Physical Chemistry 1 — Questions and Answers

Question 1: What is the unit of the rate constant for a first-order reaction?

mol/L·s
s⁻¹ (Correct answer)
L/mol·s
mol²/L²·s

Correct answer: s⁻¹

For a first-order reaction, the rate law is expressed as Rate = k[A], where Rate has units of concentration per time (e.g., mol L⁻¹ s⁻¹) and [A] has units of concentration (e.g., mol L⁻¹). To make the units consistent, the rate constant (k) must have units of inverse time. Therefore, the unit for the rate constant of a first-order reaction is s⁻¹ (or min⁻¹, hr⁻¹, etc.).

Question 2: Which law relates the pressure, volume, and temperature of an ideal gas?

Boyle's law
Charles's law
Ideal gas law (Correct answer)
Avogadro's law

Correct answer: Ideal gas law

The Ideal Gas Law is a fundamental equation that describes the behavior of an ideal gas. It relates the pressure (P), volume (V), number of moles (n), and absolute temperature (T) of a gas through the equation PV=nRT, where R is the ideal gas constant. This law combines and extends Boyle's, Charles's, and Avogadro's laws into a single comprehensive relationship.

Question 3: What is the enthalpy change for a reaction at constant pressure called?

Work done
Internal energy change
Enthalpy change (Correct answer)
Entropy change

Correct answer: Enthalpy change

The enthalpy change (ΔH) for a reaction represents the heat absorbed or released during a chemical process carried out at constant pressure. It is a state function and is a key thermodynamic quantity used to describe the energy changes in chemical reactions. If ΔH is negative, the reaction is exothermic; if positive, it is endothermic.

Question 4: What is the term for the measure of disorder in a system?

Enthalpy
Free energy
Entropy (Correct answer)
Heat capacity

Correct answer: Entropy

Entropy (S) is a thermodynamic property that quantifies the degree of randomness or disorder within a system. A system with higher entropy has more possible microstates for its particles, indicating greater disorder. The second law of thermodynamics states that the total entropy of an isolated system can only increase over time, or remain constant in ideal cases.

Question 5: Which of the following processes is exothermic?

Melting
Evaporation
Condensation (Correct answer)
Sublimation

Correct answer: Condensation

An exothermic process is one that releases energy, typically in the form of heat, to its surroundings. Condensation is the phase transition where a gas changes into a liquid. During this process, the molecules lose kinetic energy and form intermolecular bonds, releasing the latent heat of vaporization into the surroundings, thus making it an exothermic process.

Question 6: What is the significance of a catalyst in a chemical reaction?

Increases activation energy
Decreases activation energy (Correct answer)
Changes the equilibrium constant
Consumes the reactants faster

Correct answer: Decreases activation energy

A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. It achieves this by providing an alternative reaction pathway that has a lower activation energy. By lowering the activation energy, more reactant molecules possess sufficient energy to react, thereby speeding up the reaction rate.

Physical Chemistry 1

What is the unit of the rate constant for a first-order reaction?