An acid-base reaction will not take place as written when the reactant side already contains the weaker acid and weaker base. In other words, if the reaction as written would produce a stronger acid and stronger base than what you started with, equilibrium favors the reactants and the reaction essentially runs in reverse. The quick test: compare the pKa of the acid on the reactant side to the pKa of the acid on the product side. If the reactant acid has a higher pKa (meaning it’s weaker), the reaction does not proceed as written.
The Core Rule: Reactions Favor Weaker Products
Every acid-base reaction is really a competition between two acids for a proton. The stronger acid wins that competition by donating its proton more readily. This means equilibrium always shifts toward producing the weaker acid and the weaker base. If a reaction is written so that the products include a stronger acid than what’s on the reactant side, you’re looking at a reaction that won’t happen in that direction.
You can quantify this with one formula: Keq equals 10 raised to the power of ΔpKa, where ΔpKa is the pKa of the product acid minus the pKa of the reactant acid. When that difference is positive (product acid has a higher pKa), Keq is large and the reaction proceeds. When it’s negative (product acid has a lower pKa), Keq is tiny and the reaction essentially doesn’t go forward.
How to Compare Using pKa Values
The pKa scale tells you how readily an acid gives up its proton. Lower pKa means stronger acid. Here are some common reference points:
- HCl: pKa of about −7 (very strong acid)
- Acetic acid (CH₃COOH): pKa of 4.75
- Ammonium ion (NH₄⁺): pKa of 9.25
- Water (H₂O): pKa of 14
- Alcohols like ethanol: pKa of roughly 16 to 18
When you see a reaction written on paper, identify the acid on the left (the proton donor in the reactants) and the acid on the right (the species on the product side that could donate a proton back). If the left-side acid has a higher pKa than the right-side acid, the equilibrium lies to the left and the reaction won’t proceed as written.
A Classic Example That Doesn’t Work
Consider this reaction: an alcohol (like ethanol) acting as an acid to donate a proton to the conjugate base of a carboxylic acid (an acetate ion). Written out, it would look like ethanol reacting with acetate to form ethoxide and acetic acid. Ethanol has a pKa around 16, and acetic acid has a pKa around 4.75. The product acid (acetic acid, pKa 4.75) is far stronger than the reactant acid (ethanol, pKa 16). That means equilibrium massively favors the reactants. This reaction would not take place as written.
The ΔpKa here is 4.75 minus 16, which gives roughly −11. Plugging that in, Keq equals 10⁻¹¹, an incredibly small number. Virtually none of the products form.
Another Example: Ammonia in Water
When ammonia dissolves in water, you might write: NH₃ + H₂O → NH₄⁺ + OH⁻. Here, water (pKa 14) is acting as the acid, and ammonium (pKa 9.25) is the product acid. Since the product acid is stronger than the reactant acid, this reaction barely proceeds. The equilibrium constant is 1.8 × 10⁻⁵, meaning at equilibrium, the vast majority of species in solution are still unreacted ammonia and water. The reaction technically occurs to a tiny extent, but it overwhelmingly favors the reactant side.
Contrast this with HCl reacting with ammonia: HCl (pKa −7) donates a proton to NH₃, forming NH₄⁺ (pKa 9.25) and Cl⁻. The product acid has a much higher pKa than the reactant acid, giving a ΔpKa of about +16. Keq is astronomically large, and this reaction goes essentially to completion.
When pKa Values Aren’t Available
On exams, you sometimes need to predict relative acidity without a pKa table. The ARIO framework helps you rank acid strength using four factors, in order of importance:
- Atom: What atom holds the acidic hydrogen? Across a row of the periodic table, more electronegative atoms stabilize a negative charge better, making the acid stronger. Going down a column, larger atoms are more polarizable and also stabilize the charge, so HI is a stronger acid than HF despite fluorine being more electronegative.
- Resonance: Can the conjugate base spread its negative charge across multiple atoms through resonance? Carboxylic acids (pKa ~5) are far more acidic than alcohols (pKa ~16) largely because the carboxylate ion delocalizes charge over two oxygen atoms.
- Induction: Are there electronegative atoms nearby pulling electron density away from the acidic site? Trifluoroacetic acid is much stronger than acetic acid because the fluorine atoms withdraw electron density.
- Orbital: What type of orbital holds the electrons in the conjugate base? An sp orbital holds electrons closer to the nucleus than sp² or sp³, stabilizing the negative charge. This is why terminal alkynes (sp carbon) are more acidic than typical hydrocarbons.
If you determine that the reactant acid is weaker than the product acid using these criteria, the reaction won’t proceed as written.
The Solvent Leveling Effect
Sometimes a reaction that looks fine on paper can’t actually happen in a given solvent. In water, the strongest acid that can exist is the hydronium ion (H₃O⁺, pKa 0). Any acid stronger than that, like HCl (pKa −7) or nitric acid (pKa −1.3), immediately donates its proton to water and is “leveled” to H₃O⁺. This means you can’t distinguish the strength of different strong acids in water. HCl and nitric acid both dissociate 100% in dilute solution.
The practical consequence: if you write a reaction that depends on one strong acid being stronger than another strong acid, it won’t take place as written in water because both acids are leveled to the same effective strength. To observe differences between very strong acids, you’d need a less basic solvent like acetic acid, where the leveling ceiling is higher. Each solvent creates an acid-base “window,” and reactions outside that window can’t proceed as written in that solvent.
Quick Checklist for Exam Problems
When you encounter a “which reaction would not take place as written” question, work through these steps. First, identify the proton donor (acid) on the reactant side and the proton donor (conjugate acid) on the product side. Second, compare their pKa values. If the reactant acid has a higher pKa than the product acid, the reaction does not proceed as written. Third, if no pKa values are given, use the ARIO criteria to determine which acid is stronger. The reaction that tries to go from a weaker acid to a stronger acid is your answer.
A reliable shortcut: reactions between a carboxylic acid and an amine go forward easily (pKa gap of roughly 5 to 10 units). Reactions asking an alcohol to donate a proton to form a carboxylic acid almost never proceed (the alcohol is too weak an acid by about 11 pKa units). Any reaction that asks water to act as a strong enough acid to protonate something with a pKa well below 14 will also fail.