# Entropy, enthalpy, and reversibility

In my introductory chemistry course, we learned that for reversible processes, entropy is defined as $ΔS = q/T$. However, we did not spend that long on this before moving on to other things like Gibbs free energy. My questions are:
1. What exactly is a reversible process?
2. Why must we use things like standard molar entropies to calculate $\Delta S$ instead of just using $\Delta H$ according to this equation?

Next, the equation for the entropy above is not correct except at constant temperature. In reality it is $dS = dq/T$, which generally must be integrated in order to determine the entropy by this method.
For the second question, there is some confusion here. $\Delta H$ is equal to $q$ only at constant pressure.