A disposable chemical heat pack is activated inside a pair of cold hands. Initially, the heat pack contents are at 15∘C and the hands are at 15∘C. During the ensuing reaction the heat pack releases 120kJ of thermal energy, after which both the heat pack and the hands reach a common final temperature of 30∘C. Neglect any energy exchange with the external environment.
(a) If the hands (without the heat pack) are chosen as the system, state whether the hands experience heating or cooling, and indicate the sign of Q for the hands. Explain your reasoning.
(b) If the contents of the heat pack are chosen as the system, state whether that system experiences heating or cooling, providing the sign of Q and justification.
(c) Draw and label a qualitative energy-flow diagram showing the direction of energy transfer between the heat pack contents and the hands, including appropriate +Q or −Q labels for each system defined in parts (a) and (b).
(d) After both objects reach 30∘C, describe how the microscopic interactions between particles lead to the cessation of net energy transfer even though molecular collisions continue.