CPC CPC Physical Chemistry & Thermodynamics 1 — Questions and Answers
Question 1: According to the first law of thermodynamics, which statement is correct?
- Energy can be created from nothing in a chemical reaction
- The total energy of an isolated system is constant (Correct answer)
- Entropy always increases in any process
- Enthalpy equals internal energy at constant pressure
Correct answer: The total energy of an isolated system is constant
The first law states that energy is conserved: ΔU = q + w, meaning the internal energy change equals the heat added plus the work done on the system.
Question 2: The Gibbs free energy change (ΔG) at standard conditions for a spontaneous reaction must be:
- Positive
- Negative (Correct answer)
- Zero
- Equal to ΔH
Correct answer: Negative
A spontaneous process at constant temperature and pressure has ΔG < 0, indicating the system releases usable free energy.
Question 3: The van't Hoff equation relates the equilibrium constant K to temperature through which thermodynamic quantity?
- ΔCp
- ΔH° (Correct answer)
- ΔS°
- ΔG°
Correct answer: ΔH°
The van't Hoff equation, d(ln K)/dT = ΔH°/RT², shows how K changes with temperature based on the standard enthalpy of reaction.
Question 4: Which colligative property is used to determine the molar mass of a large polymer in solution?
- Boiling point elevation
- Freezing point depression
- Osmotic pressure (Correct answer)
- Vapor pressure lowering
Correct answer: Osmotic pressure
Osmotic pressure (π = MRT) is the most sensitive colligative property for large molecules because it produces measurable pressures even at very low molar concentrations.
Question 5: In the Arrhenius equation k = A·e^(−Ea/RT), increasing the temperature primarily affects the reaction rate by:
- Decreasing the activation energy
- Increasing the fraction of molecules with sufficient energy to react (Correct answer)
- Changing the pre-exponential factor A
- Reducing molecular collisions
Correct answer: Increasing the fraction of molecules with sufficient energy to react
Higher temperature increases the Boltzmann factor e^(−Ea/RT), meaning a larger fraction of molecules have kinetic energy exceeding the activation energy Ea.
Question 6: Which statement correctly describes an ideal gas according to kinetic-molecular theory?
- Molecules have finite volume and attractive forces between them
- Molecules have negligible volume and no intermolecular forces (Correct answer)
- Pressure is independent of temperature
- Molecules move in straight paths except near walls
Correct answer: Molecules have negligible volume and no intermolecular forces
Ideal gas molecules are treated as point masses with no volume and no intermolecular forces, with all kinetic energy being translational.
CPC CPC Physical Chemistry & Thermodynamics 1
According to the first law of thermodynamics, which statement is correct?