3.1.4Additional lessons
Group 1 Metals with Water — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 40 of 98
Part 1 of 6The Reaction
These metals are stored under oil because they react with air and moisture.
Products
- A Group 1 metal reacts with water to form an alkaline hydroxide solution and hydrogen gas.
General equation
- 2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g), where M is Li, Na or K.
Alkaline solution
- The hydroxide ions turn universal indicator purple or blue and give a pH well above 7.
Exothermic
- The reactions release heat, and for potassium enough heat ignites the hydrogen.
Part 2 of 6Observations
Observations
Lithium
- It floats, fizzes steadily and moves slowly on the surface; it does not melt.
Sodium
- It floats, melts into a ball because of the heat released, and moves quickly across the surface.
Potassium
- It reacts very vigorously, melts, moves rapidly and the hydrogen burns with a lilac flame.
Rubidium and caesium
- These react even more violently and are not used in school laboratories.
| Metal | Equation | Reactivity |
|---|---|---|
| Li | 2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g) | slowest |
| Na | 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) | faster |
| K | 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) | fastest |
Part 3 of 6Why Reactivity Increases
1
Each metal atom loses its single valence electron to form M⁺, so reactivity depends on how easily this electron is removed.
2
Down the group the atomic radius increases as more shells are added.
3
The valence electron is further from the nucleus and more shielded by inner shells.
4
Less energy is needed to remove it, so the first ionisation energy falls from lithium to potassium.
5
Therefore reactivity of the metals increases down Group 1.
Part 4 of 6Practical Notes
Safety
- Use very small pieces, wear eye protection and stand back, because the reactions can spit hot metal and hydroxide.
Handling
- Cut the metal with a knife on a tile and keep the leftovers in oil.
Testing hydrogen
- Hydrogen gives a squeaky pop with a lit splint.
Testing the solution
- Add universal indicator to the water before or after the reaction to show the alkaline hydroxide.
Part 5 of 6Worked Example
Question Write the balanced equation, including state symbols, for the reaction of potassium with water and state the pH of the final solution.
- Potassium forms K⁺ and water supplies OH⁻, so the products are KOH and H₂.
- Balance hydrogen2K + 2H₂O → 2KOH + H₂.
- Add statesK is solid, water liquid, KOH aqueous, H₂ gas.
Answer
- 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g);
- the solution is strongly alkaline (pH above 7).
Part 6 of 6Exam Checklist & Tips
3.1.4 – Key Points for the Exam
- Alkali metal A Group 1 metal, which forms an alkaline solution with water.
- Reactivity How readily an element takes part in chemical reactions.
- Hydroxide A compound or ion containing OH⁻.
- Exothermic A reaction that releases energy to the surroundings.
Common Mistakes
- Writing H as a product instead of H₂.
- Saying that reactivity decreases down the group.
- Omitting state symbols or unbalanced equations such as Na + H₂O → NaOH + H₂.
- Explaining reactivity by the number of valence electrons, which is the same for all Group 1 metals.
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