The reversible decomposition of hydrogen iodide is represented by 2HI(g)…
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
The reversible decomposition of hydrogen iodide is represented by
2HI(g)⇌H2(g)+I2(g)
The standard enthalpy change for the forward reaction is ΔH∘=+52kJmol−1 (endothermic). The equilibrium constants at two temperatures are
• at 500 K, Kc=1.3×10−3
• at 700 K, Kc=4.0×10−2
A 1.00-L vessel is filled at 500 K with the following initial concentrations:
• [HI]=0.800M
• [H2]=0.0500M
• [I2]=0.0500M
Assume the volume remains constant when the temperature is later raised to 700 K.
(a) Calculate the value of the reaction quotient Qc for the initial mixture at 500 K and predict the spontaneous direction of reaction at this temperature.
(b) After the system has reached equilibrium at 500 K, the temperature is suddenly increased to 700 K while the volume is kept constant. Qualitatively predict in which direction the net reaction will proceed immediately after the temperature jump, and justify your answer using both the numerical values of the equilibrium constants and the sign of ΔH∘.
(c) Explain how the concentrations of all three species at the new equilibrium at 700 K will compare with their concentrations at the 500 K equilibrium. Your explanation should integrate Le Châtelier’s principle with the temperature dependence of the equilibrium constant.