Acid-Base Reactions — Proton Transfer Chemistry
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Acid-base reactions involve the transfer of protons (H+ ions) from an acid to a base. In the Bronsted-Lowry definition, an acid donates a proton while a base accepts one. The Arrhenius model defines acids as substances that increase H+ concentration in water, and bases as those that increase OH-. Neutralization — the reaction of an acid with a base to form salt and water — is the most recognizable acid-base reaction and is fundamental to analytical chemistry, biology, and industry.
Reaction Mechanism
In aqueous neutralization, H3O+ ions from the acid combine with OH- ions from the base to form water: H3O+ + OH- -> 2H2O. This is highly exothermic (57.1 kJ/mol for strong acid-strong base). Buffer systems resist pH changes through equilibrium between a weak acid and its conjugate base. The Lewis model broadens the concept further — any electron pair donor (base) reacting with an electron pair acceptor (acid), which explains reactions involving no protons at all, like BF3 + NH3 -> F3B-NH3.
Everyday Examples
Antacids neutralize excess stomach acid (HCl, pH ~1.5) with bases like magnesium hydroxide or calcium carbonate. Vinegar (acetic acid) and baking soda (sodium bicarbonate) react to produce carbon dioxide gas — a classic kitchen chemistry demonstration. Soil pH adjustment uses lime (CaO, a base) to neutralize acidic soils for agriculture.
Endüstriyel Önemi
Acid-base chemistry underpins the production of fertilizers — sulfuric acid reacting with phosphate rock produces phosphoric acid for superphosphate fertilizers. Global sulfuric acid production exceeds 260 million tonnes annually, making it the most-produced industrial chemical. Titration, an acid-base analytical technique, is used in quality control across pharmaceutical, food, and environmental laboratories worldwide.
Safety Note
Concentrated acids and bases cause severe chemical burns. Neutralization reactions can be vigorously exothermic — always add acid to water, never water to acid, to prevent boiling and spattering. Wear goggles, gloves, and a lab coat when handling corrosive substances.
Formic Acid and Sodium Hydroxide
HCOOH + NaOH → HCOONa + H₂O
Formic acid, the simplest carboxylic acid, reacts with sodium hydroxide to form sodium formate and water. Formic acid (Ka = …
Hydrochloric Acid and Barium Hydroxide
2HCl + Ba(OH)₂ → BaCl₂ + 2H₂O
Hydrochloric acid reacts with barium hydroxide to form barium chloride and water. Barium hydroxide is one of the few strongly …
Acetic Acid and Sodium Acetate Buffer System
CH₃COOH ⇌ CH₃COO⁻ + H⁺
The acetic acid/sodium acetate buffer system resists pH changes by shifting equilibrium in response to added acid or base. When …
Acetic Acid Dissociation in Water
CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺
Acetic acid partially dissociates in water with Ka = 1.8 × 10⁻⁵, meaning only about 1.3% of molecules ionize in …
Ammonia and Hydrochloric Acid
NH₃ + HCl → NH₄Cl
Ammonia gas reacts with hydrogen chloride gas to form white fumes of ammonium chloride. In aqueous solution, ammonia acts as …
Ascorbic Acid and Sodium Hydroxide
C₆H₈O₆ + NaOH → NaC₆H₇O₆ + H₂O
Ascorbic acid (vitamin C) reacts with sodium hydroxide to form sodium ascorbate and water. Despite its name, ascorbic acid is …
Barium Hydroxide and Ammonium Thiocyanate (Endothermic)
Ba(OH)₂·8H₂O + 2NH₄SCN → Ba(SCN)₂ + 2NH₃ + 10H₂O
This highly endothermic acid-base reaction absorbs so much heat that it can freeze water beneath the flask. Mixing barium hydroxide …
Carbonic Acid and Sodium Hydroxide
H₂CO₃ + 2NaOH → Na₂CO₃ + 2H₂O
Carbonic acid, formed when CO₂ dissolves in water, reacts with two equivalents of sodium hydroxide to form sodium carbonate. This …
Aluminum Hydroxide as Amphoteric Acid with NaOH
