List of Strong and Weak Acids

Strong and weak acids are important to know both for chemistry class and for use in the lab. There are very few strong acids, so one of the easiest ways to tell strong and weak acids apart is to memorize the short list of strong ones. Any other acid is considered a weak acid.

Strong Acids

Strong acids dissociate completely into their ions in water, yielding one or more protons (hydrogen cations) per molecule. There are only 7 common strong acids.

• HCl - hydrochloric acid
• HNO₃ - nitric acid
• H₂SO₄ - sulfuric acid (HSO₄^- is a weak acid)
• HBr - hydrobromic acid
• HI - hydroiodic acid
• HClO₄ - perchloric acid
• HClO₃ - chloric acid

Examples of ionization reactions include:

HCl → H⁺ + Cl^-

HNO₃ → H⁺ + NO₃^-

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List of the Strong Acids and Key Facts

By Anne Marie Helmenstine, Ph.D.

H₂SO₄ → 2H⁺ + SO₄^2-

Note the production of positively charged hydrogen ions and also the reaction arrow, which only points to the right. All of the reactant (acid) is ionized into product.

Weak Acids

Weak acids do not completely dissociate into their ions in water. For example, HF dissociates into the H⁺ and F^- ions in water, but some HF remains in solution, so it is not a strong acid. There are many more weak acids than strong acids. Most organic acids are weak acids. Here is a partial list, ordered from strongest to weakest.

• HO₂C₂O₂H - oxalic acid 
• H₂SO₃ - sulfurous acid
• HSO₄ ^- - hydrogen sulfate ion
• H₃PO₄ - phosphoric acid
• HNO₂ - nitrous acid
• HF - hydrofluoric acid
• HCO₂H - methanoic acid
• C₆H₅COOH - benzoic acid
• CH₃COOH - acetic acid
• HCOOH - formic acid

Weak acids incompletely ionize. An example reaction is the dissociation of ethanoic acid in water to produce hydroxonium cations and ethanoate anions:

CH₃COOH + H₂O ⇆ H₃O⁺ + CH₃COO^-

Note the reaction arrow in the chemical equation points both directions. Only about 1% of ethanoic acid converts to ions, while the remainder is ethanoic acid. The reaction proceeds in both directions. The back reaction is more favorable than the forward reaction, so ions readily change back to weak acid and water.

Distinguishing Between Strong and Weak Acids

You can use the acid equilibrium constant K_a or pK_a to determine whether an acid is strong or weak. Strong acids have high K_a or small pK_a values, weak acids have very small K_a values or large pK_a values.

Strong and Weak Vs. Concentrated and Dilute

Be careful not to confuse the terms strong and weak with concentrated and dilute. A concentrated acid is one that contains a low amount of water. In other words, the acid is concentrated. A dilute acid is an acidic solution that contains a lot of solvent. If you have 12 M acetic acid, it's concentrated, yet still a weak acid. No matter how much water you remove, that will be true. On the flip side, a 0.0005 M HCl solution is dilute, yet still strong.

Strong Vs. Corrosive

You can drink diluted acetic acid (the acid found in vinegar), yet drinking the same concentration of sulfuric acid would give you a chemical burn. The reason is that sulfuric acid is highly corrosive, while acetic acid is not as active. While acids tend to be corrosive, the strongest superacids (carboranes) are actually not corrosive and could be held in your hand. Hydrofluoric acid, while a weak acid, would pass through your hand and attack your bones.

Sources

• Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. ISBN 978-0-13-039913-7.
• Porterfield, William W. (1984). Inorganic Chemistry. Addison-Wesley. ISBN 0-201-05660-7.
• Trummal, Aleksander; Lipping, Lauri; et al. (2016). "Acidity of strong acids in water and dimethyl sulfoxide". J. Phys. Chem. A. 120 (20): 3663–3669. doi:10.1021/acs.jpca.6b02253