Reaction equation

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Logo of the German Institute for Standardization DIN 32642
Area chemistry
title Symbolic description of chemical reactions
Brief description: Reaction equations
Latest edition 1992-01

In chemistry, a reaction equation is a description of a chemical reaction in the correct stoichiometric ratio. It indicates the reaction partners ( reactants and products ) of a substance conversion in symbolic notation .

A reaction equation is regarded as an equation because the same number of atoms of the respective chemical element must be represented on both sides of the equation and the sum of charges must be the same on both sides.

The two sides of the equations are usually not connected by an equal sign (=), but by an arrow that indicates the direction of conversion (→) or a double arrow ( ) that indicates an equilibrium reaction. To satisfy the stoichiometry, reaction equations contain reaction stoichiometric numbers. These numbers are usually whole numbers and as small as possible, whereby the number 1 is usually left out. According to DIN 32642 "Symbolic description of chemical reactions" one speaks of a cardinal equation .

Often times, non-stoichiometric representations are also used for reactions. According to DIN 32642, these qualitative representations are not called reaction equations, but reaction schemes. This standard also defines the terms conversion variable , formula conversion and molar reaction enthalpy .

Structure of a reaction equation

On the left side of a reaction equation are the chemical formulas of the starting materials (reactants) - on the right those of the products. A reaction arrow is written in between (e.g. ) pointing in the direction of the products. In front of the molecular formulas or formula units of the reaction partners, capitalized numbers are also placed, which indicate the ratio in which the substances are consumed or produced. The numbers are called reaction stoichiometric numbers (according to the standard: amount of the stoichiometric number ) of the substances involved. They have to be chosen in such a way that the substance quantity ratios of the reactants - their stoichiometric conditions - are correctly reproduced: For each chemical element , the same number of atoms must be present on the left side of a reaction equation as on the right side. The number “one” as the reaction stoichiometric number is not written.

For example, the combustion of methane (CH 4 ) with oxygen (O 2 ) to carbon dioxide (CO 2 ) and water (H 2 O) is given by the equation

described. In this example there is one atom each for carbon C (left in CH 4 and right in CO 2 ), for hydrogen H there are four atoms each (left in CH 4 and right 2 in both H 2 O), and for oxygen O also each four atoms (left two in both O 2 and right two in CO 2 and one each in both H 2 O).

Reaction scheme

A reaction scheme, on the other hand, does not take into account stoichiometric ratios of the reactants or only partially and only specifies which starting materials react to which products. In organic chemistry, only the carbon-containing molecules are often listed and the formulas of the low molecular weight inorganic particles involved (e.g. water) are not shown graphically. The acid-catalyzed elimination of water from 2-pentanol is an example of such a reaction scheme:

Reaction scheme:
1-pentene (left, by-product) and the main products ( E ) -2-pentene (middle) and ( Z ) -2-pentene (right) are formed from 2-pentanol through elimination of water . Not explicitly stated is (a) acid catalysis, (b) the elimination of H 2 O and (c) the stoichiometry, which is complicated in this case.

Another example of a reaction scheme is the following word equation  :



Various arrows are used in reaction equations with the following meanings:

  • Reaction arrow ( )
  • Several reaction arrows ( ) describe a reaction sequence, i.e. a sequence of several individual reactions between starting material and product
  • Back and forth reaction ( ), the reaction can proceed in one direction or the other due to changed reaction conditions.
  • Equilibrium arrow ( ), is used when a reaction equilibrium is established under the given conditions.
  • Retro synthesis arrow ( )
Curved arrows describing electron shifts: electron pair shift (left) and one-electron shift (right).
  • to identify one or two electron shifts (description of reaction mechanisms , often used in organic chemistry ):
    • Curved arrow with a whole point ( ) symbolizes the displacement of an electron pair (= two electrons).
    • Curved arrow with half point symbolizes the displacement of a single electron.

