At 448 °C the reversible reaction H₂(g) + I₂(g) ⇌ 2 HI(g) has an equilibrium constant…
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
At 448 °C the reversible reaction
H₂(g) + I₂(g) ⇌ 2 HI(g)
has an equilibrium constant of Kc = 50.5. A rigid 3.00 L vessel at this temperature is charged with 0.400 mol H₂, 0.400 mol I₂, and 0.250 mol HI, after which it is sealed and allowed to reach equilibrium.
(a) Calculate the value of the reaction quotient Qc using the initial molarities, and state whether the net reaction will proceed toward reactants or toward products before equilibrium is reached.
(b) Let x represent the change (in moles) undergone by H₂ during the shift to equilibrium. Write algebraic expressions for the equilibrium molarities of H₂, I₂, and HI in terms of x.
(c) Using the equilibrium-constant expression and your algebraic relationships from part (b), determine the numerical value of x and compute the equilibrium molarity of each species.
(d) The teacher proposes adding enough solid I₂ to the vessel to double the equilibrium molarity of I₂ found in part (c) without changing the temperature. Qualitatively predict and justify how the equilibrium molarity of HI would respond to this addition.