CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
Consider the exothermic gas-phase equilibrium
2SO2(g)+O2(g)⇌2SO3(g)(ΔH∘=−197kJ mol−1)
A concentration-versus-time probe records the following behavior for [SO₃]:
• From t = 0 s to t = 300 s, [SO₃] remains steady at 0.25 M.
• At t = 300 s a sudden external change is made. The probe registers an instantaneous drop in [SO₃] to 0.15 M.
• Over the next several minutes [SO₃] gradually rises and levels off at a new constant value of 0.18 M.
Assume the container is rigid and the total pressure is initially 1.0 atm.
(1) Identify whether the temperature of the system was increased or decreased at t = 300 s. Justify your answer based on the observed concentration changes.
(2) Explain how the sign of ΔH° for the reaction supports your conclusion in part (a).
(3) Predict qualitatively (increase, decrease, or no change) the effect of the imposed temperature change on the numerical value of the equilibrium constant. Explain.
(4) Suppose the student plots lnK versus 1/T for several temperatures spanning the original and final values. Describe how the slope of this line relates to ΔH° and indicate whether the new data point after the temperature change would lie above or below the original point. Justify your answer without performing any numerical calculation.