According to what law is the chemical equation balanced?

Chemical reaction

The Atoms are the smallest building blocks. Through the uptake or release of electrons, various Ions arise. Atoms and / or ions can form bonds with one another and create new connections through their Molecules be represented.

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Note: Molecules are particles from at least 2 binding partners of the same or different nature, e.g. oxygen molecule (O2) or water molecule (H2O).

These molecules, in turn, can enter into reactions in which their bonds are broken and new ones are made. There are four major types of reactions in inorganic chemistry:

  • Acid-base reactions (see chapter: "Acid-base chemistry")
  • Redox reactions (see chapter: "Redox chemistry")
  • Precipitation reactions (see chapter: "Precipitation Reactions")
  • Complex reactions (see chapter: "Complexes")

Most chemical reactions are reversible reactions, i.e. reactions from a forward and a reverse reaction. Due to this fact, an equilibrium is always established in reversible reactions (see chapter: "Chemical equilibrium"). Every response comes in the form of a Reaction equation shown. A reaction equation contains only the minimum number of substances participating in the reaction. Let's look at the example in Fig. 1.

Figure 1: Reaction equation for the representation of table salt (NaCl) from the elements sodium (Na) and chlorine (Cl2)

To the left of the reaction arrow are the Educts and to the right of the reaction arrow die Products. The Reaction arrow indicates the direction of the reaction. The Stoichiometry (stoichiometric number) deals with the quantitative proportions in chemical reactions and enables the calculation of the converted masses or volumes of the reactants. The stoichiometric conversions of chemical reaction partners, which can exist as atoms, molecules or ions, follow the laws of chemical bonding. In this reaction we get two particles of sodium chloride from two particles of sodium and one particle of chlorine gas. The index indicates in the empirical formula of a compound how many atoms and / or ions of the same type are bound. When setting up reaction equations, the following applies: "Law of Conservation of Mass“, Which means that the mass to the left and right of the reaction arrow must be the same. Mass cannot be lost and cannot arise suddenly. In addition, the "Law of Conservation of Charge“, Which means that the charge to the left and right of the reaction arrow must be the same. Charge cannot be lost and cannot arise out of nowhere.

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Note: Chemical reactions are represented by a reaction equation. The starting materials are to the left of the reaction arrow and the products are to the right. The reaction arrow shows the direction of the reaction. All of the particles participating in the reaction are given in their minimal number in the reaction equation. The number is represented by the stoichiometric number. The law of the conservation of mass and charge applies.

Establishing reaction equations and balancing the stoichiometric numbers can best be understood through practice. First, a simple example task that shows the most important steps.

Sample task: If there is a reaction between ammonia (NH3) and oxygen (O2) nitrogen gas (N2) and water (H2O).

Step 1: Note down the starting materials and products

NH3 + O2 → N2 + H2O

2nd step: Compensation by suitable choice of the stoichiometric coefficients: It is best to start on the far left in the reaction equation. We recognize that in ammonia (NH3) a nitrogen atom (N) is bonded. To the right of the reaction arrow, however, nitrogen is produced (N.2), which consists of two nitrogen atoms. So we have to go before the ammonia (NH3) write a “2” so that the nitrogen atoms on both sides have the same number.

2 NH3 + O2 → N2 + H2O

Next, let's look at the number of hydrogen atoms (H) to the left and right of the reaction arrow. In the total number of ammonia molecules we have six water atoms (H) on the left and two hydrogen atoms on the right. So we have to write a "3" on the left in front of the water molecule so that the hydrogen atoms are balanced.

2 NH3 + O2 → N2 + 3 H.2O

Next, let's compare the number of oxygen atoms (O) on either side of the reaction arrow. On the left we have two oxygen atoms (O) and on the right three. The common denominator of two and three is six. So we write the number 3 in front of the oxygen and multiply the water molecules by 2.

2 NH3 + 3 O2 → N2 + 6 H.2O

This increases the number of hydrogen atoms on the right. The ammonia molecules (2 NH3) must also be taken twice to compensate.

4 NH3 + 3 O2 → N2 + 6 H.2O

The nitrogen molecule on the right is finally given a “2” as a stoichiometric coefficient. Thus the reaction equation would be established.

4 NH3 + 3 O2 → 2 N2 + 6 H.2O

If the number of all atoms on the left and right is balanced, then the mass must automatically be the same on both sides. The charge is zero on both sides.

Every reaction has its own reaction energy. The thermodynamic and also more correct designation is reaction enthalpy (ΔHR.). ΔHR. can have a negative value, then one speaks of one exothermic reaction. This releases energy in the form of heat. ΔHR. can just as easily assume positive values, then one speaks of one endothermic reaction. These reactions require additional energy. ΔHR. is a difference value between the enthalpy of the products and the starting materials (ΔHR. = HProducts - HEducts). More on this topic can be found in the “Thermodynamics” chapter.

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Note: ΔHR. represents the enthalpy of reaction; -ΔHR. = exothermic reaction (energy is released); +ΔHR. = endothermic reaction (energy must be supplied).