Group 2, the alkaline earth metals — AQA A-Level Chemistry
Test yourself on Group 2, the alkaline earth metals with AQA A-Level practice questions.
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Group 2, the alkaline earth metals explained
Down Group 2 from Mg to Ba each element adds a complete electron shell, so atomic radius increases.
Read the full explanation
First ionisation energy falls: although the nuclear charge is larger, the outer pair of electrons lies further from the nucleus and is shielded by more inner shells, and those two effects outweigh the extra protons. The second ionisation energy of any of these metals is always larger than the first, because that electron is being pulled from a positively charged ion which is smaller and holds its remaining electrons more tightly, with the same number of protons now attracting fewer electrons. Melting point generally falls from calcium to barium, since the larger 2⁺ ions sit further from the delocalised electrons and the metallic bonding weakens; magnesium breaks the pattern because its crystal packing differs from the others.
The reactions of the elements Mg–Ba with water.
Group 2 metals reduce water, and they do so more readily down the group as the ionisation energies fall. Magnesium reacts only very slowly with cold water, Mg + 2H₂O → Mg(OH)₂ + H₂, and because magnesium hydroxide is only sparingly soluble the solution reaches about pH 10 rather than pH 14. Heated in steam, magnesium burns with a bright white flame to give the oxide instead: Mg + H₂O → MgO + H₂. Calcium, strontium and barium all react with cold water directly and with increasing vigour, by M + 2H₂O → M(OH)₂ + H₂, giving effervescence and a cloudy alkaline mixture. Because hydroxide solubility increases down the group, the resulting solutions become more strongly alkaline, and barium hydroxide solution approaches pH 13. Sparingly soluble Mg(OH)₂ is mild enough to swallow as an antacid.
The relative solubilities of the sulfates of the elements Mg–Ba in water.
Group 2 sulfates become less soluble down the group, the opposite way round from the hydroxides. Magnesium sulfate dissolves freely, calcium sulfate is slightly soluble, and strontium and barium sulfates are effectively insoluble. This is why barium chloride is the reagent for detecting sulfate ions: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) gives a dense white precipitate. The solution must be acidified first with dilute hydrochloric acid to destroy carbonate ions, which would also give white precipitates. Nitric acid must not be used because it oxidises sulfite ions to sulfate ions, causing a false positive. Sulfuric acid must never be used, because it supplies the very ion being tested for. This insolubility makes barium sulfate safe as a barium meal, since it is opaque to X-rays and the toxic Ba²⁺ is not absorbed from the gut.
Your focus
- Explain why the first ionisation energy of barium is lower than that of magnesium, using distance and shielding.
- Explain why the second ionisation energy of calcium is greater than its first, referring to the charge and size of the particle involved.
- Predict the order of melting points of calcium, strontium and barium and justify it from ionic size.
Show all 9 objectives
- Write balanced equations for magnesium with cold water and with steam, naming the different products.
- Explain why barium reacts with cold water faster than calcium does, using ionisation energies.
- Explain why magnesium hydroxide can be swallowed as an antacid while barium hydroxide solution cannot, referring to solubility.
- Describe the test for a sulfate ion, naming the acid used and giving the ionic equation for the precipitate.
- Explain why hydrochloric acid rather than sulfuric or nitric acid is used to acidify a solution before barium chloride is added.
- Explain why barium sulfate can be swallowed as a barium meal even though barium ions are toxic.
Group 2, the alkaline earth metals exam tips
Marking Points
- one mark for atomic radius increasing down the group because each element has one more occupied electron shell
- one mark for first ionisation energy decreasing because the outer electron is further from the nucleus and better shielded
- one mark for stating that the increase in nuclear charge is outweighed by distance and shielding
- one mark for melting point falling from calcium to barium as the larger 2⁺ ions attract the delocalised electrons less strongly
- one mark for identifying magnesium as an exception, explained by its different crystal packing
- one mark for the equation Mg + 2H₂O → Mg(OH)₂ + H₂ for the slow reaction with cold water
- one mark for the equation Mg + H₂O → MgO + H₂ for the reaction with steam
- one mark for reactivity increasing from magnesium to barium as the first ionisation energy falls
- one mark for the observation of effervescence and a solution that turns indicator alkaline
- one mark for stating that Mg(OH)₂ is only sparingly soluble, so its solution is only weakly alkaline
- one mark for stating that sulfate solubility decreases from magnesium to barium
- one mark for the ionic equation Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
- one mark for the observation of a white precipitate
- one mark for acidifying with dilute hydrochloric acid first, to remove carbonate ions
- one mark for explaining that a barium meal is safe because barium sulfate is insoluble, so Ba²⁺ is not absorbed
Examiner Tips
- 💡Marks for ionisation energy trends require the words shielding and distance, not just nuclear charge.
- 💡When comparing successive ionisation energies of the same element, say the electron is removed from a positive ion.
- 💡Read the conditions: cold water gives the hydroxide, steam gives the oxide, and the two equations are different.
- 💡Link reactivity to ionisation energy, not to atomic radius on its own.
- 💡State the acid by name; 'acidify' on its own does not identify a reagent and loses the mark.
- 💡Keep the two Group 2 solubility trends on one line of revision notes, since questions often test them against each other.
Common Mistakes
- saying first ionisation energy increases down the group because nuclear charge increases, ignoring distance and shielding
- predicting a perfectly smooth melting point trend from magnesium to barium
- describing metallic bonding as an attraction between atoms rather than between positive ions and delocalised electrons
- assuming the second ionisation energy is similar to the first, when it is much larger because the electron leaves a smaller, positive ion
- claiming the charge on the metal ion changes down the group
- stating that magnesium hydroxide solution has a pH of 14 at 25 °C, when the hydroxide is only sparingly soluble
- giving MgO as the product of the reaction with cold water, when the oxide forms only with steam
- saying the metals become less reactive down the group because atomic radius increases
- leaving hydrogen unbalanced by writing Mg + H₂O → Mg(OH)₂ + H₂
- naming the gas produced as oxygen rather than hydrogen
- assuming hydroxide and sulfate solubilities change the same way down the group, when they run in opposite directions
- acidifying with sulfuric acid, which adds the very ion being tested for
- acidifying with nitric acid, which oxidises sulfite to sulfate and gives a false positive
- describing the barium sulfate precipitate as cream or yellow instead of white
- treating barium sulfate as soluble and so predicting no precipitate