Spontaneity of reactions Knowledge-Based Practice

Questions

1. 1.

   Which of the following statements correctly describes a chemical reaction that is thermodynamically spontaneous under a given set of conditions?

   • A) It must proceed at an observable and rapid rate under those conditions.
   • B) It has a natural tendency to proceed in the forward direction without needing continuous external work.
   • C) It must be exothermic and transfer thermal energy to the surroundings.
   • D) It possesses an activation energy of zero and requires no initial energy input.
2. 2.

   According to the second law of thermodynamics, which of the following criteria must always be satisfied for any process to be spontaneous?

   • A) The change in enthalpy of the system must be strictly negative ($\Delta H_{\text{sys}} < 0$).
   • B) The entropy of the system must increase during the process ($\Delta S_{\text{sys}} > 0$).
   • C) The entropy of the surroundings must increase during the process ($\Delta S_{\text{surr}} > 0$).
   • D) The total change in entropy of the universe must be positive ($\Delta S_{\text{univ}} > 0$).
3. 3.

   A reversible chemical reaction has a positive standard Gibbs free energy change ($\Delta G^\circ > 0$) at a specified temperature. Under which condition will this reaction proceed spontaneously in the forward direction at this temperature?

   • A) When the reaction quotient is equal to the equilibrium constant ($Q = K$)
   • B) When the reaction quotient is strictly less than the equilibrium constant ($Q < K$)
   • C) When the reaction quotient is strictly greater than the equilibrium constant ($Q > K$)
   • D) When the product of temperature and entropy change equals enthalpy change ($T\Delta S^\circ = \Delta H^\circ$)
4. 4.

   A chemical reaction has a standard enthalpy change of $\Delta H^\circ = -80\text{ kJ}$ and a standard entropy change of $\Delta S^\circ = -200\text{ J/K}$. At which of the following temperatures is this reaction spontaneous under standard conditions?

   • A) 500 K
   • B) 450 K
   • C) 400 K
   • D) 350 K
5. 5.

   Under which set of thermodynamic conditions is a reaction nonspontaneous at low temperatures but spontaneous at high temperatures?

   • A) $\Delta H < 0$ and $\Delta S < 0$
   • B) $\Delta H > 0$ and $\Delta S > 0$
   • C) $\Delta H > 0$ and $\Delta S < 0$
   • D) $\Delta H < 0$ and $\Delta S > 0$

Answers and step-by-step solutions appear after you finish the test.