CHEMISTRY GLOSSARY

oxygen    →   kisik

Oxygen was discovered by Joseph Priestley (England) in 1774. The origin of the name comes from the Greek words oxy genes meaning acid and forming (acid former). It is colourless, odourless gas; pale blue liquid. Extremely reactive. Forms oxides with nearly all other elements except noble gases. It is the most abundant element in the earth’s crust and makes up almost 21 % of the atmosphere. Oxygen is obtained primarily from liquid air by fractional distillation. Small amounts are made in the laboratory by electrolysis of water. Used in steel making, welding and supporting life. Naturally occurring ozone (O₃) in the upper atmosphere shields the earth from ultraviolet radiation.

silver coulometer    →   srebrni kulometar

Silver coulometer consists of a platinum vessel which acts as a cathode and contains a solution of pure silver nitrate as an electrolyte (c(AgNO₃) = 1 mol/L). A rod of pure silver enclosed in a porous pot acts as the anode. The current density at the anode should not exceed 0.2 Acm^-2. After electrolysis, the electrolyte is taken out and the platinum vessel is washed, dried and weighed. The increase in the weight gives the amount of silver deposited (96500 C of electricity deposits 107.88 g of silver). From the mass of the silver deposited, the coulomb involved in the reaction can be calculated.

spectrophotometer    →   spektrofotometar

Spectrophotometer is an instrument for measuring the amount of light absorbed by a sample.

The absorption of light by a substance in a solution can be described mathematically by the Beer-Lambert law

where A is the absorbance at a given wavelength of light, ε is the molar absorbtivity or extinction coefficient (L mol^-1 cm^-1), unique to each molecule and varying with wavelength, b is the length of light path through the sample (cm), and c is the concentration of the compound in solution (mol L^-1).

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.

thermal resistance    →   toplinski otpor

Heat always flows from a higher to a lower temperature level. The driving force for the heat flux lies in the temperature difference ΔT between two temperature levels. Analogous to Ohm’s law, the following holds:

where H = dQ/dt is heat flux, measured in watts, ΔT is temperature difference across the thermal resistance, measured in kelvin, and R_th is thermal resistance, measured in K/W.

For example, suppose there were two houses with walls of equal thickness; one is made of glass and the other of asbestos. On a cold day, heat would pass through the glass house much faster. The thermal restistance of asbestos is then higher than of glass.

If the thermal Ohm’s law is divided by the heat capacity C, Newton’s law of cooling is obtained:

where dT/dt is rate of cooling or heating, measured in K s^-1, and C is heat capacity, measured in J K^-1.

U-tube    →   U-cijev

U-tube filled with a sodium hydroxide is used for a cleaned and dried gas.

U-tube filled with a 1 mol dm^-3 aqueous potassium nitrate solution is used as a permanent salt bridge.

U-tube is a device used for water electrolysis, for laboratory hydrogen and oxygen obtaining.

Heyrovsky-Ilkovic equation    →   Heyrovsky-Ilkovičeva jednadžba

The Heyrovsky-Ilkovic equation describes the entire current-potential curve (polarographic wave) of a reversible redox system in polarography

where R is the gas constant, T is the absolute temperature, F is the Faraday constant, n denotes the number of electrons taking part in the electrode reaction. E_1/2 is a unique potential (for a given reaction and supporting electrolyte) termed the half-wave potential.

In order to obtain E_1/2 from the above equation, we plot a graph of ln[(i_d-i)/i] against E. The intercept on the x-axis gives then an accurate value of E_1/2. The slope of the obtained straight line is equal to nF/RT from which n is determined.