4. Some important reactions of Acids

Acids are neutralised by reaction with metals, oxides, hydroxides or carbonates to form salts and other products.

Apart from metals (which is an electron loss/gain redox reaction), the other reactants listed above are considered as bases (meaning they react by accepting a proton from an acid). Water soluble bases are known as alkalis.

The reaction between acids and bases like oxides, hydroxides and carbonates are called neutralisation reactions.

• (A) metal + acid ==> a salt* + hydrogen

• (A1) e.g. zinc + hydrochloric acid ==> zinc chloride + hydrogen

  • Zn_(s) + 2HCl_(aq) ==> ZnCl_2(aq) + H_2(g)

  • Its the same equation for many other Group 2 and Transition metals e.g. Mg, Ca and Fe, Co, Ni

    • Test for hydrogen gas - squeaky pop with lit splint (can often see condensed water on side of test tube)

    • 2H_2(g) + O_2(g) ==> 2H₂O_(l) + energy!

• (A2) magnesium + sulphuric acid ==> magnesium sulphate + hydrogen

  • Mg_(s) + H₂SO_4(aq) ==> MgSO_4(aq) + H_2(g)

  • Note 1: sulphuric acid gives sulphate salt and hydrogen, 

    • see also extra example 3. Zn + H₂SO₄

  • Note 2: Nitric acid (HNO₃) doesn't usually form hydrogen with a metal, instead you get nasty brown fumes of nitrogen dioxide! but you still get the metal nitrate salt.

• (B) alkali (soluble base ) + acid ==> salt + water (the 'classic' neutralisation reaction)

• (B1) e.g.  sodium hydroxide + hydrochloric acid ==> sodium chloride + water (salt preparation method a)

  • NaOH_(aq) + HCl_(aq) ==> NaCl_(aq) + H₂O_(l)

• (B2) metal hydroxide + acid ==> a salt* + water

  • e.g.  sodium hydroxide + sulphuric acid ==> sodium sulphate + water

  • 2NaOH_(aq) + H₂SO_4(aq) ==> Na₂SO_4(aq) + 2H₂O_(l)

  • Its the same equation for any Group 1 Alkali Metal hydroxide e.g. LiOH, KOH etc.

  • or potassium hydroxide + nitric acid ==> potassium nitrate + water

  • NaOH_(aq) + HNO_3(aq) ==> NaNO_3(aq) + 2H₂O_(l)

• (B3) insoluble base + acid ==> salt + water

  • (note: oxides that react with acids to form salts are known as 'basic oxides')

  • e.g. metal oxide + acid ==> salt* + water

  • e.g. copper(II) oxide + sulphuric acid ==> copper(II) sulphate + water

  • CuO_(s) + H₂SO_4(aq) ==> CuSO_4(aq) + H₂O_(l)

• (B4) calcium hydroxide + hydrochloric acid ==> calcium chloride + water

  • Ca(OH)_2(s) + 2HCl_(aq) ==> CaCl_2(aq) + 2H₂O_(l)

• See also extra examples

  • 2. Mg(OH)₂ + HCl, 5. ZnO/HCl and 8. KOH + HBr

• (C) metal carbonate or hydrogencarbonate + acid ==> a salt* + water + carbon dioxide

• (C1) e.g.  calcium carbonate + nitric acid ==> calcium nitrate + water + carbon dioxide

• CaCO_3(s) + 2HNO_3(aq) ==> Ca(NO₃)_2(aq) + H₂O_{(l) +} CO_{2 (g)}

• Its the same equation for many other Group 2 and Transition metals e.g. Mg, Sr and Co, Ni, Cu

  • Test for carbon dioxide gas - it gives a white precipitate of calcium carbonate (cloudiness) when bubbled into limewater (calcium hydroxide solution).

  • Ca(OH)_2(aq) + CO_2(g) ==> CaCO_3(s) + H₂O_(l)

• Note: Using sulphuric acid and calcium carbonate you don't get much of a fizz! because the calcium sulphate salt formed, is not very soluble, and coats the remaining calcium carbonate inhibiting the reaction! This will happen with any reaction between an acid and a water insoluble reactant which forms an insoluble solid product!

• (C2) magnesium carbonate + sulphuric acid ==> magnesium sulphate + water + carbon dioxide

  • MgCO_3(s) + H₂SO_4(aq) ==> MgSO_4(aq) + H₂O_{(l) +} CO_{2 (g)}

  • or sodium hydrogencarbonate + nitric acid ==> sodium nitrate + water + carbon dioxide

  • NaHCO_3(s) + HNO_3(aq) ==> NaNO_3(aq) + H₂O_(l) + CO_2(g)

• See also extra examples on an extra page.

  • 1. CuCO₃/H₂SO₄, 6. HCl/CaCO₃, 7. Na₂CO₃/HCl and 9. NaHCO₃/HCl

• (D) ammonia + acid ==> ammonium salt  **

  • Note that no water is formed and see also note (c) below.

  • e.g. ammonia + hydrochloric acid ==> ammonium chloride

  • NH_3(aq) + HCl_(aq) ==> NH₄Cl_(aq)

  • or ammonia + sulphuric acid ==> ammonium sulphate

  • 2NH_3(aq) + H₂SO_4(aq) ==> (NH₄)₂SO_4(aq)

• See also extra example 4. NH₃/HNO₃

NOTE (a)*: The name of the particular salt formed depends on (i) the metal name, which becomes the first part of salt name, and (ii) the acid e.g. H₂SO₄ sulphuric acid on neutralisation makes a ... sulphate; HCl hydrochloric acid makes a ... chloride;  HNO₃ nitric acid makes a nitrate etc.

NOTE (b)**: There is a list of compound formulae and their solubility in section 8. The first part of the salt name is ammonium derived from ammonia (with metals or their compounds the metal retains its original name), but the second part of the salt name is always derived from the acid as in NOTE (a) above.

NOTE (c): Ammonia is an alkaline gas that is very soluble in water. It is a weak alkali or soluble base and is readily neutralised by acids in solution to form ammonium salts which can be crystallised on evaporating the resulting solution. Sometimes the equations are written with the 'fictitious' 'ammonium hydroxide'

e.g. NH₄OH_(aq) + HCl_(aq) ==> NH₄Cl_(aq) + H₂O_(l)

NOTE (d): There are more equations in section 9. and an extensive structured question on acid reaction equations.