Balancing the chemical equations
One of the most fundamental skills in chemistry is the ability to balance chemical equations. A chemical equation is a representation of a chemical reaction, where the reactants (starting substances, substances ENTERING the reaction) are on the left side of the equation, and the products (substances formed during the reaction) are on the right side. The equality of the left and right sides is expressed with the equal sign (=). For equilibrium reactions, we use a double arrow ↔, and if the equilibrium is shifted to one side, we use a single arrow →.
A chemical equation includes both a qualitative (what?) and quantitative (how much?) aspect of a chemical process. The equation must therefore satisfy the law of conservation of the number of atoms of the same element, i.e., the sum of atoms of a given element on the left side must equal the sum of atoms of that element on the right side. At the same time, the rule of charge conservation must be fulfilled, i.e., the total sum of charges of the reactants must equal the total sum of charges of the products. The type and number of atoms on the right and left sides of the chemical equation are identical. Consequently, the sum of the masses of the reactants equals the sum of the masses of the products.
In a chemical equation, arrows can also indicate volatile or sparingly soluble products of the reaction, for example:
Zn + H2SO4 → ZnSO4 + H2↑
AgNO3 + NaCl → AgCl↓ + NaNO3
Each chemical process can be represented by one or more equations. Absolute values of stoichiometric coefficients are given before the symbol of the element or the formula of the compound. These are rational numbers that express the relationship between the amounts of reactants and products in a given chemical reaction.
In practice, amounts of reactants and products are expressed using the mole quantities n(A); n(B)... However, it should be noted that these two concepts are not identical. The mole quantity indicates how many moles of a given substance are present in the system at a given time. The mole quantity therefore changes over time, whereas stoichiometric coefficients remain constant.
(The mole quantities of reactants may numerically match the stoichiometric coefficients—for example, if at the beginning of the reaction the system contains exactly the amount of reactants indicated by the stoichiometric coefficients. In practice, exact stoichiometric amounts of reactants are not always used—for instance, 1.2 equivalents of one reactant may be used relative to another to accelerate the reaction. Sometimes, even products may already be present at the start of the reaction.)