How to Find Acid & Base in an Equation

Chemistry instructors teach their students the adage "an acid and a base make a salt and water," which represents a simplification of the neutralization process. Identifying acids and bases from balanced equations requires an understanding of the definitions of acids and bases. Charged hydrogen atoms called protons, written H+, represent the species responsible for acidic behavior, whereas hydroxide ions, OH-, are responsible for basic or alkaline behavior. In simplest terms, any substance that increases the amount of protons in a solution represents an acid and any substance that increases the amount of hydroxide ions represents a base; this is the Arrhenius definition of acids and bases. However, these definitions cannot adequately describe all acids and bases. Thus, the Bronsted-Lowry definition describes an acid as any substance that donates a proton and a base as any substance that accepts a proton.

Instructions

1. Write out the balanced chemical equation for the acid-base reaction. A properly balanced equations contains the same number of each type of atom on each side of the reaction arrow.

2. Examine the products on the right side of the equation to identify the proton acceptor. Because a proton exhibits a positive charge, it will either turn a negatively charged ion, such as hydroxide, OH-, neutral, or turn a neutral molecule, such as water, H2O, positive. For example, in the reaction HCl + H2O ' Cl- + H3O+, H2O has accepted a proton from HCl to form H3O+. This means H2O acts as the base. In the example NH3 + H2O ' OH- + NH4+, the NH3 acts as a base because it accepts a proton from H2O.

3. Identify the acid in the reaction as the reactant that donated a proton. In the example of HCl + H2O ' Cl- + H3O+, HCl represents the acid because it donated a proton to H2O. In NH3 + H2O ' OH- + NH4+, H2O represents the acid because it donated a proton to NH3.

Tips & Warnings

• Chemists refer to compounds that can act either as a base or an acid, such as water in the previous examples, as "amphoteric" substances.

References

• Mountain Empire Community College, Water/Wastewater Distance Learning; Lesson 7; Acids and Bases
• Oklahoma State University; Department of Chemistry; The Arrhenius Definition
• New York University; Department of Chemistry; Bronsted-Lowry Acids and Bases

Resources

• Widener University: Solving an Acid Base Problem
• University of Iowa; Department of Chemistry; Acid-Base Chemistry