4 MAXE · 401 MEHC
★
University of Wisconsin–Madison
College of Letters & Science — Department of Chemistry
NUMEN LUMEN
EST. 1848
NUMEN LUMEN · GOD, OUR LIGHT
Chem 104 Exam 4
T H E R M O DY N A M I CS · E L E CT R O C H E M I ST RY · O R GA N I C C H E M I ST RY & H Y B R I D I Z AT I O N
INSTITUTION University of Wisconsin–Madison COURSE CODE CHEM 104
PROGRAM Bachelor of Science — General ACADEMIC YEAR
Chemistry Sequence
EXAM TITLE Chem 104 Exam 4 TOTAL QUESTIONS 68 Questions
COURSE TITLE General Chemistry II FORMAT Multiple Choice — Select the
Single Best Answer
EXAMINATION INSTRUCTIONS
▸ Select the single best answer for each question.
▸ Gibbs free energy, spontaneity, electrochemistry, oxidation-reduction, and organic chemistry
fundamentals are core testable content.
▸ Nernst equation calculations, cell potential predictions, and hybridization/geometry are emphasized.
▸ Correct answers and rationales appear below each question for self-assessment and review.
▸ All content reflects CHEM 104 General Chemistry II — Exam 4 learning objectives.
, SECTION I — THERMODYNAMICS, ELECTROCHEMISTRY
Questions 1 – 68
& ORGANIC CHEMISTRY
1. What is the oxidation number of free (uncombined) elements such as Na, Mg, Ar, O₂, O₃, S₈,
and H₂?
A. +1
B. −1
C. 0
D. Depends on the element
CORRECT ANSWER C—0
RATIONALE The oxidation number of any free (uncombined) element in its standard state is
always 0. This applies to monatomic species (Na, Mg, Ar), diatomic molecules (O₂,
H₂), and polyatomic elemental forms (O₃, S₈). This is a fundamental rule of
oxidation number assignment. Atoms in elements have no charge displacement
because they are bonded to identical atoms with equal electronegativity.
2. If a reaction is exothermic (ΔH < 0) AND has a positive ΔS (entropy increases), what can be
said about its spontaneity?
A. The reaction is never spontaneous
B. The reaction is spontaneous only at high temperatures
C. The reaction is entropically favored and would be spontaneous at any temperature
D. The reaction is spontaneous only at low temperatures
CORRECT ANSWER C — The reaction is entropically favored and would be spontaneous at any
temperature
RATIONALE ΔG = ΔH − TΔS. When ΔH is negative (favorable) and ΔS is positive (favorable),
both terms contribute to making ΔG negative regardless of temperature. The
reaction is both enthalpically and entropically favored, so it is spontaneous at all
temperatures. This is the most favorable thermodynamic scenario — both driving
forces work in the same direction.
,3. What is the chemical formula for methane, the simplest organic molecule?
A. CH₃
B. CH₄
C. C₂H₆
D. CH₃OH
CORRECT ANSWER B — CH₄
RATIONALE Methane has the formula CH₄ — one carbon atom bonded to four hydrogen
atoms. It is the simplest alkane and the simplest organic molecule. The carbon is
sp³ hybridized with a tetrahedral geometry and 109.5° bond angles. CH₃ (A) is a
methyl group, not a complete molecule. C₂H₆ (C) is ethane. CH₃OH (D) is
methanol.
4. What is the oxidation number of a neutral molecule?
A. The sum of the oxidation numbers of all atoms equals the charge of the molecule, which
is 0
B. Always +1
C. Always −1
D. Depends on the number of atoms
CORRECT ANSWER A — The sum of the oxidation numbers of all atoms equals the charge of the
molecule, which is 0
RATIONALE For any neutral molecule, the sum of the oxidation numbers of all atoms must
equal zero (the overall charge). This is one of the fundamental rules for assigning
oxidation numbers. For a polyatomic ion (B/C are incorrect), the sum equals the
ion's charge. This rule is essential for determining unknown oxidation numbers in
compounds like H₂SO₄ or KMnO₄.
, 5. In thermodynamics and the Nernst equation, the gas constant R has what value in
J/(mol·K)?
A. 0.08206
B. 8.314
C. 96,500
D. 1.38 × 10⁻²³
CORRECT ANSWER B — 8.314
RATIONALE R = 8.314 J/(mol·K) is the gas constant in SI energy units, used in thermodynamics
(ΔG calculations) and electrochemistry (Nernst equation). R = 0.08206
L·atm/(mol·K) (A) is used in the ideal gas law with pressure in atmospheres. 96,500
C/mol (C) is Faraday's constant (F). 1.38 × 10⁻²³ J/K (D) is Boltzmann's constant (k).
Using the correct R value with matching units is critical.
6. A carbon atom has 4 groups attached to it (four single bonds). What is its hybridization,
bond angle, and molecular shape?
A. sp² hybridization; 120° bond angles; trigonal planar
B. sp³ hybridization; 109.5° bond angles; tetrahedral
C. sp hybridization; 180° bond angles; linear
D. sp³d hybridization; 90°/120° bond angles; trigonal bipyramidal
CORRECT ANSWER B — sp³ hybridization; 109.5° bond angles; tetrahedral
RATIONALE Four electron groups around a central atom require sp³ hybridization (one s +
three p orbitals = four sp³ hybrid orbitals). The bond angles are 109.5°, and the
molecular geometry is tetrahedral. Methane (CH₄) is the classic example. sp² (A)
applies to 3 groups (120°, trigonal planar). sp (C) applies to 2 groups (180°, linear).
