Chemical equation

In chemistry, a reaction equation is the shorthand notation for a chemical reaction. They are the starting materials (better: reactants) and products of a mass conversion in formula notation again and is internationally uniform. However, since the starting materials and products are not the same during a conversion, the term reaction scheme is used for didactic reasons often. This description is according to DIN 32642 " symbolic description of chemical reactions " can be used from January 1992 only for a qualitative description of a chemical reaction. Accordingly, the two sides are not connected by an equal sign ( =), but by an arrow that indicates the direction of the turnover (→ to the right or left insertion ) or by a double arrow () indicates an equilibrium reaction.

When the amounts of the stoichiometric coefficients in the equation are integers and as small as possible, it is called according to DIN 32642 of a cardinal equation. This standard does not divide the reactants in the reactants ( starting materials) and products; they also lays down even the terms turnover variable, formula conversion and molar reaction enthalpy and contains six examples of the qualitative description by reaction schemes.

The designation as an equation has its basis in the fact that on both sides for all occurring elements the respective numbers of the atoms as well as the charge sums must be equal.

Construction of a chemical equation

On the left side are the chemical formulas of the starting materials ( reactants ) - on the right of the products. A reaction between the arrow is written (for example ), which points in the direction of the products out. Before the formulas, it is also uppercase numbers that indicate how many molecules of the substance or how much amount of substance ( in moles) of each required, consumed or produced. They are called stoichiometric coefficients ( conforming to standards: stoichiometric numbers) of the substances involved. They must be chosen such that the molar ratios of the reactants - their stoichiometric conditions - are reproduced correctly: For each chemical element have on the left side of a chemical equation the same number of atoms as to the right to be present. The number "one" as the stoichiometric coefficient is not written.

For example, the combustion of methane gas (formula: CH4) and oxygen gas ( the formula: O 2) to carbon dioxide and water by the equation

Described. In this example, an atom (left to right in CH4 and CO2), H for hydrogen per four atoms ( CH4 left and right 2 in both H2O ) and oxygen O also four atoms ( on the left are for carbon C je je two present in both O2 and CO2 in two right and one each in both H2O).

A reaction scheme, however, not taken into account stoichiometric ratios of the reactants or only partially and only indicates which reactants react to these products. For example, the following equation word is a reaction scheme:

Symbols

Arrows

In response equations different arrows with the following meanings are used:

• Reaction arrow ()
• More reaction arrows ( ) describe a reaction sequence, ie a sequence of several individual reactions between reactant and product
• Round-trip response (), the reaction can proceed by changing reaction conditions in one direction or another.
• Equilibrium arrow () is used when setting a reaction equilibrium at the specified conditions.
• Retrosynthetic arrow ()
• For the identification of one or two electrons shifts ( description of reaction mechanisms, often used in organic): Curved arrow with all lace () symbolizes the shift of an electron pair ( = two electrons).
• Curved arrow with half point symbolizes the shift of a single electron.

Notes: The Mesomeriepfeil () does not describe a chemical reaction and is thus not used in reaction equations. Of these arrows only the reaction arrow and the equilibrium arrows ( two parallel, oppositely directed arrows with half- heads) are standard according to DIN 32642; is intended to express 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.

State information

To illustrate modifications, states of aggregation or solution states can be specified after the chemical symbols or formulas in parentheses. For this purpose, in accordance with DIN 32642, the following abbreviations are used:

• G for gaseous (English: Gaseous )
• L for liquid (English: liquid)
• S for fixed (English: solid)
• Aq ' dissolved in water ' for (English: Aqueous )

Resulting solids or gases must to standards are also featured with a trailing downward () or upward () pointing arrow.

More information

Over the reaction arrow to write optionally the reaction conditions and the catalyst used. The substances heated in the reaction, this is indicated by a large delta ( Δ ) to the reaction arrow. The resulting reaction or expended energy is written on the side where it is incurred or to be expended.

For thermodynamic calculations, the enthalpy of reaction is often indicated with, for example, in the reaction equation of the hydrogen-oxygen reaction

In the formation of one mole of liquid from gaseous H2O and H2 O2 that is 286 kJ of energy is released. Here it is essential that the phase of the substances participating in the reaction is indicated by, as in the phase transition energy is also implemented. The heat of reaction AH is usually given at 25 ° C. A positive value of AH called endothermic reactions, a negative value exothermic reactions.

Shortened form

Instead of the full notation with complete empirical formulas may be omitted uninvolved reactants. For example:

It's at this precipitation reaction no difference whether in lithium or sodium sulphate is used as both salts are soluble in water and precipitate either lithium or sodium chloride. The uninvolved in this reaction cation (Li or Na ) can therefore be omitted.

Benefits of reaction equations: Revenue calculations

To calculate the metabolic rate in a reaction, the equation with the aid of quantities of material being used, the mol. Foundations of this computational method can be found in the article stoichiometry ( Fachrechnen chemistry). As an example, the reaction equation of the combustion of methane gas described above is taken. The reaction scheme is:

It means high methane and oxygen react to form carbon dioxide and water.

It states quantitatively: 1 mole of methane and 2 moles of oxygen yield 1 mole of carbon dioxide and 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 44 g 1 mole of carbon dioxide, it also states:

Since 1 mole of gas occupies 22.4 L space under normal conditions, the reaction scheme also states:

Similar calculations are possible for sales any other chemical reaction, the reaction scheme has been created. Thus, for required quantities of raw materials or theoretically recoverable quantities of product ( 100 % yield ) by reaction schemes and molar masses calculated. For example, the task How much hydrogen is formed in the reaction of 1 g of lithium with water? there is such an example in the article to stoichiometry.