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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Helmenstine, Anne Marie, Ph.D. "List of Strong and Weak Acids." ThoughtCo. //www.thoughtco.com/list-of-strong-and-weak-acids-603642 (accessed December 17, 2022).
Have you ever looked at an exam question that is asking you about acids, and it doesn’t use the word “weak” or the word “strong”? Maybe it was a question in an assignment, a mock exam or in your own exam practice.
And because it doesn’t tell you, you are left feeling unsure how to go about answering the question.
It is typical that the question won’t be specific about whether the acid is weak or strong. You are probably getting used to A-Level questions being less directive than the questions you have faced at GCSE.
So how should you decide whether your question is about a weak acid or a strong acid?
No need to despair as there will be plenty of clues within the question. We’ll consider what those clues are:
Clue #1: There are Few Strong Acids
The truth is that there are only a small number of strong acids, and you will already be familiar with most of these.
Specifically, these are the mineral acids you will know well:
- sulfuric acid
- nitric acid
- hydrochloric acid
- hydrobromic acid
- hydroiodic acid
There are a few more but the list is very small. Almost all acids are weak acids.
So, if the acid in your question is one of these you can go ahead and calculate pH directly from the acid concentration.
Clue #2: The Acid is an Organic Acid
All organic acids are weak acids. If the acid in your question is a weak acid then you need to make any calculations based upon the acid dissociation constant.
But how do you spot an organic acid?
The first way is from the formula. The majority of organic acids include the -COOH group, but any organic compound that has an acidic proton is a weak acid. In other words, any compound described as an acid that includes at least one C-C or C-H bond is a weak acid.
The second way is the name. The -COOH group, or carboxylic acid group, identifies many organic weak acids. The compounds with this group have names ending with “-oic acid”; for example ethanoic acid or benzoic acid. If your compound has this name ending it will be a weak acid. (Beware though: if your compound doesn’t have this type of name it may still be a weak acid).
Clue #3: The Question Indicates Incomplete Dissociation
If the question implies that there is a reversible reaction or an equilibrium related to the dissociation of the acid, this indicates that dissociation is incomplete. In thus case the acid in your question is a weak acid.
Clue #4: The Question Refers to a Ka Value
Mention of a Ka (or pKa) value in the question makes it clear that the question is about a weak acid. (Strong acids don’t have acid dissociation constants).
Clue #5: The Compound is Used in an Acidic Buffer
If the question goes on to imply the use of the acid within an acidic buffer, the acid is definitely a weak acid. The function of an acidic buffer relies on the reversible dissociation of an acid – hence the acid must be a weak acid.
Weak Acid vs Strong Acid
Why does it matter if the acid is weak or strong?
The way we make calculations regarding pH and concentration will depend on the type of acid.
Because a strong acid dissociates quickly it is straightforward to calculate its pH if you know its concentration, and vice versa.