Spontaneity — Prelims Strategy

Prelims Strategy

To effectively tackle NEET questions on spontaneity, a structured approach is essential:

1
Master the Gibbs Equation: — The equation $\Delta G = \Delta H - T\Delta S$ is your central tool. Understand each term and its units. Always convert units (e.g., J to kJ) to be consistent before calculation.
2
Qualitative Analysis: — Practice predicting spontaneity based on the signs of $\Delta H$ and $\Delta S$ for all four combinations (e.g., $\Delta H < 0, \Delta S > 0$ always spontaneous; $\Delta H > 0, \Delta S < 0$ never spontaneous). For cases where spontaneity is temperature-dependent, know how to determine the threshold temperature ($T = \Delta H / \Delta S$).
3
Distinguish Spontaneity from Rate: — This is a major trap. Spontaneity (thermodynamics) tells you if a reaction *can* happen; rate (kinetics) tells you *how fast* it happens. A spontaneous reaction can be very slow.
4
Equilibrium Condition: — Remember that $\Delta G = 0$ at equilibrium. Use this to find the temperature at which a phase transition occurs or a reaction shifts from spontaneous to non-spontaneous.
5
Relationship with K: — Be comfortable using $\Delta G^circ = -RT \ln K$. Pay attention to the value of R (8.314 J/mol·K) and ensure $\Delta G^circ$ is in Joules.
6
Phase Transitions: — Understand the signs of $\Delta H$ and $\Delta S$ for melting, freezing, vaporization, and condensation, and how temperature influences their spontaneity.
7
Practice Numerical Problems: — Work through a variety of problems involving calculations of $\Delta G$, $\Delta H$, $\Delta S$, and $K$. Pay close attention to signs and units. Don't shy away from problems involving logarithms and exponentials, as they are common in $K$ calculations.