What is a reagent in chemistry?

In a chemical reaction, the reactants, sometimes also called reactants, are the starting substances that will undergo chemical changes to become the products . In other words, they are those chemical species that will undergo changes in their chemical bonds and in the distribution of their valence electrons, either to form new bonds, to break old bonds, or both.

In other words, the reactants are those substances that we have BEFORE a chemical reaction occurs, unlike the products that are those that are obtained AFTER .

The reactants in a chemical equation

In a chemical equation, we can easily identify the reactants since they are the ones on the left side of the reaction arrow, as shown below:

Recognizing the reactants is as easy as this, as long as the reaction is irreversible , meaning it can only occur in one direction: from left to right.

However, care must be taken, as there are chemical reactions that can occur in both directions. Such is the case of reversible reactions, which can occur both from left to right and from right to left, which is represented by a double arrow (sometimes like this ⇌ and other times like this ⇄).

For these types of reactions, both the species on the left and the right side of the double reaction arrow can be considered both reactants and products.

To better understand this, you just have to remember that this representation of a reversible chemical reaction is just a way to save space, but, in reality, they are two separate and independent reactions. If we look at it from this point of view, any reversible reaction can be written as two opposite reactions that occur at the same time, as shown in the following example.

Example: Identification of reactants in a reversible reaction

Consider the reversible decomposition reaction of gaseous dinitrogen tetroxide (N ₂ O ₄ ). This reaction is commonly represented as a single double-arrow chemical equation:

Seeing it written like this, many tend to think that only N2O4 is a reagent. However, this equation actually represents two separate reactions:

And its reverse reaction:

Seeing the reaction represented like this, it is much easier to realize that both dinitrogen tetroxide (N ₂ O ₄ ) and nitrogen dioxide (NO ₂ ) can be reactants and products at the same time.

How to identify the reagents in the statement of a problem

Many chemistry problems begin with a description of a chemical reaction, which the student must be able to represent in the form of a chemical equation in order to later balance or adjust it and carry out the required calculations. To achieve this, it is essential to be able to unequivocally identify reactants and products from the information in the statement. In the simplest cases, the statement will explicitly identify the reactants. In case this does not happen, we present some tips and advice on how to recognize the reactants in the description of a chemical reaction. In all cases, the capital letters (A, B, C) represent any chemical substances:

• Look for expressions like “substance A reacts…” or “…A reacts with B…”. In this case, substances A and/or B represent the reactants or reactants.
• Sometimes the statement refers to a particular type of reaction, such as combustion. Phrases like “Compound A burns in the presence of oxygen…”, this implies that both A and molecular oxygen (O _2(g) ) are the reactants.
• In the case of oxidation-reduction reactions, or redox reactions, the oxidizing and reducing agents are always the reactants, while the oxidized and reduced species are the products. Thus, phrases like “Substance A oxidizes substance B…” or “Substance A reduces substance B…” imply that A and B are reacting by means of a redox reaction, so that both are the reactants of the reaction. reaction.
• In general, whenever it is mentioned that a substance undergoes a reaction, said substance corresponds to a reactant. For example, by saying that elemental chlorine dismutes to sodium chloride and sodium hypochlorite in the presence of sodium hydroxide, it is implied that the reactants are chlorine and sodium hydroxide.

Examples of Reactants in Chemical Reactions

irreversible reactions

reversible reactions

Examples of identification of items in statements

• Statement: Thermal decomposition of calcium carbonate produces calcium oxide and carbon dioxide gas. What mass of calcium oxide will be produced if 3 kg of calcium carbonate react? REAGENT: Calcium carbonate (CaCO ₃ )
• Statement: Sodium nitrate and sulfuric acid react to form nitric acid and sodium hydrogen sulfate. If we react 10 g of sodium nitrate with 9.8 g of sulfuric acid, what mass of nitric acid can we obtain? REAGENTS: Sodium nitrate (NaNO ₃ ) and sulfuric acid (H ₂ SO ₄ )
• Statement: Determine the mass of silver chloride formed from the reaction of 25 g of barium chloride with sufficient silver nitrate. REAGENTS: Barium chloride (BaCl ₂ ) and silver nitrate (AgNO ₃ )

References

Aguamarket. (n.d.). Chemical reagent . https://www.aguamarket.com/diccionario/terminos.asp?Id=1549%20&%20termino=Reactivo+qu%EDmico

Chang, R., & Goldsby, K. (2013). Chemistry (11th ed.). McGraw-Hill Interamericana de España SL

del Moral, M., & Rodriguez, J. (nd). Chemical reagents . Exampleof.com. https://www.ejemplode.com/38-quimica/4855-reactivos_quimicos.html

Chemistry.ES. (n.d.). reactive . https://www.quimica.es/enciclopedia/Reactivo.html

quimicas.net. (n.d.). Examples of Reagents . https://www.quimicas.net/2015/10/ejemplos-de-reactivos.html