Reactions of metals with oxygen, water, acids and bases
Metals lose electrons to form positive ions. They react with oxygen to give basic oxides, and with water, dilute acids and some bases to give hydrogen.
Metals lose electrons
Metals are electropositive: they tend to lose electrons and form positive ions. Most have 1, 2 or 3 electrons in their outer shell. Losing them gives a noble gas configuration: Na (2, 8, 1) → Na⁺ (2, 8) + e⁻. Non-metals are electronegative: they have 5, 6 or 7 outer electrons. They gain electrons to form negative ions: Cl (2, 8, 7) + e⁻ → Cl⁻ (2, 8, 8). When a solution of a metal salt is electrolysed, the metal ions move to the cathode, the negative electrode.
Reaction with oxygen
| Metal | How it reacts | Balanced equation |
|---|---|---|
| Sodium, potassium, calcium | React with oxygen of the air even without heating | 4Na(s) + O₂(g) → 2Na₂O(s) |
| Magnesium | Burns on heating with a dazzling white flame | 2Mg(s) + O₂(g) → 2MgO(s) |
| Aluminium | Reacts on heating; a thin oxide layer then protects the metal | 4Al(s) + 3O₂(g) → 2Al₂O₃(s) |
Basic and amphoteric oxides
Most metal oxides are basic. Those that dissolve in water form bases: Na₂O(s) + H₂O(l) → 2NaOH(aq), and CaO(s) + H₂O(l) → Ca(OH)₂(aq). Some metal oxides, such as aluminium oxide and zinc oxide, react with both acids and bases. They are called amphoteric, from a Greek word meaning both. Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O. Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O, where NaAlO₂ is sodium aluminate. Note: your NIOS lesson prints a few of these equations unbalanced (Mg + O₂, Mg + H₂O, Fe + steam) or without the hydrogen (Zn or Sn with NaOH). Learn the balanced forms given here.
Reaction with water
| Metal | Reacts with | Balanced equation |
|---|---|---|
| Sodium, potassium | Cold water, vigorously | 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g); 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) |
| Magnesium | Hot water | Mg(s) + 2H₂O(l) → Mg(OH)₂ + H₂(g) |
| Aluminium, iron | Only steam, on heating; they form oxides, not hydroxides | 2Al(s) + 3H₂O(g) → Al₂O₃(s) + 3H₂(g); 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g) |
Reaction with dilute acids and with bases
| Reaction | Balanced equation | What to note |
|---|---|---|
| Magnesium + dilute HCl | Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) | Brisk bubbling; the gas burns with a pop |
| Zinc + dilute H₂SO₄ | Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) | Used to make hydrogen in the laboratory |
| Zinc + sodium hydroxide | Zn(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + H₂(g) | Sodium zincate forms; aluminium also reacts with bases |
| Tin + sodium hydroxide | Sn(s) + 2NaOH(aq) + H₂O(l) → Na₂SnO₃(aq) + 2H₂(g) | Sodium stannate forms |
How to draw magnesium reacting with dilute HCl
- 1Draw a test tube held by a clamp on a stand, with a magnesium ribbon at the bottom.
- 2Fit a cork carrying a thistle funnel that reaches into the acid, and a short delivery tube.
- 3Show dilute HCl in the tube, with bubbles rising from the ribbon.
- 4Label: stand, clamp, test tube, magnesium ribbon, dilute HCl, thistle funnel, cork, hydrogen gas.
Sodium in a school laboratory is kept under kerosene, because it reacts with the oxygen and moisture in air. Aluminium pressure cookers and kadais last for years because a thin oxide layer protects them.
3 marks: “Complete and balance: (i) K + H₂O → (ii) Al + H₂O (steam) → (iii) Zn + HCl →” (i) 2K + 2H₂O → 2KOH + H₂ (ii) 2Al + 3H₂O → Al₂O₃ + 3H₂ (iii) Zn + 2HCl → ZnCl₂ + H₂.
2 marks: “What is an amphoteric oxide? Give one example, with equations to show its nature.” An oxide that reacts with both acids and bases to give salt and water. Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O.
1 marks: “Name the gas given off when magnesium reacts with dilute hydrochloric acid.” Hydrogen.
Cold water: Na and K. Hot water: Mg. Steam: Al and Fe. Al and Zn oxides play both sides (amphoteric).