CHEMISTRY GLOSSARY

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 manometer    →   U-manometar

U-tube manometer contains water or mercury in a U-shaped tube, and is usually used to measure gas pressure. One end of the U tube is exposed to the unknown pressure field (P) and the other end is connected to a reference pressure source (usually atmospheric pressure) (P_ref), shown in the schematic below.

If fluid C is the atmosphere, fluid B is the liquid in the U tube (e.g. water or mercury), and fluid A is a gas, then we can assume that ρB >> ρA, ρC. The pressure contributed by the weight of gas within the U tube can therefore be neglected. The gage pressure of the gas can be approximated by,

Winkler’s method    →   Winklerova metoda

Winkler’s method was once a common method used to determine the dissolved oxygen concentration by titration. Now rarely used due to the accuracy and low price of oxygen meters.

The water sample is first treated with excess manganese(II) sulfate solution and then with an alkaline solution of potassium iodide. The Mn(OH)₂ initially formed reacts with the dissolved oxygen. The amount of MnO(OH)₂ formed is determined by reaction with iodide ion in acidic solution. The iodine formed may be titrated against standard thiosulfate solution, using starch as an indicator.

X-ray diffraction    →   rendgenska difrakcija

The regular array of atoms in a crystal is a three-dimensional diffraction grating for short-wavelength waves such as X-rays. The atoms are arranged in planes with interplanar spacing d. Diffraction maxima occur in the incident direction of the wave, measured from the surface of a plane of atoms, and the wavelength λ of the radiation satisfy Braggs’s law:

Lennard-Jones potential    →   Lennard-Jonesov potencijal

The Lennard-Jones potential (or 12-6 potential) is a mathematically simple model that describes the interaction between two non-bonded and uncharged atoms (known as the van der Waals interaction). It was first proposed in 1924 by British physicist Sir John Edward Lennard-Jones (1894-1954). The Lennard-Jones Potential is given by the following equation

where V is the intermolecular potential between the two atoms or molecules, ε is the well depth and a measure of how strongly the two particles attract each other, σ is the distance at which the intermolecular potential between the two particles is zero, r is the distance of separation between centres of both particles.