30. The energy of the first triplet excited state is lower than that of the first singlet excited state for ethylene. What is the correct explanation for it?
(A) the more unpaired electrons the more stable a molecule
(B) only the singlet excited state has electron(s) in antibonding orbital(s)
(C) the triplet excited state has a larger bond order
(D) the wavefunction is antisymmetric with respect to electron interchange.
(E) Hund's rule requires that the electrons are filled parallel first.

1. Determine the sign of ΔSsys for each chemical reaction. Which one is positive? (A) Mg(s) + Cl2(g) → MgCl2(s) (B) 2H2S(g) + 3O2(g) → 2H2O(g) + 2SO2(g) (C) 2O3(g) → 3O2(g) (D) HCl(g) + NH3(g) → NH4Cl(s) (E) 2CO(g) + O2(g) → 2CO2(g)

2. To inflate a balloon, you must do pressure-volume work on the surroundings. If you inflate a balloon from a volume of 0.100 L to 1.85 L against a constant external pressure of 1.00 bar, how much work is done (in joules)? (A) -1.75 (B) -17 (C) -175 (D) +17 (E) +175

3. Lighters are usually fueled by butane (C4H10). When 2.00 mol of butane burns at constant pressure, it produces 5316 kJ of heat and does 6.00 kJ of work. What are the values of ΔH and ΔU for the combustion of one mole of butane? (A) ΔΗ = −2658 kJ； ΔU = −2655 kJ (B) ΔΗ = −2658 kJ； ΔU = -3 kJ (C) ΔΗ = –2658 kJ； ΔU = −2661 kJ (D) ΔΗ = 5316 kJ； ΔU = 5322 kJ (E) ΔΗ = −5316 kJ； ΔU=-5322 kJ

4. A strip of copper is placed in a 1 M solution of copper nitrate, and a strip of silver is placed in a 1 M solution of silver nitrate. The two metal strips are connected to a voltmeter by wires, and a salt bridge connects the solutions. The following standard reduction potentials apply: Ag+(aq) + e-  → Ag(s)      E° = +0.80 V Cu2+(aq) + 2e- → Cu(s) E° = +0.34 V When the voltmeter is removed and the two electrodes are connected by a wire, which of the following does .not. take place? (A) Negative ions pass through the salt bridge from the silver half-cell to the copper half-cell. (B) Some positive copper ions pass through the salt bridge from the copper half-cell to the silver half-cell. (C) The silver electrode increases in mass as the cell operates. (D) Electrons flow in the external circuit from the copper electrode to the silver electrode. (E) There is a net general movement of silver ions through the salt bridge to the copper half-cell.

5. Which of the following statements is .true.? (A) We can determine the exact location of an electron if we know its energy. (B) Ni has 2 unpaired electrons in its 3d orbitals in its ground state. (C) An electron in a 2s orbital can have the same n, l, and m_l quantum numbers as an electron in a 3s orbital. (D) Only three quantum numbers are needed to uniquely describe an electron. (E) In the building up of atoms, electrons occupy the 4f orbitals before the 6s orbitals.

6. A fuel cell designed to react grain alcohol with oxygen has the following net reaction: C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l) The maximum work that 2.00 mol of alcohol can yield by this process is 2640 kJ. What is the theoretical maximum voltage this cell can achieve? (Faraday constant = 96500 C mol⁻¹) (A) 1.14 V (B) 2.28 V (C) 13.7 V (D) 2.01 V (E) 0.760 V

7. The elements of Group 5A, the nitrogen family, form compounds with hydrogen that have the boiling points listed below. SbH3 -17°C, AsH3 -55°C, PH3 -87°C, NH3 -33°C The first three elements illustrate a trend where the boiling point increases as the mass increases； however, ammonia (NH3) does not follow the trend because of (A) ionic bonding. (B) hydrogen bonding. (C) metallic bonding. (D) dipole-dipole attraction. (E) London dispersion forces.

8. The rate constant of a reaction is measured at different temperatures. A plot of the natural log of the rate constant as a function of the inverse of the temperature (in kelvins) yields a straight line with a slope of -9.74 × 10³ K. What is the activation energy (Ea) for the reaction? (A) -81 kJ/mol (B) 81 kJ/mol (C) 1.2 kJ/mol (D) -1.2 kJ/mol (E) 1.2 kcal/mol

9. The first-order decay of radon has a half-life of 3.823 days. How many grams of radon remain after 7.65 days if the sample initially weighs 500.0 grams? (A) 4.21 g (B) 250 g (C) 109.6 g (D) 100 g (E) 125 g

10. A metal crystallizes in a body-centered cubic unit cell with an edge length of 1.95 × 10² pm. Assume the atoms in the cell touch along the cube diagonal. What will be the percentage of empty volume in the unit cell? (A) 68.0% (B) 0.0% (C) 45.0% (D) 32.0% (E) 26.0%

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