A sealed 1.50 L container at 298 K contains the gaseous equilibrium mixture for the reaction
C₂H₂(g) + H₂(g) ⇌ C₂H₄(g) K_c = 4.5 (at 298 K)
Immediately before an experimental disturbance (time t = –1 min), the measured concentrations are
[C₂H₂] = 0.200 M, [H₂] = 0.200 M, [C₂H₄] = 0.180 M.
At time t = 0 min an additional 0.200 mol of H₂ is injected instantaneously. The injection momentarily increases [H₂] to 0.900 M while leaving [C₂H₂] and [C₂H₄] unchanged. A student continuously records the concentrations of all three species for the next 30 minutes.
Answer the following questions about the system’s response to the disturbance.
(a) Using the definition of the reaction quotient Q, explain qualitatively whether the net reaction will initially proceed in the forward direction (toward C₂H₄) or in the reverse direction (toward C₂H₂ + H₂).
(b) Describe how the concentrations of C₂H₂, H₂, and C₂H₄ will change over the interval from t = 0 min until the system re-establishes equilibrium. In your description, indicate which species increase or decrease and why their rates of change slow as equilibrium is approached.
(c) After the system has returned to equilibrium (t ≈ 30 min), the measured concentration of C₂H₄ is found to be 0.600 M. Explain qualitatively why this final value must be larger than the initial equilibrium concentration of 0.180 M.