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AS 10

Group 2

AS Level · Inorganic chemistry · Papers 1, 2 and 3 · extended in A2 27

🎯What you need to be able to do

  • Describe, with equations, the reactions of magnesium to barium with oxygen, water and dilute hydrochloric and sulfuric acids.
  • Describe, with equations, the reactions of their oxides, hydroxides and carbonates with water and with dilute hydrochloric and sulfuric acids.
  • Describe, with equations, the thermal decomposition of the nitrates and carbonates, and the trend in thermal stability.
  • Describe the trends in these properties down the group and use them to make predictions.
  • State how the solubilities of the hydroxides and sulfates change down the group.

📚The chemistry

The Group 2 elements all have two outer electrons, ns2, and react by losing both to form M2+ ions. Everything they do is the same reaction with a trend running through it. The trend in reactivity has one cause: down the group the atoms get larger, the outer electrons are further from the nucleus and more shielded, so the first and second ionisation energies decrease and the two electrons are lost more easily. So reactivity increases down the group, from magnesium to barium.

The elements

With oxygen

\[ \mathrm{2M(s) + O_2(g) \rightarrow 2MO(s)} \]

All burn to give white ionic oxides. Magnesium burns with a brilliant white flame; calcium brick-red, strontium crimson, barium apple-green. (Barium also forms some peroxide, BaO2, in excess oxygen; you only need the oxide.)

With water

\[ \mathrm{M(s) + 2H_2O(l) \rightarrow M(OH)_2(aq) + H_2(g)} \]
  • Magnesium reacts extremely slowly with cold water, giving a few bubbles over days. With steam it burns brightly: Mg + H2O(g) → MgO + H2.
  • Calcium fizzes steadily in cold water and the solution turns cloudy, because calcium hydroxide is only slightly soluble and some precipitates.
  • Strontium and barium react faster still, giving clearer solutions, because their hydroxides are more soluble.

The solutions are alkaline, and more strongly so down the group.

With dilute acids

\[ \mathrm{M(s) + 2HCl(aq) \rightarrow MCl_2(aq) + H_2(g)} \] \[ \mathrm{M(s) + H_2SO_4(aq) \rightarrow MSO_4 + H_2(g)} \]

With hydrochloric acid all react, faster down the group, because every chloride is soluble. With sulfuric acid, magnesium reacts vigorously, but calcium, strontium and barium soon stop: their sulfates are sparingly soluble or insoluble, and a layer of sulfate forms on the metal and stops the acid reaching it. That is an example of the solubility trend below making a prediction.

The compounds

Oxides and hydroxides

The oxides are basic. With water they form hydroxides (MgO only slightly, because Mg(OH)2 is barely soluble):

\[ \mathrm{MO(s) + H_2O(l) \rightarrow M(OH)_2} \]

Oxides and hydroxides neutralise acids to give a salt and water:

\[ \mathrm{MO + 2HCl \rightarrow MCl_2 + H_2O} \] \[ \mathrm{M(OH)_2 + 2HCl \rightarrow MCl_2 + 2H_2O} \] \[ \mathrm{MO + H_2SO_4 \rightarrow MSO_4 + H_2O} \] \[ \mathrm{M(OH)_2 + H_2SO_4 \rightarrow MSO_4 + 2H_2O} \]

With sulfuric acid the insoluble sulfates of Ca, Sr and Ba again form a coating that slows or stops the reaction of the solid.

Carbonates

The carbonates are all insoluble in water. They react with acids, fizzing as carbon dioxide is given off:

\[ \mathrm{MCO_3(s) + 2HCl(aq) \rightarrow MCl_2(aq) + H_2O(l) + CO_2(g)} \] \[ \mathrm{MCO_3(s) + H_2SO_4(aq) \rightarrow MSO_4 + H_2O(l) + CO_2(g)} \]

With sulfuric acid, the reaction of CaCO3, SrCO3 and BaCO3 stops almost at once behind a layer of insoluble sulfate — which is why sulfuric acid is not used to make CO2 from marble chips.

Thermal decomposition

Heated strongly, the carbonates decompose to the oxide and carbon dioxide, and the nitrates to the oxide, nitrogen dioxide and oxygen:

\[ \mathrm{MCO_3(s) \rightarrow MO(s) + CO_2(g)} \] \[ \mathrm{2M(NO_3)_2(s) \rightarrow 2MO(s) + 4NO_2(g) + O_2(g)} \]

Nitrate decomposition is easy to spot: the white solid gives off brown fumes of NO2, and the oxygen relights a glowing splint. Carbonate decomposition is followed by passing the gas through limewater.

Thermal stability increases down the group. Magnesium carbonate decomposes fairly easily with a Bunsen burner; barium carbonate needs a much higher temperature. The same order holds for the nitrates. (The explanation, in terms of how strongly the small Mg2+ ion distorts the carbonate or nitrate ion, is an A Level outcome, in topic 27. At AS you need the trend and the equations.)

Solubility of the hydroxides and sulfates

The two trends run in opposite directions, and both are frequently examined:

Hydroxides — solubility increases down the group. Mg(OH)2 is almost insoluble; Ba(OH)2 is fairly soluble.
Sulfates — solubility decreases down the group. MgSO4 is soluble; BaSO4 is insoluble.

Consequences worth knowing: a saturated solution of the hydroxide has a higher pH down the group (about 10 for Mg(OH)2, about 13 for Ba(OH)2), because more OH− ions are in solution. And the insolubility of barium sulfate is the basis of the test for sulfate ions: add acidified barium chloride or nitrate, and a sulfate gives a white precipitate of BaSO4.

