrolytes    →   elektroliti

Electrolytes are substances which, when melted or dissolved in water, conduct electric current. By melting or dissolving they are dissociated into electrically charged particles (ions) which are able to conduct electric current. By passing of electric current the transfer of matter occurs. Positively charged particles (cations) travel towards the negative pole (the cathode) and negatively charged particles (the anions) travel towards the positive pole (the anode). Liquid metals, in which the conduction is by free electrons, are not usually regarded as electrolytes. Solid conductors of ions, as in the sodium-sulphur cell, are also known as electrolytes. Depending upon how it conducts electric current, matter can be divided into strong electrolytes, weak electrolytes and nonconductors.

electrolytic cell    →   elektrolitska ćelija

Electrolytic cell is an electrochemical cell that converts electrical energy into chemical energy. The chemical reactions do not occur spontaneously at the electrodes when they are connected through an external circuit. The reaction must be forced by applying an external electric current. It is used to store electrical energy in chemical form (rechargeable battery). It is also used to decompose or produce (synthesise) new chemicals by the application of electrical power. This process is called electrolysis, e.g., water can be decomposed into hydrogen gas and oxygen gas. The free energy change of the overall cell reaction is positive.

weak electrolyte    →   slabi elektrolit

Weak electrolytes are those electrolytes which in water solutions dissociate only partially, giving ions and which are in equilibrium with undissociated molecules. Their water solutions conduct electric current weakly. For example, acetic acid partially dissociates into acetate ions and hydrogen ions, so that an acetic acid solution contains both molecules and ions.

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).

lead-acid battery    →   olovni akumulator

Lead-acid battery is a electrical storage device that uses a reversible chemical reaction to store energy. It was invented in 1859 by French physicist Gaston Planté. Lead-acid batteries are composed of a lead(IV) oxide cathode, a sponge metallic lead anode and a sulphuric acid solution electrolyte.

In charging, the electrical energy supplied to the battery is changed to chemical energy and stored. The chemical reaction during recharge is normally written:

In discharging, the chemical energy stored in the battery is changed to electrical energy. During discharge, lead sulfate (PbSO4) is formed on both the positive and negative plates. The chemical reaction during discharge is normally written:

Lead acid batteries are low cost, robust, tolerant to abuse, tried and tested. For higher power applications with intermittent loads however, they are generally too big and heavy and they suffer from a shorter cycle life.

luminescence    →   luminiscencija

Luminescence (from Latin lumen, light) is the emission of electromagnetic radiation (UV, visible or IR) from atoms or molecules as a result of the transition of an electronically excited state to a lower energy state, usually the ground state. Luminescence can be divided into categories by duration (fluorescence or phosphorescence) or by the mechanism that creates the light (radioluminescence, electroluminescence, photoluminescence, thermoluminescence, triboluminescence, chemiluminescence, bioluminescence). The prefix identifies the energy source responsible for generating or releasing the light.

Phosphorescence is emission of light from a substance exposed to radiation and persisting as an afterglow after the source of excitation has been removed. Fluorescence, on the other hand, is an almost instantaneous effect, ending within about 10^-8 second after excitation.

mercury    →   živa

Mercury has been known since ancient times. The origin of the name comes from the Latin word hydrargyrum meaning liquid silver. It is heavy, silver-white metal, liquid at ordinary temperatures. Stable in air and water. Unreactive with alkalis and most acids. Gives off poisonous vapour. Chronic cumulative effects. Mercury only rarely occurs free in nature. The chief ore is cinnabar or mercury sulfide (HgS). Used in thermometers, barometers and batteries. Also used in electrical switches and mercury-vapour lighting products.

metal    →   metal

Metals are materials in which the highest occupied energy band (conduction band) is only partially filled with electrons.

Their physical properties generally include:

- They are good conductors of heat and electricity. The electrical conductivity of metals generally decreases with temperature.

- They are malleable and ductile in their solid state.

- They show metallic lustre.

- They are opaque.

- They have high density.

- They are solids (except mercury)

- They have a crystal structure in which each atom is surrounded by eight to twelve near neighbours

Their chemical properties generally are:

- They have one to four valence electrons.

- They have low ionisation potentials; they readily lose electrons.

- They are good reducing agents.

- They have hydroxides which are bases or amphoteric.

- They are electropositive.

Metallic characteristics of the elements decrease and non-metallic characteristics increase with the increase of valence electrons. Also metallic characteristics increase with the number of electron shells. Therefore, there is no sharp dividing line between the metals and non-metals.

Of the 114 elements now known, only 17 show primarily non-metallic characteristics, 7 others are metalloids, and 89 may be classed as metals.

non-metal    →   nemetal

Non-metals are defined as elements that are not metals.

Their physical properties generally include:

• They are poor conductors.
• They are brittle, not ductile in their solid state.
• They show no metallic lustre.
• They may be transparent or translucent.
• They have low density.
• They form molecules which consists of atoms covalently bonded; the noble gases are monoatomic.

Their chemical properties are generally:

• They usually have four to eight valence electrons.
• They have high electron affinities (except the noble gases)
• They are good oxidising agents (except the noble gases)
• They have hydroxides which are acidic (except the noble gases)
• They are electronegative.

selenium    →   selenij

Selenium was discovered by Jöns Jakob Berzelius (Sweden) in 1817. The origin of the name comes from the Greek word selene meaning moon. It is soft metalloid similar to sulfur. Ranges from grey metallic to red glassy appearance. Unaffected by water. Soluble in alkalis and nitric acid. Burns in air. Toxic by inhalation or ingestion. Selenium is obtained from lead, copper and nickel refining. Conducts electricity when struck by light. Light causes it to conduct electricity more easily. It is used in photoelectric cells, TV cameras, xerography machines and as a semiconductor in solar batteries and rectifiers. Also colours glass red.