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Posted: Wed Jan 17, 2018 4:22 pm
In question 8.17, there are two pictures. One of melting and one of condensation. In each case, indicate whether heat is absorbed or given off by the system, whether expansion work is done on or by the system, and predict the signs of q and w for the process.
For melting why is work negative and for condensation why is work positive?
Posted: Wed Jan 17, 2018 4:34 pm
For melting, work is negative because work is done by the system. For condensation, work is positive because work is done on the system. Furthermore, enthalpy and heat are correlated with work.
Posted: Wed Jan 17, 2018 9:43 pm
When going from a solid to liquid, aka melting, heat is required without a change in temperature because it is a phase change. So, q is positive. For work, say ice is the solid. Ice, the system, gains energy, while the surroundings lose energy. This implies work is negative (because the system loses energy by doing work).
When going from gas to liquid, aka condensing, heat is released (also without a change in temperature). So, q is negative. For work, say steam is the gas. When the steam condenses, it loses energy, while the surroundings gain energy. So, work is positive.
(btw this question wasn't assigned haha)
Posted: Thu Jan 18, 2018 8:49 pm
When there is a phase change, such as when going from a solid to a liquid (melting), heat is required although the temperature does not change. Melting is an endothermic reaction so q will be positive and work is done by the system because it's losing energy so w is negative.
Condensation work is positive because work is being done on the system and heat is released so energy is gained and work is positive.