✏️Worked example

2.00 g of the carbonate of a Group 2 metal, MCO3, is heated strongly to constant mass. The mass falls by 0.446 g. (a) Write an equation for the decomposition and identify M. (b) A student tries to decompose the same mass of magnesium carbonate using the same Bunsen burner. Predict, with a reason, how the result will differ. (c) The residue from (a) is added to water. Write an equation and predict the approximate pH. [Ar: C 12.0, O 16.0, Mg 24.3, Ca 40.1, Sr 87.6, Ba 137.3]

(a) MCO3(s) → MO(s) + CO2(g). The mass lost is the carbon dioxide.

\[ n(\mathrm{CO_2}) = \frac{0.446}{44.0} = 0.01014\ \mathrm{mol} = n(\mathrm{MCO_3}) \] \[ M_\mathrm{r}(\mathrm{MCO_3}) = \frac{2.00}{0.01014} = 197.3 \qquad A_\mathrm{r}(\mathrm{M}) = 197.3 - 60.0 = 137.3 \]

M is barium; the carbonate is BaCO3.

(b) Thermal stability of the Group 2 carbonates decreases up the group, so magnesium carbonate decomposes more readily — at a lower temperature, or faster at the same one. It will also lose a much larger mass: MgCO3 (Mr 84.3) contains a bigger proportion of CO2, so 2.00 g would lose 2.00 × 44.0/84.3 = 1.04 g if it decomposed completely.

(c) BaO(s) + H2O(l) → Ba(OH)2(aq). Barium hydroxide is the most soluble hydroxide in the group, so the solution is strongly alkaline, pH about 13.

Check it. The mass lost as a fraction of the mass heated is 0.446/2.00 = 22.3%, and CO2 makes up 44.0/197.3 = 22.3% of BaCO3. For CaCO3 it would be 44%, for MgCO3 52%, so the fraction alone rules out the lighter carbonates. “Heated to constant mass” is what makes the calculation valid: decomposition is complete.
Getting the two stability and solubility trends the wrong way round. Thermal stability of carbonates and nitrates increases down the group; solubility of hydroxides increases; solubility of sulfates decreases. Learn them as a set of three with the one exception. And a nitrate equation that gives only NO2, without O2, cannot balance — check the oxygen count.

📝Practise

Work through these, then reveal the answer. Each question targets a different objective from the list above.

1. Explain why the reactivity of the Group 2 elements with water increases from magnesium to barium.
Each metal reacts by losing its two outer electrons to form M2+. Down the group the atoms have more shells, so the outer electrons are further from the nucleus and more shielded by inner shells. Although the nuclear charge rises, the attraction for the outer electrons decreases, so the first and second ionisation energies decrease. The electrons are lost more easily, so the metals react more readily.
2. Write an equation, with state symbols, for the thermal decomposition of calcium nitrate, and describe what you would see.
2Ca(NO3)2(s) → 2CaO(s) + 4NO2(g) + O2(g). The white solid may melt, then brown fumes of nitrogen dioxide are given off, and a white solid (calcium oxide) remains. The oxygen would relight a glowing splint, though the brown gas makes that hard to show. Oxygen count check: left 2 × 6 = 12; right 2 + 8 + 2 = 12.
3. Explain why the reaction of calcium carbonate with dilute sulfuric acid stops quickly, while its reaction with dilute hydrochloric acid continues until one reactant is used up.
With sulfuric acid the product is calcium sulfate, which is only sparingly soluble. It precipitates as a layer on the surface of the calcium carbonate, preventing further acid reaching the carbonate, so the reaction stops. With hydrochloric acid the product, calcium chloride, is soluble, so it dissolves away and the surface stays exposed: CaCO3 + 2HCl → CaCl2 + H2O + CO2.
4. State the trends in solubility of the Group 2 hydroxides and sulfates, and describe how one of them is used to test for sulfate ions.
Hydroxides: solubility increases down the group (Mg(OH)2 almost insoluble, Ba(OH)2 fairly soluble). Sulfates: solubility decreases down the group (MgSO4 soluble, BaSO4 insoluble). Test for sulfate: acidify the solution with dilute hydrochloric (or nitric) acid, then add barium chloride (or nitrate) solution. A white precipitate of barium sulfate shows sulfate: Ba2+(aq) + SO42−(aq) → BaSO4(s). The acid removes carbonate or sulfite ions, which would otherwise also give white precipitates.
5. Radium is below barium in Group 2. Predict (a) how it reacts with water, (b) the solubility of radium sulfate, (c) the thermal stability of radium carbonate compared with barium carbonate.
(a) Very vigorously — more so than barium — forming radium hydroxide and hydrogen: Ra + 2H2O → Ra(OH)2 + H2; strongly alkaline solution. (b) Insoluble, even less soluble than BaSO4, continuing the trend. (c) More stable than barium carbonate — it would need an even higher temperature to decompose.
6. Magnesium is heated in steam. Write the equation, and explain why the product differs from that of calcium with cold water.
Mg(s) + H2O(g) → MgO(s) + H2(g), with a bright white glow. At the high temperature of the steam reaction, any magnesium hydroxide would decompose, so the product is the oxide. Calcium reacts with liquid water at room temperature, where the hydroxide is stable: Ca + 2H2O → Ca(OH)2 + H2. Magnesium needs steam at all because it is the least reactive member of the group.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Chemguide (Jim Clark) — the Group 2 pages, including the reactions with water and the solubility trends
  • Royal Society of Chemistry — practical guides for the thermal decomposition of carbonates and for flame tests