Which of the following processes are spontaneous: (a) the...

Which of the following processes are spontaneous: (a) the melting of ice cubes at $-10^{\circ} \mathrm{C}$ and 1 atm pressure; (b) separating a mixture of $\mathrm{N}_{2}$ and $\mathrm{O}_{2}$ into two separate samples, one that is pure $\mathrm{N}_{2}$ and one that is pure $\mathrm{O}_{2} ;$ (c) alignment of iron filings in a magnetic field; (d) the reaction of hydrogen gas with oxygen gas to form water vapor; (e) the dissolution of $\mathrm{HCl}(g)$ in water to form concentrated hydrochloric acid?

Which of the following processes are spontaneous: (a) the melting of ice cubes at $-10^{\circ} \mathrm{C}$ and 1 atm pressure;
(b) separating a mixture of $\mathrm{N}_{2}$ and $\mathrm{O}_{2}$ into two separate samples, one that is pure $\mathrm{N}_{2}$ and one that is pure $\mathrm{O}_{2} ;$ (c) alignment of iron filings in a magnetic field; (d) the reaction of hydrogen gas with oxygen gas to form water vapor; (e) the dissolution of $\mathrm{HCl}(g)$ in water to form concentrated hydrochloric acid?

Key Concepts

Thermodynamic Spontaneity

Thermodynamic spontaneity refers to whether a process occurs on its own without the need for external energy input. It is determined by the change in free energy of the system, with a process typically proceeding spontaneously if the overall free energy decreases, moving the system toward a more stable state.

Gibbs Free Energy (?G)

Gibbs free energy is a thermodynamic potential that predicts the direction of chemical and physical processes at constant temperature and pressure. A negative ?G indicates that a process is spontaneous because it leads to a net release of energy, whereas a positive ?G means that the process is non?spontaneous without additional energy input.

Entropy and the Second Law of Thermodynamics

Entropy measures the disorder or randomness within a system, and the second law of thermodynamics states that the total entropy of the universe tends to increase in spontaneous processes. This concept helps explain why processes that increase the overall randomness, such as mixing substances, are often spontaneous.

Phase Transitions and Equilibrium Conditions

Phase transitions, such as melting and freezing, occur under specific equilibrium conditions where the free energies of the phases are balanced. Deviations from these conditions can render the phase transition non-spontaneous unless external parameters like temperature or pressure are adjusted to favor the transition.

Entropy of Mixing

The entropy of mixing refers to the increase in disorder that results when substances are combined. Mixing typically occurs spontaneously because it leads to a higher entropy state, while separation of a well-mixed system is usually non-spontaneous due to a resulting decrease in overall entropy.

Activation Energy versus Thermodynamic Favorability

While a process may be thermodynamically favorable (with a negative ?G), it can still proceed slowly if there is a high activation energy barrier. This concept emphasizes that kinetic factors, such as the energy needed to initiate a reaction, must also be considered alongside thermodynamic criteria when determining the rate at which a spontaneous process occurs.

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