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Compression is the reverse of expansion. We know that work=-Pex(delta)V when the pressure is constant (see the book for derivation). If we were working with an expansion problem, work should come out negative, since work is being done by the system rather than on the system. Now consider the case of compression. Compression will occur when the external pressure is greater than the pressure within the system, such as a piston. In this specific case, the work is then being done on the system, rather than by the system. As a result, the work done on the system will be positive. If you want to find the change in internal energy of this system, you would use, (delta)U=q+w. In the absence of one factor-either the q or the w-(delta)U would be equal to the other term. For example, if we assume that heat was not involved in some circumstance, then (delta)U=w. If both heat and work are involved in some situation, then you use that relationship mentioned above to find the change in internal energy. A negative sign will indicate that heat was released(exothermic), and a positive sign will indicate that heat was absorbed (endothermic).
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