Chemical Thermodynamics Part 3

Concept:First law of thermodynamics: $\Delta U = q + w$.When a system absorbs heat and expands, $q > 0$ and $w < 0$, so $\Delta U$ can be positive if heat gained exceeds work done.Explanation:Water at $5^\circ \mathrm{C}$ is heated in a microwave.It first warms to $100^\circ \mathrm{C}$ (sensible heating).Then it begins to boil: liquid water changes to vapour at constant temperature.During boiling, volume increases significantly as steam forms.Hence the system does work on the surroundings: $w$ is negative.Microwave supplies energy to the water, so heat $q$ is absorbed (positive).Internal energy of steam at $100^\circ \mathrm{C}$ is higher than that of liquid water at the same temperature due to greater kinetic and potential energy of molecules.Thus $\Delta U$ is positive overall.Therefore, $q=+ve$, $w=-ve$, $\Delta U=+ve$.Answer:Option A: $q=+ve$, $w=-ve$, $\Delta U=+ve$.