MCAT PRACTICE TEST 11
,AAMC MCAT Practice Test 11— Answer Key Companion
Passage I
The Haber process of ammonia production is a classic example of a reaction limited by thermodynamic constraints. The
overall conversion is given in unbalanced form as Reaction 1.
N2(g) + H2(g) NH3(g)
Reaction 1
Because the gas volume decreases in the reaction, high pressure must be used to drive the reaction to the right. Because it is
exothermic, high temperature favors the reverse reaction. However, high temperatures are required to achieve a reaction rate f
enough for industrial use. Despite these limitations, the Haber process remains the dominant route for the production of ammon
however, satisfactory commercial production requires a catalyst.
Electrolytic cells also can be used to produce ammonia. In one such method, hydrogen is pumped into one compartment of
reactor constructed of the ceramic strontia-ceria-ytterbia (SCY); it is converted to protons according to Half-Reaction 2.
H2(g) → 2H+ + 2e–
Half-Reaction 2
The electrolytically generated protons are then transported through the solid SCY electrolyte to react with the nitrogen at the othe
electrode according to Half-Reaction 3. SCY conductors are favored for this use because their proton conductivities increase
substantially with temperature.
N2(g) + 6H+ + 6e– → 2NH3(g)
Half-Reaction 3
While the Haber process generally operates at about 450°C and 15–30 atm, an electrolytic cell operates most efficiently at
about 600°C and atmospheric pressure. In both cases the upper operating temperatures are limited by the reversible
decomposition of ammonia, which is insignificant below 300°C, but increases dramatically thereafter.
, 1. The electrochemical cell can be operated at atmospheric rather than high pressure because:
A) Le Chatelier’s principle does not apply to reactions in electrochemical cells.
B) electrical work serves to drive the reaction.
C) SCY catalyzes the reaction.
D) oxidation–reduction processes are unaffected by pressure.
ANS:: B
Elaboration: The electrolytic ammonia method can run at atmospheric pressure because electrical work (the applied potential)
supplies the free energy needed to drive the N₂ + H⁺ → NH₃ conversion in the cell. In other words, instead of using high pressure to
shift equilibrium (as in Haber), the cell uses electrical energy to push the
2. In the overall electrochemical reaction:
A) nitrogen is oxidized at the anode, and hydrogen is reduced at the cathode.
B) nitrogen is reduced at the cathode, and hydrogen is oxidized at the anode.
C) nitrogen is reduced at the anode, and hydrogen is oxidized at the cathode.
D) nitrogen is oxidized at the cathode, and hydrogen is reduced at the anode.
ANS:: B
Elaboration: In the cell the H₂ half-reaction is H₂ → 2H⁺ + 2e⁻ (oxidation) so hydrogen is oxidized at the anode.
Nitrogen gains electrons in the nitrogen half-reaction (reduction) at the cathode to form NH₃. So nitrogen is reduced at
the cathode and hydrogen oxidized at the anode
3. In industrial use, ammonia is continuously removed from the reaction mixture. This serves to drive Reaction 1
because of:
A) Boyle’s law.
B) Charles’s law.
C) Heisenberg’s principle.
D) Le Chatelier’s principle.
ANS:: D
Elaboration: Continuously removing ammonia shifts the chemical equilibrium to the right (toward product formation) — this is a
direct application of Le Chatelier’s principle (removing a product drives the reaction forward).
4. The lone pair of electrons in ammonia allows the molecule to:
A) assume a planar structure.
B) act as an oxidizing agent.
C) act as a Lewis acid in water.
D) act as a Lewis base in water.
ANS:: D
Elaboration: The lone pair on the nitrogen in NH₃ allows ammonia to donate an electron pair to acceptors (protons or Lewis acid
That behavior defines a Lewis base (electron-pair donor).redox chemistry forward.
5. What is the role of the solid-state catalyst in the Haber process?
A) It increases the amount of ammonia produced per unit time.
B) It increases the total amount of ammonia produced.
C) It decreases the amount of ammonia that decomposes per unit time.
, D) It decreases the total amount of ammonia produced.
ANS:: A
Elaboration: A catalyst lowers activation energy and speeds how fast a reaction reaches equilibrium (increases rate)
but does not change the equilibrium composition (the final amounts at equilibrium remain the same).
6. It is possible to design a reactor where the SCY conductor and the nitrogen/ammonia electrode operate at different
temperatures. Which combination of temperatures is expected to give the best results?
A) SCY temperature higher than electrode temperature
B) SCY temperature lower than electrode temperature
C) SCY temperature the same as electrode temperature
D) The temperature of the components does not make a difference.
ANS:: A
Elaboration: Proton conductivity of the SCY solid electrolyte increases strongly with temperature, so the material performs bes
higher temperature; operating at elevated T increases proton mobility and the cell’s ionic conductivity, improving the
electrolytic production efficiency.
