Elemental bromine can

CHEMISTRY · Multi-part · A Level · Chemistry · Electrode potentials · Electrode potentials

Problem

Elemental bromine can, in principle, be produced from aqueous bromide ions by a sequence of coupled reactions that do not involve the direct use of electricity. A chemical plant has access to large quantities of the five reactions listed below; all ΔG∘\Delta G^\circ values are for 298 K298\,\mathrm{K}.
LabelReactionΔG∘ (kJ mol−1)\Delta G^\circ\,(\mathrm{kJ\,mol^{-1}})
A2Br−(aq)+Cl2(g)→Br2(l)+2Cl−(aq)2\mathrm{Br^-}(\mathrm{aq})+\mathrm{Cl_2}(\mathrm{g})\rightarrow\mathrm{Br_2}(\mathrm{l})+2\mathrm{Cl^-}(\mathrm{aq})+3.2+3.2
BCl2(g)+H2(g)→2HCl(g)\mathrm{Cl_2}(\mathrm{g})+\mathrm{H_2}(\mathrm{g})\rightarrow2\mathrm{HCl}(\mathrm{g})−185-185
CH2(g)+Br2(l)→2HBr(g)\mathrm{H_2}(\mathrm{g})+\mathrm{Br_2}(\mathrm{l})\rightarrow2\mathrm{HBr}(\mathrm{g})+53+53
D2HBr(g)+Cl2(g)→2HCl(g)+Br2(l)2\mathrm{HBr}(\mathrm{g})+\mathrm{Cl_2}(\mathrm{g})\rightarrow2\mathrm{HCl}(\mathrm{g})+\mathrm{Br_2}(\mathrm{l})−91-91
EBr2(l)+2Cl−(aq)→2Br−(aq)+Cl2(g)\mathrm{Br_2}(\mathrm{l})+2\mathrm{Cl^-}(\mathrm{aq})\rightarrow2\mathrm{Br^-}(\mathrm{aq})+\mathrm{Cl_2}(\mathrm{g})−3.2-3.2
Only chlorine gas Cl2\mathrm{Cl_2}, hydrogen gas H2\mathrm{H_2}, and aqueous chloride ion Cl−\mathrm{Cl^-} are available on site in addition to the aqueous bromide feed. Gaseous hydrogen chloride HCl\mathrm{HCl} may be vented or absorbed as desired. (1) (a) Propose a sequence that uses the minimum number of the provided reactions to convert aqueous Br−\mathrm{Br^-} to elemental Br2\mathrm{Br_2} with a negative overall ΔG∘\Delta G^\circ. List the reactions in the order you would run them and indicate any stoichiometric scaling you apply. (2) (b) Show the detailed cancellation of intermediates to obtain a single net reaction expressed with the smallest whole-number coefficients. (3) (c) Compute the cumulative ΔG∘\Delta G^\circ for your net reaction from part (b) and state whether the process is spontaneous under standard conditions. (4) (d) The plant manager suggests that running only Reaction A should suffice. Using thermodynamic reasoning based on ΔG∘\Delta G^\circ, explain why Reaction A alone is an inadequate industrial route to Br2\mathrm{Br_2} despite its equilibrium constant being close to unity.

Related theory

Browse the problem library