2.2.2Organic Chemistry
Reactions of Group 2 — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 6 of 42
Part 1 of 5Reaction with Oxygen
Equation
- 2M(s) + O₂(g) → 2MO(s) where M is a Group 2 metal.
Magnesium
- Burns with a bright white flame to form white magnesium oxide, MgO.
Calcium
- Burns with a brick-red flame forming calcium oxide, CaO.
Surface layer
- Metals in air are coated with a thin layer of oxide that slows further reaction.
Flame test link
- Calcium compounds give a brick-red flame colour, barium compounds a pale green one, which helps in identification.
Part 2 of 5Reaction with Water
Reaction with Water
Magnesium and cold water
- Reacts very slowly, forming magnesium hydroxide and hydrogen.
Magnesium and steam
- Burns brightly to form magnesium oxide and hydrogen: Mg(s) + H₂O(g) → MgO(s) + H₂(g).
Calcium
- Reacts steadily with cold waterCa(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g), giving bubbles and a cloudy solution.
Strontium and barium
- React more vigorously with cold water than calcium, forming an alkaline hydroxide solution.
Alkaline products
- Group 2 hydroxides formed from the reaction with water are alkaline, and the pH increases down the group as solubility grows.
| Metal | Water reaction | Relative rate |
|---|---|---|
| Mg | Very slow with cold water; fast with steam | Slowest |
| Ca | Steady fizzing with cold water | Moderate |
| Ba | Rapid with cold water | Fastest |
Part 3 of 5Reaction with Dilute Acid
1
Add a Group 2 metal to dilute hydrochloric acid in a flask.
2
Hydrogen gas bubbles are produced, and the metal dissolves.
3
The general equation is M(s) + 2HCl(aq) → MCl₂(aq) + H₂(g).
4
Magnesium reacts steadily, calcium more vigorously and barium violently.
5
The solution warms, showing that the reaction is exothermic.
Part 4 of 5Explaining Reactivity
Cause
- The ease of losing two electrons increases down the group.
Oxidation
- In every reaction the metal is oxidised from 0 to +2.
Hydrogen
- Water or acid is the oxidising agent, with hydrogen reduced from +1 to 0.
Safety
- Store reactive metals under oil or in sealed containers away from moisture.
Part 5 of 5Exam Checklist & Tips
2.2.2 – Key Points for the Exam
- Oxidation
- Loss of electrons;
- Group 2 metals go from 0 to +2.
- Loss of electrons;
- Alkaline solution A solution containing excess OH⁻ ions.
- Exothermic Releasing heat to the surroundings.
- Flame colour The characteristic colour produced when a metal burns or is heated.
Common Mistakes
- Writing M₂O instead of MO for Group 2 oxides.
- Giving the product of magnesium and steam as magnesium hydroxide.
- Missing state symbols in equations.
- Forgetting to balance hydrogen gas in equations with water.
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