CHEMISTRY GLOSSARY

equilibrium constant    →   konstanta ravnoteže

The equilibrium constant (K) was originally introduced in 1863 by Norwegian chemists C.M. Guldberg and P. Waage using the law of mass action. For a reversible chemical reaction represented by the equation

chemical equilibrium occurs when the rate of the forward reaction equals the rate of the back reaction, so that the concentrations of products and reactants reach steady-state values.

The equilibrium constant is the ratio of chemical activities of the species A, B, C, and D at equilibrium.

To a certain approximation, the activities can be replaced by concentrations.

For gas reactions, partial pressures are used rather than concentrations

The units of K_p and K_c depend on the numbers of molecules appearing in the stoichiometric equation (a, b, c, and d).

The value equilibrium constant depends on the temperature. If the forward reaction is exothermic, the equilibrium constant decreases as the temperature rises. The equilibrium constant shows the position of equilibrium. A low value of K indicates that [C] and [D] are small compared to [A] and [B]; i.e. that the back reaction predominates.

The equilibrium constant is related to Δ_rG°, the standard Gibbs free energy change in the reaction, by

glucose    →   glukoza

Glucose (grape sugar, blood sugar), C₆H₁₂O₆, is an aldohexose (a monosaccharide sugar having six carbon atoms and an aldehyde group). An older common name for glucose is dextrose, after its dextrorotatory property of rotating plane polarized light to the right. Glucose in free (in sweet fruits and honey) or combined form (sucrose, starch, cellulose, glycogen) is is probably the most abundant organic compound in nature. During the photosynthesis process, plants use energy from the sun, water from the soil and carbon dioxide gas from the air to make glucose. In cellular respiration, glucose is ultimately broken down to yield carbon dioxide and water, and the energy from this process is stored as ATP molecules (36 molecules of ATP across all processes).

Naturally occurring glucose is D isomers (OH group on the stereogenic carbon farthest from the aldehyde group, C-5, is to the right in the Fischer projection). Although often displayed as an open chain structure, glucose and most common sugars exist as ring structures. In the α form, the hydroxyl group attached to C-1 and the CH₂OH attached to C-5 are located on opposite sides of the ring. β-glucose has these two groups on the same side of the ring. The full names for these two anomers of glucose are α-D-glucopyranose and β-D-glucopyranose.

iridium    →   iridij

Iridium was discovered by Smithson Tennant (England) in 1803. The origin of the name comes from the Latin word iris, meaning rainbow, because its salts are highly colored. It is heavy, brittle, white metal. Unreactive in air, water and acids. Attacked by fused NaOH. Metal ignites and burns readily. Iridium is found in gravel deposits with platinum. Used with osmium to tip gold pen points, to make crucible and special containers. Also to make alloys used for standard weights and measures and heat-resistant alloys. Also as hardening agent for platinum.

Hesse’s law    →   Hessov zakon

Hesse’s law says that reaction heat of some chemical change does not depend on the way in which the reaction is conducted, but only on starting and ending system state. Hesse’s law is also known as the law of constant heat summation. Hesse’s law is also known as the law of constant heat summation. The law was first put forward in 1840 by the Swiss-born Russian chemist Germain Henri Hess (1802-1850).

Hesse’s law can be used to obtain thermodynamic data that cannot be measured directly. For example, it is very difficult to control the oxidation of graphite to give pure CO. However, enthalpy for the oxidation of graphite to CO₂ can easily be measured. So can the enthalpy of oxidation of CO to CO₂. The application of Hess’s law enables us to estimate the enthalpy of formation of CO.

The equation shows the standard enthalpy of formation of CO to be -110 kJ/mol.

kilogram    →   kilogram

Kilogram (kg) is the SI base unit of mass; it is equal to the mass of the international prototype of the kilogram.

The prototype of the standard is a cylinder of platinum-iridium alloy (90:10), 39 mm in diameter and 39 mm high. Prototype of the kilogram kept by the Bureau International des Poids et Mesures (International Bureau of Weights and Measures) at Sèevres, near Paris.

