Acid-Base Neutralization and Proton Transfer

When an acid gives up its proton, what remains is its conjugate base. When a base accepts a proton, the result is its conjugate acid. Members of a pair always differ by exactly one H and one unit of charge. HCl and Cl⁻ are a pair; H₂O and OH⁻ are a pair; H₂O and H₃O⁺ are also a pair, which is why water appears on both sides of so many equations.

In CH₃COOH + NH₃ ⇌ CH₃COO⁻ + NH₄⁺, trace the hydrogen rather than pairing reactants by position. Acetic acid and acetate are one pair; ammonia and ammonium are the other. Removing H⁺ lowers a species' charge by one; adding H⁺ raises it by one. Thus removing H⁺ from neutral CH₃COOH gives −1 acetate, while adding H⁺ to neutral NH₃ gives +1 ammonium. The total charge stays zero. An oxygen atom or an electron is not transferred in this bookkeeping.

Strong acids such as HCl, HBr, HI, HNO₃, HClO₄ and commonly HClO₃ transfer their protons essentially completely to water under the conditions of introductory problems. Sulfuric acid requires a qualification: its first ionization is strong, but HSO₄⁻ loses its second proton through a separate, incomplete equilibrium. Complete neutralization by sufficient strong base can nevertheless consume two OH⁻ per H₂SO₄.

Group 1 hydroxides and dissolved Ca(OH)₂, Sr(OH)₂ and Ba(OH)₂ supply hydroxide essentially completely. Solubility and base strength are different: limited dissolution of a solid does not make its dissolved portion a weak base. A weak acid or base has an incomplete reaction with water; its fraction ionized depends on concentration. A dilute strong acid can have a higher pH than a concentrated weak acid. “Strong,” “concentrated” and “corrosive” are different descriptions.

Acid reacting with a hydroxide base commonly gives water and a salt. For 2 HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2 H₂O, two hydroxides require two donated protons. The salt formula follows Ca²⁺ and NO₃⁻ charges, then coefficients balance the reaction. In contrast, HCl + NH₃ → NH₄Cl transfers a proton without producing water in that molecular equation. “Salt plus water” is therefore a useful hydroxide pattern, not the definition of every acid-base reaction.

Neutralization describes the reaction, not a guaranteed final pH. Excess acid leaves an acidic mixture; excess base leaves a basic one. Even at stoichiometric equivalence, an acetate salt is basic and an ammonium salt acidic because their ions react with water. Strong acid–strong base equivalence is neutral in the ideal dilute model; neutral pH is 7.00 specifically at 25 °C.

Some acid reactions produce a gas rather than only water. Carbonates and bicarbonates react with acids to give carbon dioxide, water and a salt, and the fizzing is the visible clue. The mechanism is still proton transfer, the unstable carbonic acid formed simply falls apart.

For carbonate, two proton transfers give CO₃²⁻ + 2 H⁺ → CO₂(g) + H₂O(l). Bicarbonate needs only one: HCO₃⁻ + H⁺ → CO₂ + H₂O. Balance atoms and charge to distinguish these ratios. Gas escape helps drive the observed process forward; fizzing alone does not reveal how many moles reacted without a balanced equation.