How to Compare & Contrast the Properties of Acids & Bases

Strong acids like sulfuric acid are well-known for their corrosive properties.

Acids and bases are opposites; an acid will give a proton or hydrogen ion to water while a base will take it away. There are several useful ways to compare acids and bases and how they interact.

Types

The pH scale is one useful way to compare acids and bases. The pH is the negative log of the concentration or molarity of hydrogen ions in water. A pH below 7 is defined as acidic, while a pH above 7 is defined as alkaline or basic and 7 itself is neutral. It's important to remember that the pH is a log scale, so a change of 1 in the pH actually describes a tenfold increase or decrease in hydrogen ion concentration.

Function

In comparing acids to bases, it's important to think about the interaction between the water and the base or acid. When an acid reacts with water, it gives up a hydrogen ion, so the products of the reaction are H3O+ and a negatively charged ion. The negatively charged ion can now pick up hydrogen ions so it can act as a base and is thus the conjugate base of the acid. When a base reacts with water, on the other hand, it steals a hydrogen ion, so the products of the reaction are OH- and a positively charged ion. The positively charged ion can now donate a hydrogen ion back to water, so it can act as an acid and is thus the conjugate acid of the base.

Features

The stronger the acid or base, the weaker its conjugate. One way to quantify the strength of an acid or base is to use the dissociation constant, commonly written as Ka for an acid or Kb for a base. The dissociation constant is the ratio of the products of the reaction to the reactants. The bigger the ratio, the stronger the acid or base will be. Hydrochloric acid, for example, has a Ka of 10 to the 8th power, while acetic acid or vinegar has a Ka of 10 to the -4.76, so hydrochloric acid is clearly a much stronger acid than vinegar.

References

• "Chemical Principles, the Quest for Insight, 4th Edition": Peter Atkins and Loretta Jones: 2008.