Kjeldahl’s method    →   Kjeldahlov postupak

Kjeldahl’s method is an analytical method for determination of nitrogen in certain organic compounds. The method was developed by the Danish chemist Johan Kjeldahl (1849-1900).

It involves addition of a small amount of anhydrous potassium sulphate to the test compound, followed by heating the mixture with concentrated sulphuric acid, often with a catalyst such as copper sulphate. As a result ammonia is formed. After alkalyzing the mixture with sodium hydroxyde, the ammonia is separated by distillation, collected in standard acid, and the nitrogen determined by back-titration.

1. Kjeldahl flask for decomposition (500 ml – macro or 100 ml - micro)
2. funnel for alkaline solution
3. Wagner tube (drop catcher)
4. condenser
5. absorption flask with known volume of standard acid

melting point    →   talište

Melting point is the temperature at which a solid becomes a liquid at normal atmospheric pressure.

A more specific definition of melting point (or freezing point) is the temperature at which the solid and liquid phases of a substance are in equilibrium at a specified pressure (normally taken to be atmospheric unless stated otherwise). A pure substance under standard condition of pressure has a single reproducible melting point. The terms melting point and freezing point are often used interchangeably, depending on whether the substance is being heated or cooled.

Nernst’s electrode potential equation    →   Nernstova jednadžba za elektrodni potencijal

For general reaction of some redox system

dependence of electrode potential of redox system upon activity of oxidised and reduced form in solution is described in Nernst’s equation for electrode potential:

where E = to electrode potential of redox system

E° = standard electrode potential of redox system

R = universal gas constant

T = thermodymical temperature

F = Faraday’s constant

z = number of electrons exchanged in redox reaction

a_O = activity of oxidised form

a_R = activity of reduced form

n = stechiometrical coefficient of oxidised form

m = stechiometrical coefficient of reduced form

monosaccharide    →   monosaharid

Monosaccharides are carbohydrates, with the general formula C_n(H₂O)_n, that cannot be decomposed to a simpler carbohydrates by hydrolysis.

Depending on whether the molecule contains an aldehyde group (-CHO) or a ketone group (-CO-) monosaccharide can be a polyhydroxy aldehyde (aldose) or a polyhydroxy ketone (ketose). These aldehyde and ketone groups confer reduction properties on monosaccharides. They are also classified according to the number of carbon atoms they contain: trioses have three carbon atoms, tetroses four, pentoses five, hexoses six, heptoses seven, etc. These two systems of classification are often combined. For example, a six-carbon polyhydroxy aldehyde such as D-glucose is an aldohexose, whereas a six-carbon polyhydroxy ketone such as D-fructose is a ketohexose.

The notations D and L are used to describe the configurations of carbohydrates. In Fischer projections of monosaccharides, the carbonyl group is always placed on top (in the case of aldoses) or as close to the top as possible (in the case of ketoses). If the OH group attached to the bottom-most asymmetric carbon (the carbon that is second from the bottom) is on the right, then the compound is a D-sugar. If the OH group is on the left, then the compound is an L-sugar. Almost all sugars found in nature are D-sugars.

Monosaccharides can exist as either straight-chain or ring-shaped molecules. During the conversion from straight-chain form to cyclic form, the carbon atom containing the carbonyl oxygen, called the anomeric carbon, becomes a chiral center with two possible configurations (anomers), α and β. When the stereochemistry of the first carbon matches the stereochemistry of the last stereogenic center the sugar is the α-anomer when they are opposite the sugar is the β-anomer.

normal conditions    →   normalni uvjeti

Gas is under normal (or standard) conditions when: p₀ = 10⁵ Pa and T₀ = 273.15 K (0 °C). IUPAC recommends that the former use of the pressure of 1 atm as standard pressure (equivalent to 101 325 Pa) should be discontinued. At these conditions, the molar volume of gas V_m0 is 0.022 711 m³ (22.711 L).