Identifying Acid-Base Reactions (College Board AP® Chemistry): Revision Note

• There are acids that have the capability of donating more than one proton

• Acids that are able to transfer two protons are diprotic acids

H₂SO₄ (aq) + H₂O (l) → H₃O⁺ (aq) + HSO₄^– (aq)

HSO₄^– (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + SO₄^2– (aq)

• Only the first proton transfer goes to completion

• Acids that can transfer three protons, one at a time, are triprotic acids, e.g. phosphoric acid. H₃PO₄ is a weak acid, so each proton transfer is an equilibrium and most molecules stay un-ionized:

H₃PO₄ (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + H₂PO₄^– (aq)

Neutralization

• One of the most common acid-base reactions is neutralization

• Neutralization occurs when an acid reacts with a base. When the base is a metal hydroxide, the products are a salt and water

  • The salt produced in the neutralization is formed from the cation of the base and the anion from the acid

  • Therefore the name of the salt can be predicted from the acid and base that have reacted

  • The cation and the anion that make up the salt are spectator ions: they stay dissolved in the solution and are left out of the net ionic equation

Acid reacted and salt produced

Acid-base neutralization

• Main neutralization reaction

  • Acid: HCl

  • Base: NaOH

  • Water: H₂O

  • Salt: NaCl

HCl (aq) + NaOH (aq) → H₂O (l) + NaCl (aq)

• Net ionic equation

H⁺ (aq) + OH^– (aq) → H₂O (l)

• Na⁺ (aq) and Cl^– (aq) are spectator ions: they stay in solution

Neutralization with gas formation

• The reaction of an acid and a metal carbonate or a metal hydrogen carbonate is also a neutralization reaction

• The products in both cases are a salt, water and carbon dioxide

  • Carbon dioxide is released as a gas

• Examples for both cases are shown below:

acid + metal carbonate → salt + water + carbon dioxide

2HNO₃ (aq) + CuCO₃ (s) → Cu(NO₃)₂ (aq) + H₂O (l) + CO₂ (g)

acid + metal hydrogen carbonate → salt + water + carbon dioxide

HCl (aq) + NaHCO₃ (s) → NaCl (aq) + H₂O (l) + CO₂ (g)