3.3 - Reactions of Acids

Acids with metals

Dilute acids react with many metals to produce a salt and hydrogen gas:

acid + metal → salt + hydrogen

Which salt forms depends on the acid

• Hydrochloric acid gives a chloride.
• Sulfuric acid gives a sulfate.
• Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
• Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
• Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

What you see happening

• Bubbles of hydrogen gas stream off the metal's surface.
• The piece of metal gets smaller and disappears completely if there is enough acid.
• The mixture warms up (the reaction is exothermic).

Reactivity sets the rate

The rate bubbles come off shows how reactive the metal is:

• Magnesium reacts vigorously with cold dilute acid, giving a fast stream of bubbles.
• Zinc and iron react slowly in the cold, but faster if warmed.
• Copper doesn't react with dilute acid at all.

Acids with bases and carbonates

Metal oxides and metal hydroxides are bases that neutralise acids to produce a salt and water:

acid + metal oxide (or hydroxide) → salt + water

2HCl(aq) + CuO(s) → CuCl₂(aq) + H₂O(l)

H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)

HNO₃(aq) + NaOH(aq) → NaNO₃(aq) + H₂O(l)

Both soluble bases (alkalis, e.g. NaOH, KOH) and insoluble bases (e.g. CuO, MgO) neutralise acids this way.

Carbonates and hydrogencarbonates

Metal carbonates and hydrogencarbonates react with acids to produce a salt, water, and carbon dioxide:

acid + metal carbonate (or hydrogencarbonate) → salt + water + carbon dioxide

2HCl(aq) + Na₂CO₃(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

H₂SO₄(aq) + CaCO₃(s) → CaSO₄(s) + H₂O(l) + CO₂(g)

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

Ammonia

Ammonia reacts with an acid to form an ammonium salt only (no water forms because ammonia contains no oxygen):

acid + ammonia → ammonium salt

NH₃(aq) + HCl(aq) → NH₄Cl(aq)

Observations during neutralisation

• Effervescence (fizzing) occurs with carbonates and hydrogencarbonates as CO₂ gas escapes; oxides and hydroxides produce no gas.
• Solid bases disappear as they react to form soluble salts.
• The reaction mixture warms up.
• If a transition metal base is used, a colour change occurs (e.g. black copper(II) oxide disappears, leaving a blue copper(II) salt solution).

Naming salts

• A salt is a compound formed when some or all of the hydrogen ions in an acid are replaced by metal ions or ammonium ions.
• A salt's name has two parts:
  1. First part - From the metal or ammonium in the base.
  2. Second part - From the acid.

• Most Group 1, Group 2, and aluminium or zinc salts form colourless solutions and white solids.
• Transition-metal salts are typically coloured (e.g. copper(II) salts are blue in solution).

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Worked example - Salt from nitric acid and copper(II) oxide

Name the salt made when nitric acid reacts with copper(II) oxide, and write the word equation.

The neutralisation ionic equation

Neutralisation between an acid and an alkali involves hydrogen ions reacting with hydroxide ions to form water:

H⁺(aq) + OH^-(aq) → H₂O(l)

• An equation showing only the particles that react is an ionic equation.
• This single ionic equation describes every reaction between an acid and an alkali.

Spectator ions

• In the reaction between hydrochloric acid and sodium hydroxide:
  • The acid supplies H⁺(aq) and Cl^-(aq) ions.
  • The alkali supplies Na⁺(aq) and OH^-(aq) ions.
• H⁺ and OH^- react to form H₂O(l).
• Na⁺ and Cl^- remain unchanged in solution.
• Ions that remain unchanged throughout a reaction are called spectator ions, and they're omitted from the ionic equation.
• If the water is evaporated, the spectator ions combine to form the solid salt (sodium chloride).

Finding the spectator ions

Dissolved ionic compounds and aqueous acids exist as separate ions in solution:

H⁺(aq) + Cl^-(aq) + Na⁺(aq) + OH^-(aq) → Na⁺(aq) + Cl^-(aq) + H₂O(l)

Cancelling the spectator ions (Na⁺ and Cl^-) that appear on both sides gives:

H⁺(aq) + OH^-(aq) → H₂O(l)

Ionic equations with other bases

Insoluble solids do not split into separate ions in solution, so they keep the state symbol (s):