A chemist seeks a convenient laboratory route to generate small amounts of hydrogen…
CHEMISTRY · Subjective · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
A chemist seeks a convenient laboratory route to generate small amounts of hydrogen bromide gas. The following elementary equilibria are available from the literature at 298 K:
(1) Br₂(l) ⇌ Br₂(g) Kp1=0.30
(2) H₂(g) ⇌ 2 H(g) Kp2=3.2×10−3
(3) Br₂(g) ⇌ 2 Br(g) Kp3=4.1×10−1
(4) H(g) + Br(g) ⇌ HBr(g) Kp4=1.6×1010
(a) Propose a sequence of manipulations (reversal and/or multiplication) of Equilibria (1)–(4) that will yield the direct gas-phase reaction 21H2(g)+21Br2(l)⇌HBr(g). Present the manipulated reactions clearly before summing.
(b) Derive an explicit expression for Kptarget of the reaction in part (a) in terms of Kp1 through Kp4. Your derivation must explain the effect of every manipulation on the associated equilibrium constant.
(c) Compute the numerical value of Kptarget at 298 K, expressed in base-10 logarithmic form and as an ordinary number.
(d) Evaluate whether the generation of HBr(g) under these conditions is thermodynamically practical if the system initially contains 1.00 atm H₂(g) and a pool of liquid Br₂ at its vapor pressure (0.30 atm). Support your conclusion with quantitative reasoning that compares the initial reaction quotient Qp to Kptarget.