Group 2: practice questions

Question 1

Small, similarly-sized pieces of magnesium, calcium, strontium and barium are each added to separate beakers of cold water. Which statement correctly describes the trend observed?

Accepted Answer

A

Question 2

A student investigates Group 2 metals and their compounds. Small, similarly-sized pieces of calcium and barium are each added to separate beakers of cold water. (a) Write a balanced symbol equation, including state symbols, for the reaction between barium and cold water. [2] (b)…

Accepted Answer

((a)) Ba(s) + 2H2O(l) → Ba(OH)2(aq) + H2(g)  ((b)) Barium has a larger atomic/ionic radius and lower ionisation energies than calcium, so its outer electrons are lost more easily to water, giving a faster, more vigorous reaction.  ((c)) BaO(s) + 2HCl(aq) → BaCl2(aq) + H2O(l); the white solid dissolves/disappears and the mixture becomes noticeably warmer (no gas is produced).

Question 3

The table gives the approximate temperatures at which the Group 2 carbonates begin to decompose on strong heating. Carbonate Approximate decomposition temperature / °C MgCO3 400 CaCO3 840 SrCO3 1210 BaCO3 1360 (a) Describe the trend in thermal stability of the Group 2 carbonates…

Accepted Answer

((a)) Thermal stability of the Group 2 carbonates increases down the group, from magnesium to barium (the decomposition temperature increases down the group).  ((b)) Down the group the cation radius increases, so its charge density (polarising power) decreases. A smaller, more strongly polarising cation such as Mg^2+ distorts the large carbonate ion more, weakening its C–O bonds and allowing it to decompose at a lower temperature; a larger,…  ((c)) SrCO3(s) → SrO(s) + CO2(g)  ((d)) 2Mg(NO3)2(s) → 2MgO(s) + 4NO2(g) + O2(g); a brown (reddish-brown) gas, NO2, is produced.

Question 4

The table gives lattice energies and hydration enthalpies (of the constituent ions) for calcium hydroxide and barium hydroxide. Compound Lattice energy / kJ mol^-1 Hhyd(M^2+) / kJ mol^-1 Hhyd(OH^-) / kJ mol^-1 Ca(OH)2 -2540 -1650 -460 Ba(OH)2 -2160 -1350 -460 (a) Define the term…

Accepted Answer

((a)) The enthalpy change when 1 mole of a solid ionic compound is formed from its constituent ions in the gaseous state, under standard conditions.  ((b)) Hsol = -(lattice energy) + Hhyd(M^2+) + 2 Hhyd(OH^-); for Ca(OH)2, Hsol = -30 kJ mol^-1; for Ba(OH)2, Hsol = -110 kJ mol^-1.  ((c)) Hsol is more negative (more exothermic/more favourable) for Ba(OH)2 than for Ca(OH)2, so, assuming similar entropy changes for both, this is consistent with Ba(OH)2 being more soluble than Ca(OH)2.

Question 5

Which statement about the Group 2 hydroxides, from Mg(OH)2 to Ba(OH)2, is correct?

Accepted Answer

B

Question 6

Consider the elements magnesium, calcium, strontium and barium, taken in that order down Group 2.

Which row correctly describes how atomic radius, ionic radius (of the $\text{M}^{2+}$ ion) and first ionisation energy change going down the group from magnesium to barium?

Accepted Answer

A

Question 7

A strip of magnesium ribbon is held in tongs and ignited in a Bunsen flame, then plunged into a gas jar of oxygen. In a separate experiment, a sample of calcium turnings is heated strongly in a stream of dry oxygen gas. (a) Write a balanced symbol equation, including state symbo…

Accepted Answer

((a)) 2Mg(s) + O2(g) → 2MgO(s); the magnesium burns with a brilliant/blinding white flame, leaving a white solid (ash).  ((b)) 2Ca(s) + O2(g) → 2CaO(s)  ((c)) Calcium has a larger atomic radius and a lower first ionisation energy than magnesium (its outer electrons are further from the nucleus and more shielded), so its outer electrons are lost more readily to oxygen, giving a faster, more vigorous reaction.

Question 8

Solid magnesium oxide and solid barium oxide are added, separately, to distilled water at room temperature. Magnesium oxide reacts only very slowly, giving a mixture that is only faintly alkaline even after standing for some time. Barium oxide, in contrast, reacts rapidly and vi…

Accepted Answer

((a)) MgO(s) + H2O(l) → Mg(OH)2(s); BaO(s) + H2O(l) → Ba(OH)2(aq)  ((b)) Going down Group 2 the ionic radius of M^2+ increases, so its charge density decreases; the lattice energy of the oxide (formed from M^2+ and the small O^2- ion) therefore becomes less exothermic (less negative) down the group. MgO, with the small, high-charge-density Mg^2+ ion,…

Question 9

Excess powdered magnesium and excess powdered barium are added to separate test tubes, each containing dilute sulfuric acid. (a) Write a balanced symbol equation, including state symbols, for the reaction of magnesium with dilute sulfuric acid. [2] (b) The magnesium reacts stead…

Accepted Answer

((a)) Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)  ((b)) BaSO4 is essentially insoluble in water. As barium reacts with the acid, a layer of insoluble solid BaSO4(s) forms on the surface of the metal; this coating prevents the acid from reaching the unreacted barium underneath, so the reaction becomes very slow/appears to stop even th…  ((c)) The solubility of the Group 2 sulfates decreases down the group: MgSO4 is freely soluble, CaSO4 is only sparingly soluble, and SrSO4 and BaSO4 are increasingly (and, for BaSO4, essentially) insoluble.

Question 10

Barium nitrate must be heated to a considerably higher temperature than magnesium nitrate before it decomposes into the metal oxide, nitrogen dioxide and oxygen.

Which explanation for this difference in thermal stability is correct?

Accepted Answer

B

Group 2: Chemistry 9701 (Cambridge International AS & A Level)

Syllabus coverage

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Carbonate	Approximate decomposition temperature / $^\circ\text{C}$
$\text{MgCO}_3$	400
$\text{CaCO}_3$	840
$\text{SrCO}_3$	1210
$\text{BaCO}_3$	1360

Compound	Lattice energy / $\text{kJ mol}^{-1}$	$\Delta H_{hyd}(\text{M}^{2+})$ / $\text{kJ mol}^{-1}$	$\Delta H_{hyd}(\text{OH}^-)$ / $\text{kJ mol}^{-1}$
$\text{Ca(OH)}_2$	-2540	-1650	-460
$\text{Ba(OH)}_2$	-2160	-1350	-460