Sulfur trioxide, SO3, is commercially produced on a massive scale for use in manufacturing. It is al — Equilibrium Chemistry Question
Problem Context
Sulfur trioxide, SO3, is commercially produced on a massive scale for use in manufacturing. It is also produced as a by-product of burning fossil fuels and is an air pollutant involved in the formation of acid rain.
Model Answer
S is the central atom, surrounded by three O atoms. A valid diagram must show a total of 24 valence electrons. S can be shown with an expanded octet (e.g., three double bonds, or two double bonds and one single bond, or one double bond and two single bonds representing resonance forms). All atoms must have appropriate lone pairs to show a total of 24 valence electrons.
Scoring:
- 1 point is earned for any valid Lewis electron-dot diagram.
Model Answer
Trigonal planar.
Scoring:
- 1 point is earned for the shape that is consistent with the completed Lewis diagram in part (a).
Model Answer
The reaction will be thermodynamically favored only at high temperatures.
The reaction will be thermodynamically favored when ∆G° is negative (∆G° = ∆H° − T∆S°). ∆H° for this reaction is positive, so at low temperatures (when the value of T∆S° is small), ∆G° is positive and the reaction is not thermodynamically favored.
Because ∆S° for this reaction is positive (2 moles of gas decompose to produce 3 moles of gas, increasing entropy), high-temperature conditions give a large positive value for T∆S°. This will result in a negative ∆G°, and the reaction will be thermodynamically favored.
Scoring:
- 1 point is earned for correctly linking ∆H° to unfavorability at low temperatures.
- 1 point is earned for assigning the correct sign to ∆S° and linking it to favorability at high temperatures.
Model Answer
The student should draw an X on the time axis approximately halfway between 0 and t1 (where the partial pressure curves first become flat/horizontal, indicating that concentrations/pressures are constant and equilibrium has been established).
Scoring:
- 1 point is earned for an acceptably positioned X. || Additional SO2(g) was injected/added to the reaction vessel.
Scoring:
- 1 point is earned for the correct answer. || The addition of SO2(g) at time t1 increases the concentration/partial pressure of a product, making Qp > Kp. To restore equilibrium, the net reaction shifts in the reverse direction (to the left), consuming products (SO2 and O2) and producing more reactant (SO3).
Scoring:
- 1 point is earned for a correct explanation indicating that the system shifts left to establish a new equilibrium, consuming O2 and producing more SO3.
Model Answer
Kp = (P_SO2)^2 * P_O2 / (P_SO3)^2
Scoring:
- 1 point is earned for the correct Kp expression. || Kp = (0.33)^2 * (0.17) / (0.50)^2 = 0.074
OR
Kp = (0.41)^2 * (0.14) / (0.57)^2 = 0.072
Scoring:
- 1 point is earned for substituting appropriate values from the graph before or after t1.
- 1 point is earned for correctly calculating the value of Kp (values between 0.072 and 0.074 are acceptable).