Thermodynamics and Equilibrium Essentials Quiz Quiz

Challenge your understanding of fundamental thermodynamic principles and chemical equilibrium concepts, including definitions, equations, and real-world applications. Enhance your grasp of entropy, spontaneity, equilibrium shifts, and key laws relevant to thermodynamics and chemical reactions.

1. First Law of Thermodynamics

   Which of the following best describes the first law of thermodynamics when a gas is compressed in a piston and its temperature increases?

   1. Energy cannot be created or destroyed, only transformed.
   2. Pressure and volume remain constant.
   3. Work done by the system is always zero.
   4. Entropy of a system always increases.

   Explanation: The first law of thermodynamics states that energy is conserved; it can be transformed from one form to another but not created or destroyed. In the example, the work done to compress the gas is converted into internal energy, raising the temperature. Entropy (option B) is related to the second law, not the first. Work done by the system can be nonzero (option C), especially during compression. Pressure and volume do not have to remain constant (option D); both can change during such processes.

2. Entropy and Spontaneity

   When ice melts at room temperature, which thermodynamic factor primarily drives the spontaneity of this process?

   1. Decrease in temperature
   2. Decrease in enthalpy (ΔH u003C 0)
   3. Increase in entropy (ΔS u003E 0)
   4. Increase in pressure

   Explanation: Melting ice involves a transition from an ordered solid to a disordered liquid, resulting in an increase in entropy, which drives spontaneity at room temperature. A decrease in enthalpy is not the main factor, as melting is actually endothermic (ΔH u003E 0), so option A is incorrect. Temperature usually increases spontaneity at higher values but is not the primary thermodynamic factor here (option C). Pressure changes are minimal in common melting scenarios (option D), so they do not drive the process.

3. Le Chatelier’s Principle

   If more hydrogen gas is added to the system at equilibrium: N2(g) + 3H2(g) ⇌ 2NH3(g), how will the equilibrium shift?

   1. No change in position
   2. Toward the products (right)
   3. Toward the reactants (left)
   4. Equilibrium constant decreases

   Explanation: According to Le Chatelier’s Principle, adding more reactant (hydrogen gas) drives the equilibrium toward the products to counteract the change. Shifting toward the reactants (option B) would be the case if more product was added. No change (option C) would occur if neither reactants nor products were added. The equilibrium constant (option D) only changes with temperature, not reactant concentrations.

4. Gibbs Free Energy

   For a chemical reaction at constant temperature and pressure, which condition ensures the reaction is spontaneous?

   1. Entropy is completely unchanged
   2. Gibbs free energy change (ΔG) is negative
   3. Gibbs free energy change (ΔG) is positive
   4. The enthalpy is at a maximum

   Explanation: A negative Gibbs free energy change means the reaction is spontaneous under constant temperature and pressure. A positive ΔG (option A) indicates a nonspontaneous process. Maximum enthalpy (option C) is not a criterion for spontaneity, and entropy being unchanged (option D) does not determine spontaneity; changes in both enthalpy and entropy contribute to ΔG.

5. Equilibrium Constant Expression

   Given the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), what is the correct equilibrium constant (Kc) expression?

   1. Kc = ([SO2] + [O2]) / [SO3]
   2. Kc = [SO3] / ([SO2][O2])
   3. Kc = [SO2]^2 [O2] / [SO3]^2
   4. Kc = [SO3]^2 / ([SO2]^2 [O2])

   Explanation: The equilibrium constant is written as the product of the concentrations of the products raised to their coefficients divided by the product of the reactants’ concentrations raised to theirs. Option A correctly expresses this. Option B reverses the correct numerator and denominator. Option C fails to use the correct stoichiometric powers, and option D incorrectly uses addition instead of multiplication in the expression.