1. The problem is to understand the entropy equation in thermodynamics. 2. Entropy ($S$) is a measure of the disorder or randomness in a system. 3. The fundamental formula for entropy change ($\Delta S$) when heat ($Q$) is transferred reversibly at temperature ($T$) is: $$\Delta S = \frac{Q_{rev}}{T}$$ 4. Important rules: - $Q_{rev}$ is the heat exchanged in a reversible process. - Temperature $T$ must be in Kelvin. 5. For a system going from state 1 to state 2, the total entropy change is: $$\Delta S = S_2 - S_1 = \int_{1}^{2} \frac{dQ_{rev}}{T}$$ 6. In statistical mechanics, entropy is related to the number of microstates ($\Omega$) by Boltzmann's equation: $$S = k_B \ln \Omega$$ where $k_B$ is Boltzmann's constant. 7. Summary: Entropy quantifies disorder and is calculated by heat transfer over temperature or by counting microstates. Final answer: The entropy change formula is $$\Delta S = \frac{Q_{rev}}{T}$$ for reversible processes.