Notes: The mesomeric arrow ( ) does not describe a chemical reaction and is therefore not used in reaction equations. Of these arrows, only the reaction arrow and the equilibrium arrows (two parallel, oppositely directed arrows with half-tips) conform to the DIN 32642 standard; If it is to be expressed that the equilibrium constant of a reaction is very large or very small, this can be expressed by different arrow lengths of the equilibrium arrows.

Status information

For clarification, modifications, states of aggregation or states of solutions can be specified after the chemical symbols or formulas in round brackets. According to DIN 32642, the following abbreviations are used for this:

  • g for gaseous (ger .: gaseous )
  • l for liquid (ger .: liquid )
  • s for fixed (ger .: solid )
  • aq for 'dissolved in water' (English: aqueous )

Solids or gases formed may also be marked with an arrow pointing downwards ( ) or upwards ( ) in accordance with the standard .

More information

The reaction conditions and the catalyst used are optionally written over the reaction arrow . If the substances are heated for the reaction, this is indicated by a large delta (Δ) above the reaction arrow. The resulting or expended reaction energy is written on the page where it arises or has to be expended.

For thermodynamic calculations which is often molar enthalpy of reactionH R ) specified by, for example, in the reaction equation of the detonating gas reaction

With the formula conversion of two moles of gaseous H 2 and one mole of gaseous O 2 to two moles of liquid H 2 O, 572 kJ of energy are released. Here it is essential that the phase of the substances involved in the reaction is also specified, since energy is also converted during the phase transitions . The enthalpy of reaction is usually given at 25 ° C. A positive value of Δ H R denotes endothermic reactions, a negative value exothermic reactions.

Abbreviated form

Instead of the full notation with complete empirical formulas, uninvolved reaction partners can be omitted, e.g. B .:

instead of:

In this precipitation reaction it makes no difference whether lithium or sodium sulfate is used, since both salts are soluble in water and neither lithium nor sodium chloride precipitate. The cation (Li + or Na + ) that is not involved in this reaction can therefore be omitted.

Another abbreviation that is used in systems with several reactions occurring is the stoichiometric matrix , which compactly summarizes the stoichiometry of several reaction equations.

Using Reaction Equations: Sales Calculations

In order to calculate the metabolic rate in a reaction, the reaction equation is used with the help of the amount of substance in moles . The basics of this calculation method can be found in the article stoichiometry (technical calculation chemistry) The reaction equation for the combustion of methane gas described above is taken as an example. The reaction scheme is:

It says qualitatively: methane and oxygen react to form carbon dioxide and water.

It says quantitatively: 1 mole of methane and 2 moles of oxygen result in 1 mole of carbon dioxide + 2 moles of water.

Since 1 mole of C weighs 12 g, 1 mole of methane 16 g, 1 mole of oxygen 32 g, 1 mole of water 18 g and 1 mole of carbon dioxide 44 g, it also says:

16 g methane + 64 g oxygen result in 44 g carbon dioxide + 36 g water.
80 g of starting materials (starting materials) result in 80 g of end materials (products). For every 16 g of oxidized methane, 44 g of carbon dioxide are produced.

Since 1 mol of gas takes up 22.4 L space under normal conditions , the reaction scheme also says:

22.4 L methane + 44.8 L oxygen result in 22.4 L carbon dioxide + 44.8 L water vapor.

Similar conversion calculations are possible for any other chemical reaction for which the reaction scheme has been created. The required quantities of raw materials or theoretically achievable product quantities (with 100% yield) can be calculated using reaction schemes and molar masses. Exercise example: How much hydrogen is produced when 1 g of lithium reacts with water?


Web links

Wiktionary: reaction equation  - explanations of meanings, word origins, synonyms, translations
Wikibooks: General and Inorganic Chemistry / Reaction  Scheme - Learning and Teaching Materials
  • Online calculator for determining the coefficients of a stoichiometric equation, including a description of the mathematical background
  • Online calculator to determine the coefficients of a stoichiometric equation, indicating the masses that react with each other. (Accessed April 7, 2013)

Individual evidence

  1. Stoichiometric computing