The key difference between weak and strong acid is that weak acids ionize partially in water whereas strong acids ionize completely.
The strength of an acid is its ability to ionize or donate the hydrogen ion in an aqueous solution reacting with water. The more an acid ionizes, the stronger it is, and the less production of hydrogen ions indicates a weak acid. This is the difference between a strong and a weak acid. The concentration of ions in an aqueous solution of an acid tells how strong or weak an acid it is. So, you can have a concentrated solution of a weak acid, and it is perfectly possible to have a dilute solution of a strong acid.
CONTENTS
1. Overview and Key Difference
2. What is Weak Acid
3. What is Strong Acid
4. Side by Side Comparison – Weak vs Strong Acid in Tabular Form
5. Summary
What is Weak Acid?
Weak acid refers to the chemical compounds that partially dissociates into ions in an aqueous solution. They do not release the total amount of releasable hydrogen ions (H+) into the solution. For these acids, the acid dissociation constant or Ka is a small value. The pH of these solutions lies around 3 to 5. Mainly, this is because these weak acids cannot raise the hydrogen ion content in the solution as much as a strong acid does.
Figure 01: Weak Acid speciation Diagram (for the weak acid HA)
Since the weak acid dissociates partially, the aqueous solution contains three major components; hydrogen ions, unionized acid molecules and the conjugated base (anion bound to the hydrogen ion in the acid molecule). Some common examples include sulfurous acid, phosphoric acid, nitrous acid, etc.
What is Strong Acid?
Strong acid refers to the molecules that completely dissociates into ions in an aqueous solution. Therefore, these compounds completely ionize in water. The value of acid dissociation constant or Ka is a high value for this type of acids. Stronger the acid, easier the release of hydrogen ions. Mainly, this happens due to the high polarity of the H-A bond in which H is a hydrogen atom, and A is the conjugate base. In order to be highly polar, the electronegativity difference between the atoms in each side of this bond should be high.
Figure 02: Chemical Structure of the Strong Acid HNO3 (Nitric Acid)
Moreover, this hydrogen ion release also depends on the size of the anion (conjugated base) to which the hydrogen ion is attached. If the anion is large and more stable, then it can easily release the hydrogen ion. Unlike weak acids, these strong acids release all possible hydrogen ions to the aqueous solution. The pH value of these aqueous solutions is very small; can range from 1 to 3.
What is the Difference Between Weak and Strong Acid?
Weak acids ionize partially in water whereas strong acids ionize completely. Therefore, ionization is the key difference between weak and strong acid. Moreover, one other difference between weak and strong acid is that weak acid does not remove all the releasable hydrogen atoms. On the contrary, strong acid releases all possible hydrogen atoms.
Besides, there exists a difference between weak and strong acid based on their pH value as well. That is; the pH value of weak acids ranges from 3 to 5 while that of strong acids ranges from 1 to 3. Also, acid dissociation constant contributes to another difference between weak and strong acid. Acid dissociation constant of weak acid is small compared to strong acid.
Summary – Weak vs Strong Acid
Acids are molecules that can release hydrogen ions to an aqueous solution. We can classify all the acids as strong acids, moderately strong acids and weak acids. In brief, the key difference between weak and strong acid is that weak acids ionize partially in water whereas strong acids ionize completely.
Reference:
1. Helmenstine, Anne Marie, Ph.D. “List of Strong and Weak Acids.” ThoughtCo, Sep. 24, 2018. Available here
2. Helmenstine, Anne Marie, Ph.D. “Weak Acid Definition and Examples in Chemistry.” ThoughtCo, Jun. 18, 2018. Available here
Image Courtesy:
1.”Weak acid speciation”By P. Gans and transferred by LeaW., (Public Domain) via Commons Wikimedia
2.”CNX Chem 18 09 HNO3″By OpenStax (CC BY 4.0) via Commons Wikimedia