The gas-phase reaction H_2(g)+I_2(g) 2 ,HI(g) is allowed to reach equilibrium at 400…
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
The gas-phase reaction
H2(g)+I2(g)⇌2HI(g)
is allowed to reach equilibrium at 400 K in a rigid container. Concentrations at this initial equilibrium are
[H₂] = 0.20 M [I₂] = 0.20 M [HI] = 1.60 M
After a single disturbance is applied and the system is again allowed to come to equilibrium at 400 K, the new concentrations are
[H₂] = 0.30 M [I₂] = 0.30 M [HI] = 1.40 M
The standard enthalpy change for the reaction is ΔH° = +52 kJ mol⁻¹.
(1) Calculate the value of the equilibrium constant Kc,1 for the original equilibrium mixture.
(2) Calculate the value of the equilibrium constant Kc,2 for the mixture after it re-establishes equilibrium following the disturbance.
(3) Using only the numerical results from parts (a) and (b), determine the single type of stress that most plausibly accounts for the observed change. Explain your reasoning.
(4) Given the sign of ΔH°, state whether the temperature of the system was increased or decreased during the disturbance. Justify your answer by reference to thermodynamic principles.