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Group 2: Question 2

Syllabus 10.1

Structured AS 6 marks

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) The barium reacts noticeably faster and more vigorously than the calcium. Explain this difference in terms of atomic/ionic radius and ionisation energy. [2]

(c) In a separate experiment, solid barium oxide is added to an excess of dilute hydrochloric acid. Write a balanced symbol equation for this reaction, and state one observation that would be made. [2]

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Worked solution

Part (a): Barium with cold water

Barium is a Group 2 metal, so it forms a 2+2+ ion and reacts with water to give the metal hydroxide and hydrogen gas:

Ba(s)+2H2O(l)→Ba(OH)2(aq)+H2(g)\text{Ba(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Ba(OH)}_2\text{(aq)} + \text{H}_2\text{(g)}

Barium hydroxide is written as Ba(OH)2\text{Ba(OH)}_2 (not “BaOH”) because the Ba2+\text{Ba}^{2+} ion needs two OH−\text{OH}^- ions to balance its charge. Ba(OH)2\text{Ba(OH)}_2 is quite soluble, so it is shown as (aq)\text{(aq)}.

Part (b): Why barium reacts faster than calcium

Going down Group 2, the atomic radius (and the radius of the resulting 2+2+ ion) increases, and the first and second ionisation energies decrease, because the outer electrons are further from the nucleus and more shielded by inner shells. Barium, being below calcium, therefore has a larger atomic radius and lower ionisation energies than calcium. Its outer two electrons are more easily lost to water molecules, so barium reacts faster and more vigorously than calcium.

Part (c): Barium oxide with dilute hydrochloric acid

Barium oxide is a basic oxide, so it undergoes a simple acid-base (neutralisation) reaction with dilute hydrochloric acid to give a salt and water:

BaO(s)+2HCl(aq)→BaCl2(aq)+H2O(l)\text{BaO(s)} + 2\text{HCl(aq)} \rightarrow \text{BaCl}_2\text{(aq)} + \text{H}_2\text{O(l)}

Two moles of HCl\text{HCl} are needed per mole of BaO\text{BaO} because each O2−\text{O}^{2-} ion in the oxide must be neutralised by two H+\text{H}^+ ions (forming H2O\text{H}_2\text{O}), while the resulting Ba2+\text{Ba}^{2+} ion needs two Cl−\text{Cl}^- ions to balance its charge.

Observation: the white solid barium oxide dissolves/disappears as it reacts, giving a colourless solution, and the mixture becomes noticeably warmer (the reaction is exothermic). No gas is produced, since barium oxide is an oxide, not a carbonate.

Final answers

  • (a) Ba(s)+2H2O(l)→Ba(OH)2(aq)+H2(g)\text{Ba(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Ba(OH)}_2\text{(aq)} + \text{H}_2\text{(g)}
  • (b) Barium’s larger atomic/ionic radius and lower ionisation energies (than calcium’s) make its outer electrons easier to lose, so it reacts faster and more vigorously.
  • (c) BaO(s)+2HCl(aq)→BaCl2(aq)+H2O(l)\text{BaO(s)} + 2\text{HCl(aq)} \rightarrow \text{BaCl}_2\text{(aq)} + \text{H}_2\text{O(l)}; the solid dissolves and the mixture warms up, with no gas produced.