Al(OH)₃ + NaOH → NaAlO₂ + 2H₂O
Aluminum hydroxide dissolves in sodium hydroxide solution, demonstrating its acidic character. The Al(OH)₃ acts as a Lewis acid, accepting OH⁻ …
Aluminum Chloride and Chloride Ion (Lewis Acid-Base)
AlCl₃ + Cl⁻ → AlCl₄⁻
Aluminum chloride acts as a Lewis acid, accepting a lone pair from chloride ion to form the tetrachloroaluminate anion. AlCl₃ …
Acetic Acid and Sodium Hydroxide Neutralization
CH₃COOH + NaOH → CH₃COONa + H₂O
Acetic acid (vinegar) reacts with sodium hydroxide to form sodium acetate and water. Because acetic acid is a weak acid …
Hydrochloric Acid and Sodium Hydroxide Neutralization
HCl + NaOH → NaCl + H₂O
The quintessential strong acid-strong base neutralization reaction producing sodium chloride and water. The net ionic equation is simply H⁺ + …
Nitric Acid and Potassium Hydroxide Neutralization
HNO₃ + KOH → KNO₃ + H₂O
Nitric acid reacts with potassium hydroxide to produce potassium nitrate and water. This is a strong acid-strong base neutralization yielding …
Sulfuric Acid and Sodium Hydroxide Neutralization
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
Sulfuric acid, a diprotic acid, requires two moles of sodium hydroxide for complete neutralization. The reaction produces sodium sulfate (Glauber's …
Phosphoric Acid and Sodium Hydroxide (First Neutralization)
H₃PO₄ + NaOH → NaH₂PO₄ + H₂O
The first neutralization step of the triprotic phosphoric acid produces monosodium phosphate and water. Phosphoric acid has three ionizable protons …
Magnesium Hydroxide and Hydrochloric Acid (Milk of Magnesia)
Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
Magnesium hydroxide (milk of magnesia) neutralizes hydrochloric acid to form magnesium chloride and water. As a sparingly soluble base, magnesium …
Phosphoric Acid Complete Neutralization
H₃PO₄ + 3NaOH → Na₃PO₄ + 3H₂O
Complete neutralization of triprotic phosphoric acid requires three equivalents of sodium hydroxide, producing trisodium phosphate (TSP). The resulting solution is …
Aluminum Hydroxide as Amphoteric Base with HCl
Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O
Aluminum hydroxide dissolves in hydrochloric acid, demonstrating its basic character. As an amphoteric substance, Al(OH)₃ can act as either an …
Carbonic Acid Dissociation (Blood Buffer)
H₂CO₃ ⇌ HCO₃⁻ + H⁺
The carbonic acid/bicarbonate buffer system is the primary blood buffer maintaining pH between 7.35 and 7.45. Carbon dioxide dissolves in …
Hydrofluoric Acid and Sodium Hydroxide
HF + NaOH → NaF + H₂O
Hydrofluoric acid, despite being a weak acid (Ka = 6.8 × 10⁻⁴), reacts completely with sodium hydroxide to form sodium …
Boron Trifluoride and Ammonia (Lewis Acid-Base)
BF₃ + NH₃ → BF₃·NH₃
Boron trifluoride, a classic Lewis acid with an empty p-orbital on boron, accepts an electron pair from ammonia's lone pair …
Zinc Hydroxide as Amphoteric Base with HCl
Zn(OH)₂ + 2HCl → ZnCl₂ + 2H₂O
Zinc hydroxide, like aluminum hydroxide, is amphoteric and dissolves in hydrochloric acid to form zinc chloride and water. The white …
Zinc Hydroxide as Amphoteric Acid with NaOH
Zn(OH)₂ + 2NaOH → Na₂[Zn(OH)₄]
Zinc hydroxide dissolves in excess sodium hydroxide to form the soluble tetrahydroxozincate complex. This demonstrates zinc hydroxide's acidic character. The …
Benzoic Acid and Sodium Hydroxide
C₆H₅COOH + NaOH → C₆H₅COONa + H₂O
Benzoic acid, the simplest aromatic carboxylic acid, reacts with sodium hydroxide to form water-soluble sodium benzoate. Benzoic acid itself is …
Sulfurous Acid and Sodium Hydroxide
H₂SO₃ + 2NaOH → Na₂SO₃ + 2H₂O
Sulfurous acid reacts with two equivalents of sodium hydroxide to form sodium sulfite and water. Sulfurous acid is formed when …
Oxalic Acid and Sodium Hydroxide
H₂C₂O₄ + 2NaOH → Na₂C₂O₄ + 2H₂O
