reversible expansion and slow changes

[Lynne Zhao 2B]

Hi, could someone explain why slow expansions would be reversible and sudden or fast expansions would be irreversible?

[Anthony Tam]

Hi so for reversible reactions, the pressure it not constant. You adjust the pressure of the external pressure by a infinitely small amount and wait for the internal pressure of the system to adjust so that they are equal. You repeat this process over and over again and each time the system is working against a pressure basically equal to the internal pressure. This causes the process to be slow. In an irreversible reaction, the external pressure is constant and usually less than the internal pressure, so any changes in the internal system can happen really quickly (since the system is working against a lower pressure). Hope this makes sense and helps!