CHEMISTRY GLOSSARY

energy    →   energija

Energy (E, U) is the characteristic of a system that enables it to do work. Like work itself, it is measured in joules (J).

The internal energy of a body is the sum of the potential energy and the kinetic energy of its component atoms and molecules.

Potential energy is the energy stored in a body or system as a consequence of its position, shape, or state (this includes gravitation energy, electrical energy, nuclear energy, and chemical energy).

Kinetic energy is the energy of motion and is usually defined as the work that will be done by a body possessing the energy when it is brought to rest. For a body of mass m having a speed v, the kinetic energy is mv²/2. Kinetic energy is most clearly exhibited in gases, in which molecules have much greater freedom of motion than in liquids and solids.

In an isolated system energy can be transferred from one form to another but the total energy of the system remains constant.

enthalpy    →   entalpija

Enthalpy (H) is a thermodynamic property of a system defined by

where U is the internal energy of the system, p its pressure, and V its volume. J.W. Gibbs put the concept of an ensemble forward in 1902. In a chemical reaction carried out in the atmosphere the pressure remains constant and the enthalpy of reaction (ΔH), is equal to

For an exothermic reaction ΔH is taken to be negative.

Faraday’s laws of electrolysis    →   Faradayevi zakoni elektrolize

Faraday’s laws of electrolysis are two laws found by British chemist and physicist Michael Faraday (1791-1867) in his experiments on electrolysis:

1. The quantity of matter extracted on the electrode is proportional to the quantity of charge (Q = I·t) which has flown in electrolysis time.

where z = number of electrons changed in reaction and F = Faraday’s constant which equals 96 487 C mol^-1.

2. The masses of the elements liberated by the same quantity of electricity are directly proportional to their chemical equivalents.

96 487 C will discharge 1 mol Ag and 1/2 mol Cu. The relevant half reactions are:

Gibbs free energy    →   Gibbsova slobodna energija

Gibbs free energy (G) is an important function in chemical thermodynamics, defined by

where H is the enthalpy, S the entropy, and T the thermodynamic temperature. Gibbs free energy is the energy liberated or absorbed in a reversible process at constant pressure and constant temperature. Sometimes called Gibbs energy and, in older literature, simply free energy.

Changes in Gibbs free energy, ΔG, are useful in indicating the conditions under which a chemical reaction will occur. If ΔG is negative the reaction will proceed spontaneously to equilibrium. In equilibrium position ΔG = 0.

half-life of a reactant    →   poluživot reaktanta

For a given reaction the half-life, t_1/2, of a reactant is the time required for its concentration to reach a value that is the arithmetic mean of its initial and final (equilibrium) value.

Half-life is constant for first-order reactions.

Half-life is not constant for second-order reactions but rather it varies with initial concentration and k.

Henry’s law    →   Henryjev zakon

Henry’s law was discovered in 1801 by the British chemist William Henry (1775-1836). At a constant temperature the mass of gas dissolved in a liquid at equilibrium is proportional to the partial pressure of the gas. It applies only to gases that do not react with the solvent.

where p_i is the partial pressure of component i above the solution, x_i is its mole fraction in the solution, and K_x is the Henry’s law constant (a characteristic of the given gas and solvent, as well as the temperature).

histidine    →   histidin

Histidine is an electrically charged amino acids with basic side chains. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Histidine is perhaps the most common and versatile catalytic residue in proteins. The imidazole sidechain of histidine has a pKa of approximately 6.0. This means that, at physiologically relevant pH values, relatively small shifts in pH will change its average charge. The unprotonated imidazole is nucleophilic and can serve as a general base, while the protonated form can serve as a general acid. In addition, it is often a ligand for transition metal ions such as iron and zinc.

• Abbreviations: His, H
• IUPAC name: 2-amino-3-(1H-imidazol-5-yl)propanoic acid
• Molecular formula: C₆H₉N₃O₂
• Molecular weight: 155.15 g/mol

Hooke’s law    →   Hookeov zakon

Hooke’s law stating that the deformation of a body is proportional to the magnitude of the deforming force, provided that the body’s elastic limit (see elasticity) is not exceeded. If the elastic limit is not reached, the body will return to its original size once the force is removed. The law was discovered by English physicist Robert Hooke in 1676.

If a body on elastic spring is displaced from its equilibrium position (i.e. if the spring is stretched or compressed), a restitution force tries to return the body back in its equilibrium position. The magnitude of that force is proportional to the displacement of the body

Where F is the restitutional (elastic) force, x is the displacement of the body and k is the spring constant, which depends on dimensions, shape and material of the spring.

Ilkovic equation    →   Ilkovičeva jednadžba

Ilkovic equation is a relation used in polarography relating the diffusion current (i_d) and the concentration of the depolarizer (c), which is the substance reduced or oxidized at the dropping mercury electrode. The Ilkovic equation has the form

Where k is a constant which includes Faraday constant, π and the density of mercury, and has been evaluated at 708 for max current and 607 for average current, D is the diffusion coefficient of the depolarizer in the medium (cm²/s), n is the number of electrons exchanged in the electrode reaction, m is the mass flow rate of Hg through the capillary (mg/sec), and t is the drop lifetime in seconds, and c is depolarizer concentration in mol/cm³.

The equation is named after the scientist who derived it, the Slovak chemist, Dionýz Ilkovič 1907-1980).

inertial reference frames    →   inercijski referentni sustavi

When two frames of reference are moving relative to each other at constant velocity, they are said to be inertial reference frames. The observers from two such inertial frames measure, in general, different velocities of a moving particle. On the other hand, they measure the same acceleration for the particle. The laws of physics must have the same form in all inertial reference frames (the principle of invariance).