Final answer:
When Janica places an ice cube in a hot thermos of coffee, the system's energy remains constant, with kinetic energy transferring from the hot water to the ice to melt it, thereby causing an increase in the system's entropy without changing the total energy.
Step-by-step explanation:
When Janica places an ice cube in a hot thermos of coffee, the energy in the system remains constant, assuming the thermos is perfectly insulated and no heat is exchanged with the surroundings. This is in accordance with the principle of conservation of energy. During the process, kinetic energy is transferred from the hot water molecules to the less energetic water molecules in the ice cube, causing the ice to melt. This heat transfer does not increase the overall energy in the system, but it does facilitate a phase change and increase the entropy of the system as the highly ordered solid structure of ice is converted into a more disorderly liquid state.