Write and simplify the closed-system energy balance (Equation 7.3-4) for each of the following processes, and state whether nonzero heat and work terms are positive or negative. Begin by defining the system. The solution of Part (a) is given as an illustration. (a) The contents of a closed flask are heated from 25 degree C to 80 degree C. Solution. The system is the flask contents. Q + W = Delta U + Delta E_k + Delta E_p W = 0 (no moving parts or generated currents) Delta E_k = 0 (system is stationary). Delta E_p = 0 (no height change) Q = Delta U Q > 0 (heat is transferred to the system) (b) A tray filled with water at 20 degree C is put into a freezer. The water turns into ice at -5 degree C. (c) A chemical reaction takes place in a closed adiabatic (perfectly insulated) rigid container. (d) Repeat Part (c), only suppose that the reactor is isothermal rather than adiabatic and that when the reaction was carried out adiabatically, the temperature in the reactor increased.