,AAMC MCAT Practice Test 11— Answer Key Companion
Passage I
The Haber process of ammonia production is a classic example of a reaction limited by thermodynamic constraints. The
overall conversion is given in unbalanced form as Reaction 1.
N2(g) + H2(g) NH3(g)
Reaction 1
Because the gas volume decreases in the reaction, high pressure must be used to drive the reaction to the right. Because it is
exothermic, high temperature favors the reverse reaction. However, high temperatures are required to achieve a reaction rate f
enough for industrial use. Despite these limitations, the Haber process remains the dominant route for the production of ammon
however, satisfactory commercial production requires a catalyst.
Electrolytic cells also can be used to produce ammonia. In one such method, hydrogen is pumped into one compartment of
reactor constructed of the ceramic strontia-ceria-ytterbia (SCY); it is converted to protons according to Half-Reaction 2.
H2(g) → 2H+ + 2e–
Half-Reaction 2
The electrolytically generated protons are then transported through the solid SCY electrolyte to react with the nitrogen at the othe
electrode according to Half-Reaction 3. SCY conductors are favored for this use because their proton conductivities increase
substantially with temperature.
N2(g) + 6H+ + 6e– → 2NH3(g)
Half-Reaction 3
While the Haber process generally operates at about 450°C and 15–30 atm, an electrolytic cell operates most efficiently at
about 600°C and atmospheric pressure. In both cases the upper operating temperatures are limited by the reversible
decomposition of ammonia, which is insignificant below 300°C, but increases dramatically thereafter.
, 1. The electrochemical cell can be operated at atmospheric rather than high pressure because:
A) Le Chatelier’s principle does not apply to reactions in electrochemical cells.
B) electrical work serves to drive the reaction.
C) SCY catalyzes the reaction.
D) oxidation–reduction processes are unaffected by pressure.
ANS:: B
Elaboration: The electrolytic ammonia method can run at atmospheric pressure because electrical work (the applied potential)
supplies the free energy needed to drive the N₂ + H⁺ → NH₃ conversion in the cell. In other words, instead of using high pressure to
shift equilibrium (as in Haber), the cell uses electrical energy to push the
2. In the overall electrochemical reaction:
A) nitrogen is oxidized at the anode, and hydrogen is reduced at the cathode.
B) nitrogen is reduced at the cathode, and hydrogen is oxidized at the anode.
C) nitrogen is reduced at the anode, and hydrogen is oxidized at the cathode.
D) nitrogen is oxidized at the cathode, and hydrogen is reduced at the anode.
ANS:: B
Elaboration: In the cell the H₂ half-reaction is H₂ → 2H⁺ + 2e⁻ (oxidation) so hydrogen is oxidized at the anode.
Nitrogen gains electrons in the nitrogen half-reaction (reduction) at the cathode to form NH₃. So nitrogen is reduced at
the cathode and hydrogen oxidized at the anode
3. In industrial use, ammonia is continuously removed from the reaction mixture. This serves to drive Reaction 1
because of:
A) Boyle’s law.
B) Charles’s law.
C) Heisenberg’s principle.
D) Le Chatelier’s principle.
ANS:: D
Elaboration: Continuously removing ammonia shifts the chemical equilibrium to the right (toward product formation) — this is a
direct application of Le Chatelier’s principle (removing a product drives the reaction forward).
4. The lone pair of electrons in ammonia allows the molecule to:
A) assume a planar structure.
B) act as an oxidizing agent.
C) act as a Lewis acid in water.
D) act as a Lewis base in water.
ANS:: D
Elaboration: The lone pair on the nitrogen in NH₃ allows ammonia to donate an electron pair to acceptors (protons or Lewis acid
That behavior defines a Lewis base (electron-pair donor).redox chemistry forward.
5. What is the role of the solid-state catalyst in the Haber process?
A) It increases the amount of ammonia produced per unit time.
B) It increases the total amount of ammonia produced.
C) It decreases the amount of ammonia that decomposes per unit time.
, D) It decreases the total amount of ammonia produced.
ANS:: A
Elaboration: A catalyst lowers activation energy and speeds how fast a reaction reaches equilibrium (increases rate)
but does not change the equilibrium composition (the final amounts at equilibrium remain the same).
6. It is possible to design a reactor where the SCY conductor and the nitrogen/ammonia electrode operate at different
temperatures. Which combination of temperatures is expected to give the best results?
A) SCY temperature higher than electrode temperature
B) SCY temperature lower than electrode temperature
C) SCY temperature the same as electrode temperature
D) The temperature of the components does not make a difference.
ANS:: A
Elaboration: Proton conductivity of the SCY solid electrolyte increases strongly with temperature, so the material performs bes
higher temperature; operating at elevated T increases proton mobility and the cell’s ionic conductivity, improving the
electrolytic production efficiency.
