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A chemical equation is the way a chemical reaction is represented in written form. In other words, it consists of the representation, by means of written symbols, of the chemical substances that exist before and after a process of chemical change occurs.
In a chemical equation, atoms are represented by their chemical symbols, while other chemical species such as homonuclear substances (O 2 , O 3 , P 4 , etc.), ionic chemical compounds (sodium chloride, bromide potassium, etc.) or covalent (water, methane, benzene, etc.), as well as the individual ions, are represented by their respective molecular or empirical formulas, as the case may be.
In the chemical equation, the different laws that govern stoichiometry can be observed in action, such as the law of definite proportions and the law of conservation of matter. The law of definite proportions is present in the form of the molecular and empirical formulas of different chemical substances.
On the other hand, the stoichiometric coefficients that are used to adjust or balance the chemical equations seek that all the atoms that were before the chemical reaction continue to be present at the end of said reaction. That is, the stoichiometric coefficient adjustment process guarantees that the representation of the reaction does not violate the law of conservation of matter by preventing atoms from disappearing or appearing during the chemical reaction.
Parts of a chemical equation
Chemical equations are written in a way analogous to mathematical equations. This in the sense that they consist of two members, one that is written on the left side and the other on the right side, which are separated by a symbol that relates them to each other. The following figure shows the different parts of a chemical equation that represents a generic chemical reaction, each of which is described below.
reactants
In a chemical equation, all the substances that are written to the left of the reaction arrow (or, more precisely, to the opposite side from where the arrow points) correspond to those substances that are present before the reaction occurs. These substances are called reactants or reactants, since they are, effectively, the substances that will react with each other to become the products.
Products
In contrast to reactants, all substances written on the right side of the reaction arrow (or, more formally, on the side the arrow points to) are called products. This is because they are the substances that appear after the chemical reaction has already occurred.
The reaction arrow
The reaction arrow is the symbol that represents the relationship between reactants and products. In fact, the direction in which it points defines which substances correspond to reactants and which substances correspond to products. In most cases, the reaction arrow consists of a single arrow pointing from left to right, like the one shown in the figure above. However, these arrows can be drawn pointing in any direction, so the chemical equations do not necessarily have to be represented in a line.
In addition to the above, there are also various types of arrows that represent different types of chemical changes.
- In some cases, instead of one arrow there are two pointing in opposite directions (⇌, ⇋, ⇄ or ⇆). This symbol indicates that the reaction is reversible, or that it can occur in both directions. Sometimes one of the two arrows (the one pointing to the right or the left) is longer than the other, indicating that the equilibrium is more shifted to one side than the other.
The following chemical equation represents a reversible acid/base reaction:
- In other cases, a single arrow with two heads (⟷) is drawn. This type of reaction arrow indicates a class of process called resonance and is often used in organic chemistry.
In many cases, the particular conditions under which a chemical reaction occurs are represented in the chemical equation above or below the reaction arrow. Data such as temperature, pressure, the presence of a catalyst or solvent are often represented on the reaction arrow, as shown in the following equation:
stoichiometric coefficients
The stoichiometric coefficients indicate the number of atoms or molecules of reactants involved in a chemical reaction, as well as the corresponding number of atoms or molecules that form from the products. When the stoichiometric coefficient is absent, it is understood that it is worth 1, just as in mathematics any variable in an equation that does not have a coefficient is understood to be multiplied by 1.
The relationships between the stoichiometric coefficients in a chemical equation represent the molar relationships between all the chemical species involved in the reaction. The same chemical reaction can be represented by different chemical equations that differ in the particular set of stoichiometric coefficients. However, in all cases the relationship between all the coefficients will always be the same for all the chemical equations that represent the same reaction.
Because talking about half an atom or a third of a molecule is meaningless, stoichiometric coefficients are often chosen to be whole numbers. However, for various reasons, it is sometimes preferred to use fractional coefficients.
aggregation states
It is common for chemical equations to also include information about the state of aggregation, concentration, or other data of interest about each chemical species in parentheses and as a subscript next to their respective molecular or empirical formula.
The most common examples are:
- (s) indicates that the substance is in a solid state.
- (l) indicates that the substance is in a liquid state.
- (g) indicates that the substance is in a gaseous state.
- (ac.) is the abbreviation of aqueous and indicates that the substance is dissolved in water.
- (alc.) indicates that the substance is dissolved in alcohol.
Interpretation of chemical equations
A generic chemical equation like the one presented at the beginning of this article is interpreted as “a atoms/molecules/ions/moles of A react with b atoms/molecules/ions/moles of B to produce c atoms/molecules/ions/moles of C and d atoms/molecules/ions/moles of D”.
Some specific examples of chemical equations are presented in the next section, along with their interpretation.
Examples of Chemical Equations
Equation of a combustion reaction
This equation reads: “2 molecules of butane gas (C 4 H 10 ) react with 13 molecules of oxygen gas to produce 8 molecules of carbon dioxide gas and 10 molecules of water .”
Equation of a precipitation reaction
This equation represents a precipitation reaction that you might read: “2 moles of aqueous silver ions react with 1 mole of aqueous sulfide ions to form 1 mole of solid silver sulfide.”
Equation of a combination reaction
This is the oxidation reaction of metallic titanium to form titanic oxide. This equation reads: “1 atom of solid titanium combines with one molecule of oxygen gas to form one molecule of titanic oxide or titanium dioxide.”
References
Chang, R., Manzo, Á. R., Lopez, PS, & Herranz, ZR (2020). Chemistry (10th ed .). New York City, NY: MCGRAW-HILL.
Writing and balancing chemical equations. (2020, October 30). Retrieved from https://espanol.libretexts.org/@go/page/1818
MASTER ORGANIC CHEMISTRY (February 12, 2020). The 8 Types of Arrows In Organic Chemistry, Explained . Retrieved from https://www.masterorganicchemistry.com/2011/02/09/the-8-types-of-arrows-in-organic-chemistry-explained/
Raviolo, Andres, & Lerzo, Gabriela. (2016). Teaching Stoichiometry: Using Analogies and Conceptual Understanding . Chemical Education, 27(3), 195-204. Retrieved from https://doi.org/10.1016/j.eq.2016.04.003
