PCAT Chemistry 3 — Questions and Answers
Question 1: The rate-determining step of a reaction is best described as:
- The step with the lowest activation energy
- The fastest elementary step in a mechanism
- The slowest elementary step in a mechanism (Correct answer)
- The step that produces the most products
Correct answer: The slowest elementary step in a mechanism
The rate-determining step is the slowest step in a reaction mechanism and controls the overall rate of the reaction.
Question 2: Which of the following is a characteristic of a Lewis acid?
- It donates a proton (H+)
- It accepts a proton (H+)
- It donates an electron pair
- It accepts an electron pair (Correct answer)
Correct answer: It accepts an electron pair
A Lewis acid is an electron pair acceptor, which is a broader definition than the Brønsted-Lowry acid definition that requires proton donation.
Question 3: According to Hess's Law, the enthalpy change of an overall reaction:
- Is always exothermic
- Equals the sum of enthalpy changes of all steps (Correct answer)
- Cannot be calculated from individual steps
- Is independent of the path taken but depends on pressure
Correct answer: Equals the sum of enthalpy changes of all steps
Hess's Law states that the total enthalpy change for a reaction is the sum of enthalpy changes for each individual step, regardless of the pathway.
Question 4: Which of the following molecules has a trigonal pyramidal molecular geometry?
- BF3
- NH3 (Correct answer)
- H2O
- CO2
Correct answer: NH3
NH3 has a trigonal pyramidal geometry because nitrogen has three bonding pairs and one lone pair, which pushes the bonds downward.
Question 5: When the pressure on a gas is doubled at constant temperature, its volume:
- Doubles
- Quadruples
- Is halved (Correct answer)
- Remains the same
Correct answer: Is halved
According to Boyle's Law (P1V1 = P2V2), when pressure doubles at constant temperature, the volume is halved.
Question 6: The process by which a solid converts directly to a gas without passing through the liquid phase is called:
- Evaporation
- Condensation
- Sublimation (Correct answer)
- Deposition
Correct answer: Sublimation
Sublimation is the phase transition from solid directly to gas, as seen with dry ice (solid CO2) at room temperature and atmospheric pressure.
Question 7: Which of the following represents a correctly balanced net ionic equation for a strong acid-strong base neutralization?
- HCl + NaOH → NaCl + H2O
- H+ + OH- → H2O (Correct answer)
- H3O+ + OH- → 2H2O
- Na+ + Cl- → NaCl
Correct answer: H+ + OH- → H2O
The net ionic equation eliminates spectator ions, leaving only H+ + OH- → H2O as the essential reaction between a strong acid and strong base.
PCAT Chemistry 3
The rate-determining step of a reaction is best described as: