Chemical reactions and the Law of Conservation of Mass

Law of Conservation of Mass

In the year 1785, Antoine Lavoisier formulated the Law of Conservation of Mass or Matter. This fundamental scientific principle states that during a chemical reaction, the quantity of matter that intervenes as a reactant is equal to the quantity of matter that appears as a product. In practical terms, this means that during chemical reactions, atoms are neither created nor destroyed; instead, existing chemical bonds are broken to form different bonds that create new substances. As a result, the number of atoms for each individual element involved in the reaction must remain constant from the beginning to the end of the process.

Methods of Balancing Chemical Equations

If, upon performing an initial count of atoms in a chemical equation, the quantities on the reactant side do not coincide with those on the product side, the equation must be balanced. Several methods are commonly employed to achieve this balance, including the "Oxido reducción" (oxidation-reduction or redox) method and the "Tanteo o inspección simple" (Trial and Error or Simple Inspection) method.

The Tanteo or Simple Inspection Method

The method of "Tanteo" or simple inspection consists of writing out the chemical equation and then performing a systematic count of the atoms for each element on both the reactant and product sides. Finally, coefficients are added incrementally in front of the chemical formulas until the number of atoms for each element is equalized across both sides. It is a critical rule that under no circumstances should the subscripts of the chemical formulas be modified, as changing subscripts would change the chemical identity of the substances involved.

Activity: Balancing Chemical Equations

This section details a variety of chemical equations that were presented for balancing, along with their fully developed and balanced solutions.

Phosphorus and Oxygen Reaction: The unbalanced form is $P + O_2 \rightarrow P_2O_3$. The balanced solution is $4P + 3O_2 \rightarrow 2P_2O_3$.

Sodium and Water Reaction: The unbalanced form is $Na + H_2O \rightarrow NaOH + H_2$. The balanced solution is $2Na + 2H_2O \rightarrow 2NaOH + H_2$.

Manganese(VII) Oxide Decomposition: The unbalanced form is $Mn_2O_7 \rightarrow MnO_2 + O_2$. The balanced solution is $2Mn_2O_7 \rightarrow 4MnO_2 + 3O_2$.

Antimony and Hydrochloric Acid Reaction: The unbalanced form is $Sb + HCl \rightarrow SbCl_3 + H_2$. The balanced solution is $2Sb + 6HCl \rightarrow 2SbCl_3 + 3H_2$.

Lead(II) Sulfide and Oxygen Reaction: The unbalanced form is $PbS + O_2 \rightarrow PbO + SO_2$. The balanced solution is $2PbS + 3O_2 \rightarrow 2PbO + 2SO_2$.

Hydrochloric Acid and Calcium Hydroxide Neutralization: The unbalanced form is $HCl + Ca(OH)_2 \rightarrow CaCl_2 + H_2O$. The balanced solution is $2HCl + Ca(OH)_2 \rightarrow CaCl_2 + 2H_2O$.

Methane and Water Steam Reforming: The unbalanced form is $CH_4 + H_2O \rightarrow CO + H_2$. The balanced solution is $CH_4 + H_2O \rightarrow CO + 3H_2$.

Classification of Chemical Reactions by Structural Change

Chemical reactions are often categorized based on how the atoms are rearranged between reactants and products:

Sintesis o Combinación (Synthesis or Combination): This occurs when two or more reactants unite to form a single product. The general symbolic representation is $A + B \rightarrow AB$. An example of this is the formation of water: $2H_2 + O_2 \rightarrow 2H_2O$.

Descomposición (Decomposition): In this reaction type, a single reactant separates or breaks down into two or more products. The general symbolic representation is $AB \rightarrow A + B$. An example is the decomposition of water: $2H_2O \rightarrow 2H_2 + O_2$.

Sustitución Simple (Simple Substitution): This happens when one element replaces another within a compound. The general symbolic representation is $A + BC \rightarrow AC + B$. A relevant example is the reaction between zinc and hydrochloric acid: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$.

Classification by Reaction Directionality

Reactions can also be classified based on whether they can proceed in both directions or only one:

Reversibles (Reversible Reactions): These are reactions that can occur in both directions, meaning the products of the reaction can themselves react to reform the original reactants. An example is the synthesis of ammonia: $N_2 + 3H_2 \rightleftharpoons 2NH_3$.

Irreversibles (Irreversible Reactions): These are reactions that occur in only one single direction. Once the products are formed, they cannot easily return to the initial state or original reactants. A common example is the combustion of carbon: $C + O_2 \rightarrow CO_2$.

Specific Reaction Types: Combustion and Neutralization

Combustión (Combustion): This is a process where a substance combines with oxygen, releasing energy in the form of heat. An example is the combustion of methane: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

Neutralización (Neutralization): This specific type of reaction occurs when an acid and a base react with each other. The typical products are a salt and water. A classic example is the reaction between hydrochloric acid and sodium hydroxide: $HCl + NaOH \rightarrow NaCl + H_2O$.

Classification by Energy Exchange

Reactions are further classified by whether they absorb or release energy during the process:

Endotérmicas (Endothermic Reactions): These are reactions that absorb heat or energy from their surrounding environment. A biological example is photosynthesis: $6CO_2 + 6H_2O + \text{energía} \rightarrow C_6H_{12}O_6 + 6O_2$, where the product formed is glucose.

Exotérmicas (Exothermic Reactions): These are reactions that release heat or energy into their surroundings. A representative example is the combustion of methane which releases energy ($E$): $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + E$.