MCAT Chemistry 3 — Questions and Answers
Question 1: Which of the following best explains why ionic compounds generally have high melting points?
- They contain covalent bonds that require high energy to break
- Strong electrostatic attractions between oppositely charged ions require large amounts of energy to overcome (Correct answer)
- They have high molecular weights causing strong dispersion forces
- Hydrogen bonding between ions is very strong
Correct answer: Strong electrostatic attractions between oppositely charged ions require large amounts of energy to overcome
Ionic compounds form a crystal lattice held together by strong electrostatic forces between cations and anions, requiring substantial energy (high temperature) to disrupt.
Question 2: What product is formed when an aldehyde undergoes nucleophilic addition with an alcohol?
- Carboxylic acid
- Ketone
- Hemiacetal (Correct answer)
- Ester
Correct answer: Hemiacetal
When one equivalent of alcohol attacks the carbonyl carbon of an aldehyde, a hemiacetal is formed with both an -OH and an -OR group on the same carbon.
Question 3: Which of the following is true about an enzyme at its optimal pH?
- The enzyme has maximum catalytic activity (Correct answer)
- The enzyme is permanently denatured
- The enzyme's active site is blocked by inhibitors
- The enzyme converts substrate to product at its slowest rate
Correct answer: The enzyme has maximum catalytic activity
At its optimal pH, the ionization states of the enzyme's active site residues are ideal for substrate binding and catalysis, maximizing reaction rate.
Question 4: A galvanic cell uses the reaction: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). Which statement is correct?
- Zinc is reduced at the cathode
- Copper is oxidized at the anode
- Zinc is oxidized at the anode (Correct answer)
- The cell produces no electrical current
Correct answer: Zinc is oxidized at the anode
In a galvanic cell, the more active metal (zinc) is oxidized at the anode: Zn → Zn²⁺ + 2e⁻, while Cu²⁺ is reduced at the cathode.
Question 5: Which of the following represents an example of a conjugate acid-base pair?
- H₂O and H₃O⁺
- HCl and NaOH
- NH₃ and NH₄⁺ (Correct answer)
- H₂SO₄ and SO₄²⁻
Correct answer: NH₃ and NH₄⁺
NH₃ and NH₄⁺ differ by a single proton (H⁺), making them a conjugate acid-base pair where NH₄⁺ is the conjugate acid of NH₃.
Question 6: The rate constant of a reaction doubles when temperature increases from 25°C to 35°C. This is best explained by:
- A decrease in activation energy at higher temperatures
- An increase in the fraction of molecules with energy exceeding the activation energy (Correct answer)
- A change in the reaction mechanism at higher temperatures
- An increase in the concentration of reactants
Correct answer: An increase in the fraction of molecules with energy exceeding the activation energy
According to the Arrhenius equation and Maxwell-Boltzmann distribution, higher temperatures shift more molecules to higher energies, increasing the fraction that can overcome the activation barrier.
Question 7: Which isomers are mirror images of each other and are non-superimposable?
- Constitutional isomers
- Geometric isomers
- Enantiomers (Correct answer)
- Conformational isomers
Correct answer: Enantiomers
Enantiomers are stereoisomers that are non-superimposable mirror images of each other, differing in the spatial arrangement of groups around a chiral center.
MCAT Chemistry 3
Which of the following best explains why ionic compounds generally have high melting points?