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|Group||Compound name||Common name||#C||#H||#N|| pK a1 of the |
| pK a2 of the |
BH 2 2+
| Mole |
| Melting |
| Boiling |
| Density@20°C |
An acid dissociation constant, K a , is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation of acid–base reactions. In aqueous solution, the equilibrium of acid dissociation can be written symbolically as:
HA + H 2 O = A - +H 3 O +
where HA is an acid that dissociates into A - , (known as the conjugate base of the acid) and a hydrogen ion which combines with a water molecule to make a hydronium ion.
The chemical species HA, A - and H 3 O + are said to be in equilibrium when their concentrations do not change with the passing of time. The dissociation constant is usually written as a quotient of the equilibrium concentrations (in mol/L), denoted by [HA], [A - ] and [H 3 O + ]
K a = [A - ]*[H 3 O + ] / [HA]*[H 2 O]
In all, but the most concentrated, aqueous solutions of an acid the concentration of water can be taken as constant and can be ignored. The definition can then be written more simply
HA = A - + H + and K a = [A - ]*[H + ] / [HA]
This is the definition in common usage. For many practical purposes it is more convenient to discuss the logarithmic constant, pK a
pK a = -log 10 K a
The larger the value of pK a , the smaller the extent of dissociation at any given pH - that is, the weaker the acid.
A weak acid has a pK a value in the approximate range -2 to 12 in water.
Strong acids has pK a values of less than about -2; the dissociation of a strong acid is effectively complete such that concentration of the undissociated acid is too small to be measured. pK a values for strong acids can, however, be estimated by theoretical means.
After rearranging the expression defining K a , and putting pH = -log 10 [H + ], one obtains
pH = pK a + [A - ] / [HA] and further
pH - pK a = log [A - ] / [HA]
Then, a solution with 50% dissociation has pH equal to the pK a of the acid.
Polyprotic acids are acids that can lose more than one proton. Then we have more than one dissiciation constant; K a1 , K a2 , etc.. and similar pK a1 , pK a2 , etc.
All data given in the figures apply to dilute aqueous solutions at ambient temperature.
For amines, the pk a value is given for the conjugate bases BH + and BH 2 2+ .
BH + = B + H +
The pK b for a base may be calculated from the pK a value of its conjugate acid:
pK w = pK a + pK b
At 25°C the pK w is 14 and
pK b = 14 - pK a
Definition of organic compounds
Hydrocarbon: An organic compound consisting entirely of hydrogen and carbon.
Alkane: An acyclic saturated hydrocarbon, with the general formula C n H 2n+2 . Also called paraffin.
Aromatic hydrocarbon : A cyclic (ring-shaped), planar (flat) molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms. The simplest of the aromatics have 6 carbon atoms and contains 3 double bounds. A one ring aromatic without any substituents is called benzene, with the formula C6H6.
Alkyl: An alkyl group is an alkane substituent missing one hydrogen, with general formula C n H 2n+1.
Phenyl: An phenyl group is a benzene substituent missing one hydrogen, with general formula C 6 H 5 .
Amine: A compound or functional group that contain a basic nitrogen atom with a lone pair. It can be a primary (R-NH2), a secondary (R',R''-NH) or a tertiary amine (R',R'',R'''-N), where R represent an alkyl or other organic substituent. For 1-amines in this document the R represents an alkyl group, in which the NH2-group is placed at the end of the the alkane chain.
Diamine: An amine with two amino groups.
Pyrrole: A heterocyclic aromatic organic compound, a five-membered ring with two double bounds, with the formula C4H4NH. Substituted derivatives are called pyrroles.
Dihydropyrrole: A compound formally derived from the aromate pyrrole by partial hydrogenation, containing one double bound. Also called pyrroline.
Pyridine: A heterocyclic six-membered ring compound with the chemical formula C5H5N. It is structurally similar to benzene, with one methine group (=CH-) replaced by a nitrogen atom.
Piperidine: A heterocyclic amine consisting of a six-membered ring containing five methylene bridges (–CH2–) and one amine bridge (–NH–). The molecular formula is (CH2)5NH.
Quinoline: A heterocyclic aromatic organic compound, consisting of a benzene ring fused to a pyridine ring, with the molecular formula C9H7N and the nitrogen atom in position 1.
Isoquinoline: An analog to quinoline with the nitrogen atom in position 2.
Material properties of gases, fluids and solids - densities, specific heats, viscosities and more.
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Values for the negative logarithm of the acid dissociation constant, pKa, of inorganic acids and bases, as well as hydrated metal ions.
The rules of logarithms - log10 and loge for numbers ranging 1 to 1000.
Nomenclature rules for different groups of organic compounds and functional groups, together with examples of use of the rules.
Introduction to pH - the acidic and basic (alkaline) definition.
For oxygen containing organic compounds this is given: pKa (the negative logarithm of the acid dissociation constant), molecular structures, molar weights, density and melting and boiling points.
The most common strong acids and bases, and some examples of weak acids and bases, together with definition of strong and weak acids and bases.
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