What is normal in chemistry

Equivalent concentration

The Equivalent concentration (Formula symbol: ceq), obsolete normality (Unit symbol: N), is an indication of the concentration in chemistry. It is a special molar concentration that has a reference factor of 1 /z underlying. Here is z the stoichiometric valence, which is also called the equivalent number. In the event of z = 3, the equivalent concentration is three times as large as the molar concentration, because every whole particle, so to speak ztimes is counted. The expression 1 /z are also called equivalent particles or equivalents. ceq is a measure of how many equivalent particles of a substance are in a certain volume of the solution.

The equivalent concentration is particularly important in neutralization reactions, redox reactions, in dimensional analysis and in ionic reactions.

In a redox reaction, a permanganation of MnO4- Pick up 5 electrons, but give off a chloride ion only one electron. The molar mass of potassium permanganate has to be divided by 5, then the amount has to be dissolved in exactly one liter of distilled water to get the equivalent concentration of this oxidizing agent of 1 neq (Mole electron uptake) / liter. 1 neq(= 1 eq) electron uptake, therefore corresponds to 1/5 molar mass of KMnO4 and this is described as: neq (KMnO4) = n (1/5 KMnO4).

In acid / base titrations there are acids with one, two (e.g. sulfuric acid) or three protons (e.g. phosphoric acid). If these acids are titrated with caustic soda, one, two or three parts of caustic soda are needed to neutralize an acid, depending on the number of equivalents of the acid. Therefore, in acid / base titrations, the molar mass is divided by the number of protons that can be released or absorbed and this amount of substance is distilled in one liter. Water dissolved to the equivalent concentration of 1 mole neq To get protons. Then exactly: neq (H2SO4) = neq (NaOH)

definition

The equivalent concentration is defined by the equation:

$ c_ {eq} = c \ cdot z $

Here is ceq the equivalent concentration and c the molar concentration of the solution as well as z the stoichiometric valence, also the equivalent number.

The stoichiometric value - and thus also the equivalent concentration of a certain solution - can depend on the chemical reaction, i.e. the use of the solution, without this itself changing. Furthermore, the equivalent concentration is temperature-dependent.

The usual unit is mole / liter.

Solutions with ceq = 1 mol / L used to be called "normal solutions". One also spoke of 0.1N solutions, if ceq = 0.1 mol / L, etc.

Another possibility of definition is via the number of equivalents or Val dissolved in one liter of a solution.

Normality is then defined by the equation:

$ N = \ frac {n_ {val}} {V} $.

Here is N normality, $ n_ {val} $ the amount of equivalents and V the volume.

Examples

Saline solutions

Sodium carbonate (Na2CO3) consists of two sodium ions (Na+) and a carbonate ion. Thus, a 1 molar (M) sodium carbonate solution corresponds to a 2 normal (N) sodium carbonate solution based on the sodium ions (z = 2).

Acid-base reactions

In acid-base reactions, equivalent particles are protons (H.+) in acidic solutions or hydroxide ions (OH) in basic solutions. A sulfate ion (SO42−), for example, two protons can accumulate, which corresponds to the valency of the acid ion. As a result, the solution contains twice as many equivalent particles (here protons) as the molecules of the substance itself.

$ \ mathrm {H_2SO_4 \ longrightarrow SO_4 ^ {2-} + 2 \ H ^ +} $,

d. i.e., 1 mol / l (H.2SO4) = 2 N (H.2SO4) or in other words, is a 1 normal H.2SO4-Solution ½ molar (1 N corresponds to ½ M).

Redox reactions

In redox reactions, on the other hand, the equivalent is the amount of substance of the oxidizing or reducing agent that can accept or release exactly 1 mol of electrons. An example:

$ \ mathrm {Mn + 4 \ H_2O \ longrightarrow MnO_4 ^ - + 7 \ e ^ - + 8 \ H ^ +} $

In this reaction, manganese is the reducing agent and 1 mole of manganese releases 7 moles of electrons. It therefore gives up 1/7 mol of manganese exactly 1 mol of electrons. The equivalent particle here is 1/7 Mn.

Historical

The use of standard solutions with an equivalent concentration of 1 mol / l (one-normal solution) or 0.1 mol / l was introduced into analytical chemistry in particular by Friedrich Mohr (1806 to 1879), especially in his textbook “Chemisch -analytical titration method ". [1]

See also

swell

  • Hans R. Christen, Gerd Meyer: Basics of general and inorganic chemistry. Salle + Sauerländer, 1997, ISBN 3793554937
  • Frank H. Stepheson: Math in the laboratory. Elsevier Verlag, Munich 2004, ISBN 3-8274-1596-9

Individual evidence

  1. ^ Wilhelm Strube: The historical path of chemistry. Aulis Verlag Deubner & Co KG, Cologne 1989, ISBN 3-7614-1180-4, p. 220.