Hydrogen iodide decomposes according to
2 HI(g) ⇌ H₂(g) + I₂(g)
At a certain temperature, Kc=54.3. A sealed 2.00\,L flask at this temperature initially contains the following moles of each species:
nHI=3.00molnH2=0.60molnI2=0.20mol
A chemist investigates how changing the written form of the chemical equation affects the numerical value of the reaction quotient for the same physical mixture.
(a) Calculate the value of Qc for the decomposition reaction as written above. Clearly show the concentration of each species used in your calculation.
(b) Compare Qc with Kc and state the direction in which the system will spontaneously shift to reach equilibrium. Support your answer quantitatively.
(c) The chemist rewrites the process as
HI(g) ⇌ 21H₂(g) + 21I₂(g)
Using the same physical mixture, calculate the corresponding reaction quotient Qc′ for this new written form of the equation. Show all steps.
(d) Without performing any additional equilibrium-constant calculation beyond simple reasoning, decide whether the mixture will shift toward reactants or products when analyzed with the rewritten equation from part (c). Explain how your value of Qc′ supports your conclusion.
(e) Finally, the chemist reverses the equation in part (c) while keeping the stoichiometric coefficients unchanged. Calculate the resulting reaction quotient Qc′′. Then compare Qc′′ to the appropriate equilibrium constant and predict the direction of the spontaneous shift under this third written form.