Acid dissociation constant
The acidity constant K is a material constant and gives information about the extent to which a substance in an equilibrium reaction with water under protolysis responded:
Here, HA is a Brønsted acid ( according to John Nicolaus Brønsted ) that a proton H can give to a solvent such as water, an anion A- remains. More generally, the Brønsted'sche definition also applies to non-aqueous systems, in this case for an arbitrary, protonisable solvent Y:
KS is the equilibrium constant for this reaction, and thus a measure of the strength of an acid. The stronger the acid, the more the reaction is shifted to the right side; i.e., the higher the concentrations of [ HY ] and [A- ]. The equilibrium constant is usually given as its negative logarithm, the pKa value ( pKa also, from the English. Acid = acid). This means that the smaller the pKa value, the stronger the acid.
Derivation of the acid constants
The acidity constant is derived as the equilibrium constant of a chemical reaction from the Gibbs free energy G ( also free enthalpy ) ago. Once this is known, then for the equilibrium constant K th of any chemical reaction:
Where R is the universal gas constant, T is temperature, and e is Euler's number. The formula thus describes the observable temperature dependence of the acidity constants.
Is defined as the product of the activities and is a dimensionless quantity. If mixing effects neglected, is considered. This is possible in solutions up to 1 mmol / l with no major errors. Constants can therefore be set up with the activities as well as with the concentrations. However, they have a different value. Due to the historical development of chemistry from a practical science, the concentration- related constants are usually indicated, as these have been experimentally found in front of the thermodynamic grounds.
Acid strength
The property of a particular substance to react as an acid, is inextricably linked to its potential ability to transfer a proton ( H ) to a reaction partner. We call such a reaction protolysis. The strength of an acid describes the extent of this ability. However, this is dependent on the ability of a reactant to receive the proton. If the acid strength of different acids are compared, it is useful to consider the interaction with a standard reaction partner. This is usually the water connection and the solvent is most important in many processes in nature. The reaction equation of an acid HA in water and can be represented as follows:
In this reaction rapidly an equilibrium. Here in addition to HA has also H3O on the ability of a proton to a reactant transmitted: They are both acids. H2O and on the other hand A- have the ability to absorb a proton, and therefore they are both referred to as bases. If you separate conceptually the standard reactant water and H3O from HA and remain A- left. Since the concentrations of these components are attached to a balance, the extent of the ability of the HA to be an acid, linked to the extent of the ability of A to be a base. For example, if HA has great potential to give a proton and accept A- a small potential, a proton is called HA is a strong acid. The balance ( 1) would be on the right side. If the acid HA has a huge potential to make a proton ( ie a low pKa value ), then the corresponding base has to the extent low potential (ie, a high pK value) take A- one, a proton. For example, water applies: pK pK = pKW = 14
The acidity constant (or pK a ) is a measure of the strength of an acid. The acidity is greater the lower its pKa. The pKa value is numerically equal to the pH of a solution when HA and A- for equilibrium (1 ) are present in the same concentration.
In aqueous solutions, strong acids and strong bases very protolysieren completely H3O - and OH - ions. Thus, the different acid strengths of hydrochloric and perchloric acid can not be distinguished in water based on the pH. This is called a leveling effect ( v. French: niveler = same make ) of water. In order to distinguish even very strong acids relative to the acid strength is determined equilibrium constants in non-aqueous solutions and transfers it approximately to the solvent water.
The standard reaction partners of water has a special property of being able to react as an acid and base:
This so-called autoprotolysis allows the determination of the extent of the ability of a base to accept a proton from water, and is explained in more detail with base constant.
Acid -base reaction
Between an acid HA and its base A - is in aqueous solution the following equilibrium reaction:
According to the law of mass action, the position of the equilibrium is described by the equilibrium constant K:
As the concentration of water (C (H2O) ) in the reaction remains substantially constant, can be C ( H 2 O) to include in the constant K. Finally, This results in the acidity constant K with the unit mol / l:
Often the negative decadic logarithm of Ks, the so-called pK a value is specified:
The smaller the pKa, the stronger the acid. Thus, for example, nitric acid (HNO3, dissociation of 96 % at a concentration of 1 mol / L), the pKa -1.32, acetic acid ( dissociation of 0.4 % at a concentration of 1 mol / L ) has a pKa of 4, 75 In water in which at 25 degrees Celsius ( standard conditions) in one liter of 10-7 mol molecules are dissociated, is the pK a value of 15.74.
Accordingly, there is a base constant ( pKb ). The smaller the pKb value, the stronger the desire to take protons of the base. The pKa can convert to the base constant of the corresponding Base:
Polybasic acids
In a polybasic acid is a gradual proteolysis. For each there is a Protolysationsstufe own acidity constant or pKa. For the individual Protolyseschritte applies in general: Ks1 > KS2 > Ks3 (or PKS1 < PKS2 < PKS3 ) because of the increasing ionic charge of the resulting Säurerestanions the further proteolysis is less favored energetically.
As an example, applies to the phosphoric acid:
At pH 7.20 are equal concentrations of dihydrogen phosphate and hydrogen ions, the concentration of undissociated phosphoric acid and phosphate ions are a million times smaller. These relationships makes you look in phosphate buffers advantage.
Sulfuric acid is five orders of magnitude more acidic than acid:
Concentrated sulfuric acid is used in lead-acid battery as the electrolyte. Under these equilibrium conditions no free sulfate ions exist more.
Determination of pKa values
The determination of pK values of acid values in the range of 4 to about 10 can be determined at the half equivalent point on acid-base titrations and determination of the pH. Here the acid and its conjugate base is present in the same concentration. At this point, it follows from the Henderson-Hasselbalch equation: pH = pKa.
Acidity of organic acids
When organic acids decide three main structural properties of the acid strength:
Some substituents have both mesomeric and inductive effect, such as the halogens or nitro groups. Halogens exhibit a strong I, but a weak M effect; the nitro group acts as both electron-withdrawing (-I effect), as well as a - M effect, i.e., the two effects act in the same direction.
PKa and pKb values of some compounds
The following table lists pKa and pKb values of some acids and bases at standard conditions: