BALANCING CHEMICAL EQUATIONS: METHODS AND EXAMPLES - CHEMISTRY - 2024

The balancing chemical equations implies that all elements in the equation have the same number of atoms on each side. To achieve this it is necessary to use the balancing methods to assign the appropriate stoichiometric coefficients to each species present in the reaction.

A chemical equation is the representation, by symbols, of what happens in the course of a chemical reaction between two or more substances. The reactants interact with each other and, depending on the reaction conditions, one or more different compounds will be obtained as a product.

When describing a chemical equation, the following should be taken into account: first, the reactants are written on the left side of the equation, followed by a one-way arrow or two opposite horizontal arrows, depending on the type of reaction carried out. cape.

Balancing methods of chemical equations

It is based on the stoichiometry of the reaction and tries to try with different coefficients in order to balance the equation, provided that the smallest possible integers are chosen with which the same number of atoms of each element is obtained on both sides. of the reaction.

The coefficient of a reactant or product is the number that precedes its formula, and it is the only number that can be changed when balancing an equation, since if the subscripts of the formulas are changed, the identity of the compound will be changed. in question.

Count and compare

After having identified each element of the reaction and placing it on the correct side, we proceed to count and compare the number of atoms of each element present in the equation and determine those that must be balanced.

Then, the balancing of each element is continued (one at a time), by placing integer coefficients preceding each formula containing unbalanced elements. Normally the metallic elements are balanced first, then the non-metallic elements, and lastly the oxygen and hydrogen atoms.

Thus, each coefficient multiplies all the atoms in the preceding formula; so while one element balances the others can become unbalanced, but this is corrected as the reaction balances.

Finally, it is confirmed by a last count that the entire equation is correctly balanced, that is, that it obeys the law of conservation of matter.

Algebraic balancing of chemical equations

To use this method, a procedure is established to treat the coefficients of the chemical equations as unknowns of the system that must be solved.

In the first place, a specific element of the reaction is taken as a reference and the coefficients are placed as letters (a, b, c, d…), which represent the unknowns, according to the existing atoms of that element in each molecule (if a species does not contain that element is placed "0").

After obtaining this first equation, the equations for the other elements present in the reaction are determined; there will be as many equations as there are elements in said reaction.

Finally, the unknowns are determined by one of the algebraic methods of reduction, equalization or substitution and the coefficients that result in the correctly balanced equation are obtained.

Balancing redox equations (ion-electron method)

The general (unbalanced) reaction is placed first in its ionic form. Then this equation is divided into two half-reactions, the oxidation and the reduction, balancing each one according to the number of atoms, their type and their charges.

For example, for reactions that occur in acid medium, molecules are added H ₂ O to balance the oxygen atoms is added and H ⁺ to balance the hydrogen atoms.

On the other hand, in an alkaline medium, an equal number of OH ^- ions are added to both sides of the equation for each H ⁺ ion, and where H ⁺ and OH ^- ions arise, they join to form H ₂ O molecules.

Add electrons

Then as many electrons as necessary must be added to balance the charges, after balancing the matter in each half-reaction.

After each half-reaction has been balanced, these are added together and the final equation is balanced by trial and error. In the event of a difference in the number of electrons in the two half-reactions, one or both must be multiplied by a coefficient that equals this number.

Finally, it must be corroborated that the equation includes the same number of atoms and the same type of atoms, in addition to having the same charges on both sides of the global equation.

Examples of balancing chemical equations

Source: wikimedia.org. Author: Ephert.

This is an animation of a balanced chemical equation. Phosphorous pentoxide and water are converted to phosphoric acid.

P4O10 + 6 H2O → 4 H3PO4 (-177 kJ).

Second example

You have the combustion reaction of ethane (unbalanced).

C ₂ H ₆ + O ₂ → CO ₂ + H ₂ O

Using the trial and error method to balance it, it is observed that none of the elements has the same number of atoms on both sides of the equation. Thus, one begins by balancing the carbon, adding a two as a stoichiometric coefficient that accompanies it on the product side.

C ₂ H ₆ + O ₂ → 2CO ₂ + H ₂ O

Carbon has been balanced on both sides, so the hydrogen is balanced by adding a three to the water molecule.

C ₂ H ₆ + O ₂ → 2CO ₂ + 3H ₂ O

Finally, since there are seven oxygen atoms on the right side of the equation and it is the last element left to balance, the fractional number 7/2 is placed in front of the oxygen molecule (although integer coefficients are generally preferred).

C ₂ H ₆ + 7 / 2O ₂ → 2CO ₂ + 3H ₂ O

Then it is verified that on each side of the equation there is the same number of atoms of carbon (2), hydrogen (6) and oxygen (7).

Third example

The oxidation of iron by dichromate ions in an acid medium (unbalanced and in its ionic form) occurs.

Fe ²⁺ + Cr ₂ O ₇ ^2- → Fe ³⁺ + Cr ³⁺

Using the ion-electron method for its balancing, it is divided into two half-reactions.

Oxidation: Fe ²⁺ → Fe ³⁺

Reduction: Cr ₂ O ₇ ^2- → Cr ³⁺

Since the iron atoms are already balanced (1: 1), an electron is added to the product side to balance the charge.

Fe ²⁺ → Fe ³⁺ + e ^-

Now the Cr atoms are balanced, adding a two from the right side of the equation. Then, when the reaction occurs in an acid medium, seven molecules of H ₂ O are added on the product side to balance the oxygen atoms.

Cr ₂ O ₇ ^2- → 2Cr ³⁺ + 7H ₂ O

To balance the H atoms, fourteen H ⁺ ions are added to the reactant side and, after equalizing the matter, the charges are balanced by adding six electrons to the same side.

Cr ₂ O ₇ ^2- + 14H ⁺ + 6e ^- → 2Cr ³⁺ + 7H ₂ O

Finally, both half-reactions are added, but since there is only one electron in the oxidation reaction, all this must be multiplied by six.

6Fe ²⁺ + Cr ₂ O ₇ ^2- + 14H ⁺ + 6e ^- → Fe ³⁺ + 2Cr ³⁺ + 7H ₂ O + 6e ^-

Finally, the electrons on both sides of the global ionic equation must be eliminated, verifying that their charge and matter are correctly balanced.

References

1. Chang, R. (2007). Chemistry. (9th ed). McGraw-Hill.
2. Hein, M., and Arena, S. (2010). Foundations of College Chemistry, Alternate. Recovered from books.google.co.ve
3. Tuli, GD, and Soni, PL (2016). The Language of Chemistry or Chemical Equations. Recovered from books.google.co.ve
4. Speedy Publishing. (2015). Chemistry Equations and Answers (Speedy Study Guides). Recovered from books.google.co.ve