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An expansion is reversible when the temperature is constant. This process is gradual and requires more work. It is reversible because everything is at equilibrium and therefore entropy is equal to zero. During a reversible expansion, work is negative and the system is losing energy by expanding; however, the lost energy is being replaced by the surrounding heat which is why q is positive. On the other hand, expansion is irreversible when pressure is constant. This process occurs spontaneously and suddenly, so it requires less work and entropy is greater than zero.
Reversible reactions are always at equilibrium (entropy = 0) because all energy lost from expansion is replaced by the surrounding heat! In contrast, irreversible reactions occur quickly - too quickly for the energy to be replaced.
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