Chemistry Labs

International Chemistry Olympiad · 2021

Problems

  1. Problem 1Hydrogen molecules reaching a metal surface dissociate and adsorb as H atoms, HX2(g)→2 H(ad)\ce{H2(g) -> 2H(ad)}. The adsorbed atoms are either absorbed into the bulk, H(ad)→H(ab)\ce{H(ad) -> H(ab)}, or recombine and desorb. The rates per surface site are r1=k1PHX2(1−θ)2r_1 = k_1 P_{\ce{H2}}(1-\theta)^2 (adsorption), r2=k2θ2r_2 = k_2\theta^2 (desorption) and r3=k3θr_3 = k_3\theta (absorption), where θ\theta is the fraction of sites occupied by H atoms. Adsorption and desorption are fast compared with absorption (r1,r2≫r3r_1, r_2 \gg r_3) and θ\theta is constant. (a) Express r3r_3 as a function of PHX2P_{\ce{H2}} and show that it can be written r3=k3CPHX21+CPHX2r_3 = k_3\dfrac{\sqrt{C P_{\ce{H2}}}}{1+\sqrt{C P_{\ce{H2}}}}; express CC through k1k_1 and k2k_2. (b) A metal sample of area S=1.0×10−3S = 1.0\times 10^{-3} m2^2 is placed in a 1.0 L container with HX2\ce{H2} at PHX2=1.0×104P_{\ce{H2}} = 1.0\times 10^{4} Pa; the density of adsorption sites is N=1.3×1018N = 1.3\times 10^{18} m−2^{-2} and T=400T = 400 K. As absorption proceeds, PHX2P_{\ce{H2}} decreases at the constant rate v=4.0×10−4v = 4.0\times 10^{-4} Pa s−1^{-1}. Calculate the amount of H atoms absorbed per unit area per unit time, AA [mol s−1^{-1} m−2^{-2}]. (c) At 400 K, C=1.0×102C = 1.0\times 10^{2} Pa−1^{-1}; calculate k3k_3.Solutions: 1
  2. Problem 3Ignore the absorption of the cell and the solvent; all solutions are at 25 °C. An aqueous solution X was prepared from the weak acid HA and NaA; in X the concentrations are [AX−]=1.00×10−2[\ce{A-}] = 1.00\times 10^{-2}, [HA]=1.00×10−3[\ce{HA}] = 1.00\times 10^{-3} and [HX+]=1.00×10−4[\ce{H+}] = 1.00\times 10^{-4} mol L−1^{-1}, linked by the equilibrium HA⇌HX++AX−\ce{HA <=> H+ + A-}. The absorbance of X was A1A_1 at wavelength λ1\lambda_1 (optical path ll). Then X was diluted to twice its initial volume using hydrochloric acid of pH 2.500; after the dilution and re-establishment of equilibrium, the absorbance at λ1\lambda_1 was still A1A_1. Determine the ratio εHA/εAX−\varepsilon_{\ce{HA}}/\varepsilon_{\ce{A-}} of the absorption coefficients of HA and AX−\ce{A-} at λ1\lambda_1.Solutions: 1