CHEMISTRY GLOSSARY

solar cell    →   sunčeva ćelija

Solar cell, or photovoltaic cell, is a device that captures sunlight and transforms it directly to electricity. All solar cells make use of photovoltaic effect, so often they are called photovoltaic cells. Almost all solar cells are built from solid-state semiconducting materials, and in the vast majority of these the semiconductor is silicon.

The photovoltaic effect involves the generation of mobile charge carriers-electrons and positively charged holes-by the absorption of a photon of light. This pair of charge carriers is produced when an electron in the highest filled electronic band of a semiconductor (the valence band) absorbs a photon of sufficient energy to promote it into the empty energy band (the conduction band). The excitation process can be induced only by a photon with an energy corresponding to the width of the energy gap that separates the valence and the conduction band. The creation of an electron-hole pair can be converted into the generation of an electrical current in a semiconductor junction device, wherein a layer of semiconducting material lies back to back with a layer of either a different semiconductor or a metal. In most photovoltaic cells, the junction is p-n junction, in which p-doped and n-doped semiconductors are married together. At the interface of the two, the predominance of positively charged carriers (holes) in the p-doped material and of negatively charged carriers (electrons) in the n-doped material sets up an electric field, which falls off to either side of the junction across a space-charge region. When absorption of a photon in this region generates an electron-hole pair, these charge carriers are driven in opposite directions by the electric field, i.e. away from the interface and toward the top and bottom of the two-layer structure, where metal electrodes on these faces collect the current. The electrode on the top layer (through which light is absorbed) is divided into strips so as not to obscure the semiconducting layers below. In most widely used commercial solar cells, the p-doped and n-doped semiconductive layers are formed within a monolithic piece of crystalline silicon. Silicon is able to absorb sunlight at those wavelengths at which it is most intense-from the near-infrared region (wavelengths of around 1200 nm) to the violet (around 350 nm).

Solvay’s process    →   Solvayev postupak

Solvay ’s process is an industrial process for producing sodium carbonate from sodium chloride and ammonia and carbon dioxide.

Carbon dioixide is produced by the thermal decomposition of limestone, CaCO₃(s).

Quicklime, formed as a by-product of the thermal decomposition of limestone, is treated with water to form calcium hydroxide.

Calcium hydroxide is heated with ammonium chloride to form ammonia and calcium chloride (by product).

Carbon dioxide reacts with ammonia to form ammonium carbonate.

Ammonium carbonate further reacts with carbon dioxide to form ammonium bicarbonate.

Ammonium bicarbonate then react with sodium chloride to form sodium bicarbonate.

Dry sodium bicarbonate is heated in rotary furnace to give anhydrous sodium carbonate or soda ash.

The carbon dioxide produced is recycled back into the process.

spin    →   spin

Spin is the intrinsic angular momentum of an elementary particle, or system of particles such as nucleus, that is also responsible for the magnetic moment; or, a particle or nucleus possessing such a spin. The spins of nuclei have characteristic fixed values. Pairs of neutrons and protons align to cancel out their spins, so that nuclei with an odd number of neutrons and/or protons will have a net non-zero rotational component characterized by a non-zero quantum nuclear spin number.

Stern-Gerlach experiment: a beam of silver atoms is split into two beams when it traverses a nonuniform magnetic field. Atoms with spin quantum number m_s=+1/2 follow one trajectory, and those with m_s=+1/2 follow another.

square planar molecular geometry    →   kvadratna planarna geometrija molekule

Square planar is a molecular shape that results when there are four bonds and two lone pairs on the central atom in the molecule. An example of a square planar molecule is xenon tetrafluoride (XeF₄). This molecule is made up of six equally spaced sp³d² (or d²sp³) hybrid orbitals arranged at 90° angles. The shape of the orbitals is octahedral. Two orbitals contain lone pairs of electrons on opposite sides of the central atom. The remaining four atoms connected to the central atom give the molecule a square planar shape.

standard    →   standard

Standards are materials containing a known concentration of an analyte. They provide a reference to determine unknown concentrations or to calibrate analytical instruments.

The accuracy of an analytical measurement is how close a result comes to the true value. Determining the accuracy of a measurement usually requires calibration of the analytical method with a known standard. This is often done with standards of several concentrations to make a calibration or working curve.

A primary standard is a reagent that is extremely pure, stable, has no waters of hydration, and has a high molecular weight.

A secondary standard is a standard that is prepared in the laboratory for a specific analysis. It is usually standardised against a primary standard.

standard deviation    →   standardna devijacija

Standard deviation (σ) is a measure of the dispersion of a set of data from its mean. Standard deviation is a statistical term that measures the amount of variability or dispersion around an average

Suppose there are many measurements of a quantity presumed to be similar, like the size of peas in a pod. If the number of readings for each size were plotted, a bell-shaped curve would probably result, with a few small and large peas and most clustered around the average size. Around two-thirds of all measurements fall in the range spanned by the standard deviation, a measure of the spread.

starch    →   škrob

Starch (C₆H₁₀O5)_x is a polysaccharide used by plants to stockpile glucose molecules. It is the major component of flour, potatoes, rice, beans, corn, and peas. Starch is a mixture of two different polysaccharides: amylose (about 20 %), which is insoluble in cold water, and amylopectin (about 80 %), which is soluble in cold water. Amylose is composed of unbranched chains of D-glucose units joined by α(1→4)-glycosidic linkages. Unlike amylose, which are linear polymers, amylopectin contains α(1→6)-glycoside branches approximately every 25 glucose units.

Starch digestion begins in the mouth via the action of amylase, a digestive enzyme present in saliva. The process is completed in the small intestine by the pancreatic amylase. The final products of starch digestion, glucose molecules, are absorbed into the intestinal bloodstream and transported to the liver. Like most enzymes, glycosidases are highly selective in their action. They hydrolyze only the α-glycoside links in starch and leave the β-glycoside links in cellulose untouched. Starch is important food stuff and is used in adhesives, and sizes, in laundering, pharmacy and medicine.

state of matter    →   agregatno stanje

State of matter is one of the tree physical states in which matter can exist, i.e. solid, liquid or gas. Plasma is sometimes regarded as the fourth state of matter. By means of heating a solid substance will cross to liquid state at its melting point. If we heat up a liquid and beyond, at its boiling point it will cross to gaseous state - vapour.

stereoisomer    →   stereoizomer

Stereoisomers are compounds that have identical chemical constitution, but differ as regards the arrangement of the atoms or groups in space. Stereoisomers fall into two broad classes: optical isomers (enantiomers) and geometric isomers (cis-trans).

stoichiometry    →   stehiometrija

Stoichiometry is the relative proportions elements from compounds or in which substances react. Every chemical reaction has its characteristic proportions. For example, when methane unites with oxygen in complete combustion, 1 mol of methane requires 2 mol of oxygen.

At the same time, 1 mol of carbon dioxide and 2 mol of water are formed as reaction products.

Alternatively, 16 g of methane and 64 g of oxygen produce 44 g of carbon dioxide and 36 g of water.

The stoichiometric relationship between the products and reactants can be used to in calculations.