Which one of the following is a weak acid hno3 hcl, hi hf hclo4

Important to Know, Both for Chemistry Class and for Use in the Lab

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.

Key Takeaways

• Strong acids completely dissociate into their ions in water, while weak acids only partially dissociate.
• There are only a few (7) strong acids, so many people choose to memorize them. All the other acids are weak.
• The strong acids are hydrochloric acid, nitric acid, sulfuric acid, hydrobromic acid, hydroiodic acid, perchloric acid, and chloric acid.
• The only weak acid formed by the reaction between hydrogen and a halogen is hydrofluoric acid (HF). While technically a weak acid, hydrofluoric acid is extremely powerful and highly corrosive.

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-

H2SO4 → 2H+ + SO42-

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 Ka or pKa to determine whether an acid is strong or weak. Strong acids have high Ka or small pKa values, weak acids have very small Ka values or large pKa 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

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