★
University of Wisconsin–Madison
College of Letters & Science — Department of Chemistry
NUMEN LUMEN
EST. 1848
NUMEN LUMEN · GOD, OUR LIGHT
Chem 104 Exam 4
T H E R M O DY N A M I CS · E L E CT R O C H E M I ST RY · O R GA N I C C H E M I ST RY & H Y B R I D I Z AT I O N
INSTITUTION University of Wisconsin–Madison COURSE CODE CHEM 104
PROGRAM Bachelor of Science — General ACADEMIC YEAR
Chemistry Sequence
EXAM TITLE Chem 104 Exam 4 TOTAL QUESTIONS 68 Questions
COURSE TITLE General Chemistry II FORMAT Multiple Choice — Select the
Single Best Answer
EXAMINATION INSTRUCTIONS
▸ Select the single best answer for each question.
▸ Gibbs free energy, spontaneity, electrochemistry, oxidation-reduction, and organic chemistry
fundamentals are core testable content.
▸ Nernst equation calculations, cell potential predictions, and hybridization/geometry are emphasized.
▸ Correct answers and rationales appear below each question for self-assessment and review.
▸ All content reflects CHEM 104 General Chemistry II — Exam 4 learning objectives.
, SECTION I — THERMODYNAMICS, ELECTROCHEMISTRY
Questions 1 – 68
& ORGANIC CHEMISTRY
1. What is the oxidation number of free (uncombined) elements such as Na, Mg, Ar, O₂, O₃, S₈,
and H₂?
A. +1
B. −1
C. 0
D. Depends on the element
CORRECT ANSWER C—0
RATIONALE The oxidation number of any free (uncombined) element in its standard state is
always 0. This applies to monatomic species (Na, Mg, Ar), diatomic molecules (O₂,
H₂), and polyatomic elemental forms (O₃, S₈). This is a fundamental rule of
oxidation number assignment. Atoms in elements have no charge displacement
because they are bonded to identical atoms with equal electronegativity.
2. If a reaction is exothermic (ΔH < 0) AND has a positive ΔS (entropy increases), what can be
said about its spontaneity?
A. The reaction is never spontaneous
B. The reaction is spontaneous only at high temperatures
C. The reaction is entropically favored and would be spontaneous at any temperature
D. The reaction is spontaneous only at low temperatures
CORRECT ANSWER C — The reaction is entropically favored and would be spontaneous at any
temperature
RATIONALE ΔG = ΔH − TΔS. When ΔH is negative (favorable) and ΔS is positive (favorable),
both terms contribute to making ΔG negative regardless of temperature. The
reaction is both enthalpically and entropically favored, so it is spontaneous at all
temperatures. This is the most favorable thermodynamic scenario — both driving
forces work in the same direction.
,3. What is the chemical formula for methane, the simplest organic molecule?
A. CH₃
B. CH₄
C. C₂H₆
D. CH₃OH
CORRECT ANSWER B — CH₄
RATIONALE Methane has the formula CH₄ — one carbon atom bonded to four hydrogen
atoms. It is the simplest alkane and the simplest organic molecule. The carbon is
sp³ hybridized with a tetrahedral geometry and 109.5° bond angles. CH₃ (A) is a
methyl group, not a complete molecule. C₂H₆ (C) is ethane. CH₃OH (D) is
methanol.
4. What is the oxidation number of a neutral molecule?
A. The sum of the oxidation numbers of all atoms equals the charge of the molecule, which
is 0
B. Always +1
C. Always −1
D. Depends on the number of atoms
CORRECT ANSWER A — The sum of the oxidation numbers of all atoms equals the charge of the
molecule, which is 0
RATIONALE For any neutral molecule, the sum of the oxidation numbers of all atoms must
equal zero (the overall charge). This is one of the fundamental rules for assigning
oxidation numbers. For a polyatomic ion (B/C are incorrect), the sum equals the
ion's charge. This rule is essential for determining unknown oxidation numbers in
compounds like H₂SO₄ or KMnO₄.
, 5. In thermodynamics and the Nernst equation, the gas constant R has what value in
J/(mol·K)?
A. 0.08206
B. 8.314
C. 96,500
D. 1.38 × 10⁻²³
CORRECT ANSWER B — 8.314
RATIONALE R = 8.314 J/(mol·K) is the gas constant in SI energy units, used in thermodynamics
(ΔG calculations) and electrochemistry (Nernst equation). R = 0.08206
L·atm/(mol·K) (A) is used in the ideal gas law with pressure in atmospheres. 96,500
C/mol (C) is Faraday's constant (F). 1.38 × 10⁻²³ J/K (D) is Boltzmann's constant (k).
Using the correct R value with matching units is critical.
6. A carbon atom has 4 groups attached to it (four single bonds). What is its hybridization,
bond angle, and molecular shape?
A. sp² hybridization; 120° bond angles; trigonal planar
B. sp³ hybridization; 109.5° bond angles; tetrahedral
C. sp hybridization; 180° bond angles; linear
D. sp³d hybridization; 90°/120° bond angles; trigonal bipyramidal
CORRECT ANSWER B — sp³ hybridization; 109.5° bond angles; tetrahedral
RATIONALE Four electron groups around a central atom require sp³ hybridization (one s +
three p orbitals = four sp³ hybrid orbitals). The bond angles are 109.5°, and the
molecular geometry is tetrahedral. Methane (CH₄) is the classic example. sp² (A)
applies to 3 groups (120°, trigonal planar). sp (C) applies to 2 groups (180°, linear).
