XClO3(s) → XCl(s) + 3/2 O2(g) The equation above represents the decomposition of a compound containi — Thermodynamics Chemistry Question
Problem Context
XClO3(s) → XCl(s) + 3/2 O2(g)
The equation above represents the decomposition of a compound containing an unknown element, X. A 1.39 g sample of XClO3(s) was completely decomposed by heating. The gas produced by the reaction was captured over water in a gas-collection tube at 24.0°C. The total volume of gas in the tube was 506 mL, and the total pressure inside the tube was determined to be 739.5 torr. The vapor pressure of water is 22.4 torr at 24.0°C.
Model Answer
P(O2) = P(gas) - P(vp,water) = (739.5-22.4) torr = 717.1 torr. One point is earned for correct numerical result. Look for the answer to the tenth of a decimal place. Deduct significant figure point if absent.
Model Answer
n = (717.1/760)(0.506) / [(0.0821)(297)] = 1.96×10⁻² mol. One point is earned for correct setup. One point is earned for correct numerical result.
Model Answer
1.96×10⁻² mol O2 × (1 mol XClO3 / 1.5 mol O2) = 1.31×10⁻² mol XClO3. One point is earned for correct numerical result.
Model Answer
1.39 g XClO3 / 1.31×10⁻² mol = 106 g/mol. One point is earned for correct numerical result.
Model Answer
106 g/mol - 35.45 g/mol - 3(16.0 g/mol) = 23.0 g/mol; element = Na. One point for calculation of correct atomic mass. One point for correct selection of element. Because the molar mass is not asked for it is not subject to significant figure rule.
Model Answer
-21.1 kJ / 0.470 mol = -44.9 kJ/mol_rxn. One point for correct numerical value. The value must have a negative sign.
Model Answer
Sum of the bond energies of products is greater; since the reaction is exothermic, more energy must be released in the formation of product bonds than is required in the breaking of reactant bonds. One point is earned for correct choice with explanation. ΔG° cannot be used to explain bond energy.
Model Answer
It has no effect. ΔG° is unaffected by the introduction of a catalyst. A catalyst affects only the rate at which equilibrium is reached, not the position of equilibrium. One point is earned for correct choice with explanation.