Oxalic acid is a diprotic organic acid that reacts with two equivalents of sodium hydroxide to form sodium oxalate and …
Water Autoionization
H₂O + H₂O ⇌ H₃O⁺ + OH⁻
Water undergoes autoionization where one molecule acts as an acid (donates H⁺) and another acts as a base (accepts H⁺). …
Perchloric Acid and Sodium Hydroxide
HClO₄ + NaOH → NaClO₄ + H₂O
Perchloric acid, the strongest common mineral acid, reacts with sodium hydroxide to form sodium perchlorate and water. Perchloric acid is …
Nitric Acid and Ammonia (Ammonium Nitrate)
HNO₃ + NH₃ → NH₄NO₃
Nitric acid reacts with ammonia to form ammonium nitrate, the world's most widely used nitrogen fertilizer. This exothermic reaction must …
Hydrochloric Acid and Calcium Hydroxide
2HCl + Ca(OH)₂ → CaCl₂ + 2H₂O
Hydrochloric acid reacts with calcium hydroxide (slaked lime) to form calcium chloride and water. Calcium hydroxide is a dibasic compound …
Hydrochloric Acid and Sodium Carbonate
2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂
Hydrochloric acid reacts with sodium carbonate (washing soda) to produce sodium chloride, water, and carbon dioxide gas. The carbonate ion …
Lactic Acid and Sodium Hydroxide
CH₃CHOHCOOH + NaOH → CH₃CHOHCOONa + H₂O
Lactic acid, produced during anaerobic metabolism in muscles, reacts with sodium hydroxide to form sodium lactate and water. Lactic acid …
Phosphoric Acid Second Neutralization
H₃PO₄ + 2NaOH → Na₂HPO₄ + 2H₂O
Partial neutralization of phosphoric acid with two equivalents of NaOH produces disodium hydrogen phosphate. This salt forms solutions near pH …
Hydrochloric Acid and Sodium Acetate
HCl + CH₃COONa → CH₃COOH + NaCl
A strong acid reacts with the salt of a weak acid to liberate the weak acid. Hydrochloric acid protonates acetate …
Phosphate Buffer System
H₂PO₄⁻ ⇌ HPO₄²⁻ + H⁺
The dihydrogen phosphate/hydrogen phosphate buffer system operates near pH 7.2, making it ideal for biological applications. With pKa2 = 7.2, …
Ammonia and Ammonium Chloride Buffer System
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
The ammonia/ammonium buffer system maintains pH near 9.25 (the pKb of ammonia corresponds to pKa of NH₄⁺ = 9.25). Ammonia …
Calcium Carbonate and Hydrochloric Acid (Antacid)
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Calcium carbonate, found in Tums antacid tablets, neutralizes hydrochloric acid in the stomach to form calcium chloride, water, and carbon …
Sodium Bicarbonate and Hydrochloric Acid
NaHCO₃ + HCl → NaCl + H₂O + CO₂
Sodium bicarbonate (baking soda) reacts with hydrochloric acid to produce sodium chloride, water, and carbon dioxide gas. The bicarbonate ion …
Citric Acid and Sodium Hydroxide
C₆H₈O₇ + 3NaOH → Na₃C₆H₅O₇ + 3H₂O
Citric acid is a triprotic acid that reacts with three equivalents of NaOH to form trisodium citrate and water. The …
Sulfuric Acid and Potassium Hydroxide
H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
Sulfuric acid reacts with potassium hydroxide to form potassium sulfate and water. This diprotic neutralization requires two equivalents of base. …
Hydrochloric Acid and Lithium Hydroxide
HCl + LiOH → LiCl + H₂O
Hydrochloric acid reacts with lithium hydroxide to form lithium chloride and water. Lithium hydroxide is used as a CO₂ absorbent …
Tartaric Acid and Sodium Hydroxide
C₄H₆O₆ + 2NaOH → Na₂C₄H₄O₆ + 2H₂O
Tartaric acid is a chiral diprotic acid naturally found in grapes that reacts with two equivalents of NaOH to form …
Sulfuric Acid and Ammonia (Ammonium Sulfate)
H₂SO₄ + 2NH₃ → (NH₄)₂SO₄
Sulfuric acid reacts with ammonia to form ammonium sulfate, a major nitrogen-sulfur fertilizer containing 21% nitrogen and 24% sulfur. The …
Sulfuric Acid and Calcium Carbonate
H₂SO₄ + CaCO₃ → CaSO₄ + H₂O + CO₂
Sulfuric acid reacts with calcium carbonate to form calcium sulfate, water, and carbon dioxide. Unlike HCl, this reaction can be …