Chemistry Labs

International Chemistry Olympiad · 2019

Problems

  1. Problem 2Molecular hydrogen (H2) is an alternative to carbon dioxide-emitting fuels, so lowering the cost and environmental impact of its production is a major challenge; water splitting is a promising candidate technology. Data at 298 K: ΔfH∘\Delta_f H^\circ (kJ mol−1^{-1}): H2(g) 0, H2O(l) −285.8, H2O(g) −241.8, O2(g) 0; Sm∘S_m^\circ (J mol−1^{-1} K−1^{-1}): H2(g) 130.6, H2O(l) 69.9, H2O(g) 188.7, O2(g) 205.2. (a) Write the balanced equation for the splitting of liquid water with a stoichiometric coefficient of 1 for water and show numerically whether the reaction is thermodynamically favourable at 298 K. (b) Water splitting can be performed electrochemically with two electrodes in an acidic water bath; write the half-reactions at each electrode and derive the condition on the applied voltage ΔEapplied\Delta E_{\text{applied}} relative to the thermodynamic threshold ΔEth\Delta E_{\text{th}} for the process to be favourable at 298 K. (c) For a Pt cathode the minimum voltage ΔEmin⁡\Delta E_{\min} depends on the anode: IrOx 1.6 V, NiOx 1.7 V, CoOx 1.7 V, Fe2O3 1.9 V. Give the expression for the power efficiency ηelec\eta_{\text{elec}} of water electrolysis, calculate it for Pt/Fe2O3 and name the most efficient anode.Solutions: 1
  2. Problem 3Silver chloride is a milk-white solid (quotes from a lesson by L. J. Gay-Lussac). Data at 298 K: pKs1(AgCl)=9.7pK_{s1}(\ce{AgCl}) = 9.7; pKs2(AgX2CrOX4)=12pK_{s2}(\ce{Ag2CrO4}) = 12; formation constant of [Ag(NHX3)Xn]+[\ce{Ag(NH3)_n}]^+: βn=107.2\beta_n = 10^{7.2}; E∘(AgX+/Ag)=0.80E^\circ(\ce{Ag+/Ag}) = 0.80 V. (a) Calculate the solubility ss of AgCl(s)\ce{AgCl(s)} in water. (b) When ammonia is added to silver chloride a complex of stoichiometry nn is formed; write the equilibrium and its constant KK, then determine nn knowing that 0.10.1 mol of AgCl\ce{AgCl} in 1 dm3^3 of water just dissolves when [NHX3]=1.78[\ce{NH3}] = 1.78 mol dm−3^{-3}. (c) The Mohr method titrates ClX−\ce{Cl-} by AgX+\ce{Ag+} in the presence of KX2CrOX4\ce{K2CrO4}: three drops (≈0.5\approx 0.5 cm3^3) of KX2CrOX4\ce{K2CrO4} at c=7.76×10−3c = 7.76\times 10^{-3} mol dm−3^{-3} are added to V0=20.00V_0 = 20.00 cm3^3 of a NaCl solution titrated by AgNOX3\ce{AgNO3} at c=0.050c = 0.050 mol dm−3^{-3}; a red precipitate appears at VAg=4.30V_{\ce{Ag}} = 4.30 cm3^3. Calculate c(ClX−)c(\ce{Cl-}) and the residual [ClX−]res[\ce{Cl-}]_{\text{res}} when AgX2CrOX4\ce{Ag2CrO4} starts to precipitate.Solutions: 1