Thermodynamics unit test
A test on this unit alone, marked as a percentage and a letter grade — for the test your class is actually sitting, rather than for May. Answer everything, then submit once: seeing the answer to question 3 before attempting question 4 makes the final percentage meaningless.
Standard enthalpy of formation
Endothermic vs exothermic
System vs surroundings
Temperature dependence of ΔG
ΔH°rxn from formation enthalpies
Calorimetry
Hess's Law
Why entropy of the universe matters
State function
Enthalpy of reaction from bond energies
Coffee-cup calorimetry assumptions
Gibbs free energy
Short answer 1. Define or explain: Predicting the sign of ΔS
3 ptsShort answer 2. Define or explain: Entropy
3 ptsShort answer 3. Define or explain: Thermodynamic vs kinetic favourability
3 ptsShort answer 4. Define or explain: Heat capacity and q = mcΔT
3 ptsFree response
10 ptsWhite phosphorus is composed of P4 molecules with a tetrahedral structure: each P atom is bonded to the other three P atoms by single bonds. An incomplete Lewis diagram shows those six single bonds but omits the nonbonding electrons. The reaction of white phosphorus with oxygen is thermodynamically favorable at 298 K: Equation 1: P4(s) + 5 O2(g) → P4O10(s) The entropy change of this reaction, ΔS°, is negative, and the enthalpy change, ΔH°, is also negative. P4O10(s) reacts exothermically with water to form phosphoric acid: Equation 2: P4O10(s) + 6 H2O(l) → 4 H3PO4(aq) A chemist carries out a calorimetry experiment on equation 2 and records: Mass of P4O10 0.100 g Mass of H2O 100.0 g Initial temperature 22.00°C Final temperature 22.38°C Molar mass of P4O10 283.9 g/mol Specific heat of H2O 4.18 J/(g·°C) P4(s) also reacts with Cl2(g): Equation 3: P4(s) + 6 Cl2(g) → 4 PCl3(g) ΔH°1 = −1148 kJ/mol_rxn Equation 4: PCl3(g) + Cl2(g) ⇌ PCl5(g) ΔH°2 = −88 kJ/mol_rxn A particle-level diagram represents the contents of the vessel at equilibrium at 546 K for equation 4. It contains 2 PCl3 molecules, 6 Cl2 molecules and 4 PCl5 molecules, and each particle represents a partial pressure of 1.00 atm.
(a) The released exam asks the student to complete the Lewis diagram for P4 by drawing the nonbonding electrons. State how many nonbonding electrons belong on each phosphorus atom, and show the valence-electron count that justifies it.
(b)(i) The entropy change of the reaction in equation 1, ΔS°, is negative. Using particle-level reasoning, explain why the entropy decreases as the reaction progresses.
(b)(ii) The enthalpy change of the reaction, ΔH°, is also negative. A student claims that the favorability of the reaction is driven by enthalpy and not by entropy. Is the student’s claim correct? Justify your answer by using the relationship between ΔG°, ΔH° and ΔS°.
(c)(i) Calculate the amount of heat, q, released during the calorimetry experiment, in kJ. Assume that the specific heat of the solution is the same as that of water.
(c)(ii) Calculate the value of ΔH°rxn for equation 2 in kJ/mol_rxn. Include the sign in your answer.
(d) The chemist performs a second trial in which some of the solid P4O10 stuck to the weighing paper and was not transferred to the calorimeter. Given that P4O10 is the limiting reactant, would ΔT for the second trial be greater than, less than, or equal to the value in the first trial? Justify your answer.
(e) Calculate the standard enthalpy of formation of PCl5(g), represented by equation 5: (1/4) P4(s) + (5/2) Cl2(g) → PCl5(g).
(f)(i) If each particle in the diagram represents a partial pressure of 1.00 atm, what is the value of Kp for the equilibrium mixture at 546 K?
(f)(ii) Does the value of Kp increase, decrease, or remain the same when the temperature is increased to 596 K? Justify your answer based on ΔH°2.