Drop a strip of magnesium in a flame and it burns with a bright light. Drop a strip of beryllium and it hardly seems to care. Same group, very different behaviour. The reason is in the unit: reactivity grows down Group 2, and beryllium has a protective coat.
Unit 17 of the PECTAA Chemistry 12 book is Group 2 Elements. Section 17.2 gives the reactions with oxygen, water and acids. This note is for FSc Part 2 (2nd year, Class 12) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.
What to know
17.2.1 Reactions with oxygen. All Group 2 metals burn in oxygen to form metal oxides, MO.
- Beryllium reacts with oxygen at high temperatures (above 600 °C) to form BeO. A thin, strong oxide layer usually protects it, so it is less reactive than the other group members. 2Be + O₂ → 2BeO.
- Magnesium burns brightly in air or oxygen to form MgO. 2Mg + O₂ → 2MgO.
- Calcium reacts more vigorously and gives a brick red flame. 2Ca + O₂ → 2CaO.
The oxides are generally white solids and basic. Their basic character increases down the group. The basic character in water comes from the oxide ion reacting with water to make hydroxide ions: O²⁻ + H₂O → 2OH⁻.
17.2.2 Reactions with water. Except beryllium, the Group 2 metals react with water to give a metal hydroxide, M(OH)₂, and hydrogen gas.
- Beryllium does not react with water or steam, even when heated, because of its protective oxide layer.
- Magnesium reacts very slowly with cold water: Mg + 2H₂O → Mg(OH)₂ + H₂. With steam it reacts faster and forms the oxide: Mg + H₂O(g) → MgO + H₂.
- Calcium reacts with cold water and steam more vigorously than magnesium: Ca + 2H₂O → Ca(OH)₂ + H₂. Calcium hydroxide is sparingly soluble and forms a white precipitate.
The book says the hydroxides of Group 2 are less basic than those of Group 1. Their basic character increases down the group because the solubility increases.
17.2.3 Reactions with acids. Group 2 metals react with dilute acids to give a salt and hydrogen gas: M + 2HCl → MCl₂ + H₂. Reactivity increases down the group.
- Beryllium forms BeCl₂ and hydrogen with dilute acids, but its oxide layer can slow the reaction.
- Magnesium reacts readily with dilute hydrochloric and sulphuric acids: Mg + 2HCl → MgCl₂ + H₂ and Mg + H₂SO₄ → MgSO₄ + H₂.
- Calcium reacts vigorously: Ca + 2HCl → CaCl₂ + H₂. With sulphuric acid the reaction can stop early. The CaSO₄ formed is sparingly soluble and coats the metal with a protective layer.
Key points
| Reaction | Be | Mg | Ca |
|---|---|---|---|
| With oxygen | Above 600 °C, BeO; protected by oxide layer | Burns brightly, MgO | More vigorous, brick red flame, CaO |
| With water | No reaction, even with steam | Very slow with cold water; faster with steam (MgO) | Cold water and steam, more vigorous; Ca(OH)₂ sparingly soluble |
| With dilute acid | BeCl₂ and H₂, can be slowed | Reacts readily | Reacts vigorously; with H₂SO₄ stops early |
- Trend. Reactivity increases down the group.
- Protective layers. Be is protected by its oxide layer. Ca is slowed in sulphuric acid by a layer of CaSO₄.
- Products. Oxygen gives the oxide. Water gives the hydroxide and hydrogen. Acid gives the salt and hydrogen.
Worked examples
Example 1. Write the equation for calcium burning in oxygen.
- Calcium and oxygen give calcium oxide.
- 2Ca + O₂ → 2CaO.
- The flame is brick red.
Example 2. Write the equations for magnesium with cold water and with steam.
- Cold water: Mg + 2H₂O(ℓ) → Mg(OH)₂ + H₂.
- Steam: Mg + H₂O(g) → MgO + H₂.
- The reaction with steam is faster.
Example 3. Why does beryllium not react with water?
- A thin, strong oxide layer covers the metal.
- The layer protects it, even with steam.
Example 4 (in the style of the book Quick Check). Write a balanced equation for barium with water.
- Barium is in Group 2, so it gives a hydroxide and hydrogen.
- Ba + 2H₂O → Ba(OH)₂ + H₂.
- Barium is lower in the group than calcium, so by the trend it is more reactive.
Common mistakes in the exam
- Saying beryllium reacts with water. The book says it does not, even with steam.
- Forgetting to balance O₂. Write 2Mg + O₂ → 2MgO, not Mg + O₂ → MgO.
- Giving the oxide with cold water. Magnesium with cold water gives Mg(OH)₂. With steam it gives MgO.
- Putting H₂O in the wrong state. Use (ℓ) for cold water and (g) for steam.
- Reversing the trend. Reactivity increases down the group.
- Saying calcium reacts fully with sulphuric acid. The CaSO₄ layer stops it early.
Exam technique
- Write the word equation, then the balanced equation, then one line on the trend.
- For "why" questions about beryllium, name the protective oxide layer.
- Include state symbols, as the book does.
- Check your board's notice for the sections in your paper.
Practice questions
- Write the equation for magnesium burning in oxygen. Answer: 2Mg + O₂ → 2MgO.
- Which colour of flame does calcium give in oxygen? Answer: Brick red.
- Write the equation for calcium with dilute hydrochloric acid. Answer: Ca + 2HCl → CaCl₂ + H₂.
- Why does the reaction of calcium with sulphuric acid stop early? Answer: The sparingly soluble CaSO₄ coats the metal as a protective layer.
- How does the basic character of the oxides change down the group? Answer: It increases.
Ready to practise this? Continue in 2nd Year Chemistry.