Final answer:
In chemistry thermodynamics, heat and work have negative signs when energy is transferred from the system to the surroundings, and vice versa. Heat absorbed by the system is positive, while heat released is negative. Work by the system is negative, and work on the system is positive.
Step-by-step explanation:
In chemistry thermodynamics, the sign conventions for heat and work are established to maintain consistency when describing energy transfers between a system and its surroundings. By convention, both heat flow and work have a negative sign when energy is transferred from the system to the surroundings, and vice versa.
That is, if heat is absorbed by the system from the surroundings, it is considered positive; if heat is released by the system to the surroundings, it is considered negative.
For work, the same rule applies. Work done by the system on the surroundings is considered negative, which reflects the system losing energy. When work is done on the system by the surroundings (e.g., compressing a gas), it is considered positive, as this adds energy to the system.
The equation w = -PextΔV reflects this convention for work associated with pressure-volume changes, with w representing work, Pext the external pressure, and ΔV the change in volume.
In summary, a negative sign for heat means energy release, and a positive sign means energy absorption by the system. Similarly, a positive work value signifies work done on the system and a negative work value reflects work done by the system. Understanding these conventions is critical for correctly interpreting and applying thermodynamic principles to chemical reactions and processes.