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Home >> Chemistry Dictionary >>Fractional Crystallisation - Freeze Drying
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Formic acid.
HCOOH.
Formoxy.
The group-O.CHO.
Formula.
A way representing a chemical compound using symbols for the atom present. Subscripts are used for the numbers of atoms. The molecular formula simply give the types and numbers of atoms present. For example, the molecular formula of ethanoic acid is C2H4O2. The empirical formula give these atoms in their simplest ratio: for ethanoic acid it is CH2O. The structural formula give an indication of the way the atoms are arranged. Commonly, this is done by dividing the formula into groups; ethanoic acid can be written CH3.CO.OH (or more usually simply CH5COOH). Structural formulae can also show the arrangement of atoms or groups in space.
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Formyl.
The group C(O)H. Systematically the methanoyl group.
Fossil fuel.
Coal oil and natural gas the fuels used by man as a source of energy. They are formed from the remains of living organisms and all are having a high carbon or hydrogen content.
Fraction.
A mixture of liquids with similar boiling points collected by fractional distillation.
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Fractional crystallisation.
A method of separating a less soluble substance from a more soluble substance when present together in a solution. A solution contains two substances in equal amounts, the one with lower solubility crystallizes out first. In case a solution contains two substances whose solubilities are nearly equal at some temperature, then the one with greater amount crystallizes out first.
Fractional distillation.
The process of separating a mixture into series of fractions of different volatilities by means of distillation.
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Francium:
Symbol Fr. A radioactive element belonging to group I of the periodic cable, a.n. 87; r.d. 2.4; m.p. 27 ± 1ºC; b.p. 677 ± 1ºC. The element is found in uranium and thorium ores. All 22 known isotopes are radioactive, the most stable being francium-223.
Frasch process.
A method of obtaining sulphur from under ground deposits using a tube consisting of three concentric pipes. Superheated steam is passed down the outer pipe to melt the sulphur, which is forced up through the middle pipe by compressed air fed through the inner tube. The steam in the outer casing keeps the sulphur molten in the pipe.
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Free electron.
An electron that is not attached to an atom, molecule, or ion, but is free to move under the influence of an electric field.
Free energy. Gibbs Function, G. A thermodynamic quantity representing the energy that would be liberated or absorbed during a reversible process. Defined, under conditions of constant temperature and pressure, by G=H-TS, where H is the heat content (enthalpy). T the thermodynamic temperature, and S the entropy. Referred to chemical processes, the important quantity is not the absolute magnituce of G. but the change in free energy. D G (also called the chemical affinity), during a reaction, which is given by
DG = D A – T. D S.
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By convention, if a reaction gives out heat. DH will be negative (as the system is losing that to the surroundings). Therefore, if T.DS is not large compared to DH, DG will also be negative indicating that the reacting will proceed to chemical equilibrium. When equilibrium has been attained, DG = O, and if DG is positive the reacting will only occur if energy is supplied in some way to force it away from equilibrium. As the entropy, S is a measure of a molecular disorder of a system, and as a change of state involves a change of molecular orderliness, the term T.DS is dependent upon changes of state.
The Helmholtz Free Energy. F, is defined as U-TS, where U is the internal energy. Also, DF=DU-T.DS and for a reversible isothermal process DF represents the maximum work available. DF is sometimes called the ‘work function’.
Free radical.
An atom or group of atoms with an unpaired valence electron. Free radicals can be produced by photolysis or pyrolysis in which a bond is broken without forming ions (see hemolytic fission). Because of their free valency, most free radicals are extremely reactive.
Freeze.
To change a liquid substance into a solid when the substances is liquid at room temperature e.g., benzene freezes at 5ºC.
Freeze drying.
A process used in dehydrating food, blood plasma, and other heat-sensitive substances. The product is deep-frozen and the ice trapped in it is removed by reducing the pressure and causing it to sublime. The water vapour is then removed, leaving an undamaged dry product.
Freezing mixture.
A mixture of components that produces a low temperature. For example, a mixture of ice and sodium chloride give a temperature of - 20ºC.
Freezing point depression.
The depression caused by dissolving 1 mole of the solute in 1 litre of solvent is termed the molecule depression or cryoscopic constant of the solvent. See colligative properties.
Fries rule.
The most stable arrangement of double bonds in a polynuclear compound in the one in which the maximum number of rings possess the benzenoid structure (having three double bond).
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