Magnesium, calcium, strontium and barium are all Group 2 metals. When these metals react with dilute hydrochloric acid, a salt and hydrogen gas are produced. Explain how and why the vigour of this reaction changes as you move down Group 2 from magnesium to barium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Group 2 metals all react with dilute hydrochloric acid. The general equation for these reactions is: M + 2HCl → MCl₂ + H₂, where M represents a Group 2 metal.
Model answer (5 marks)
The vigour of the reaction increases down the group.
1. As you move down Group 2 the atoms have an additional electron shell, so the atomic radius increases.
2. The outer‑most electrons are further from the nucleus and are shielded by more inner‑shell electrons.
3. Consequently the effective nuclear charge felt by the valence electrons decreases.
4. With a weaker attraction, the outer electrons are easier to remove.
5. Therefore the metals form M²⁺ ions more readily and react more vigorously with HCl, producing more H₂ gas.
1. As you move down Group 2 the atoms have an additional electron shell, so the atomic radius increases.
2. The outer‑most electrons are further from the nucleus and are shielded by more inner‑shell electrons.
3. Consequently the effective nuclear charge felt by the valence electrons decreases.
4. With a weaker attraction, the outer electrons are easier to remove.
5. Therefore the metals form M²⁺ ions more readily and react more vigorously with HCl, producing more H₂ gas.
Examiner tips
- State the trend in reaction vigour first. Use the terms ‘electron shell’, ‘atomic radius’, ‘shielding’, ‘effective nuclear charge’ and ‘reactivity’. Explain how each factor leads to easier loss of electrons and faster reaction.
- common_mistakes
- :
- Failing to mention the increase in atomic radius. Using ‘size’ without linking it to shielding. Not connecting the loss of electrons to the formation of M²⁺ ions.
Mark scheme (5 marks)
- The vigour of the reaction increases going down the group (from magnesium to barium)
- Each successive element has one more electron shell / larger atomic radius going down the group
- The outer electrons experience greater shielding from the nucleus going down the group
- The electrostatic attraction between the nucleus and the outer electrons decreases going down the group
- Outer electrons are lost more easily, so the metal forms cations more readily / is more reactive
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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