Question

Identify each energy exchange as primarily heat or work and determine whether the sign of δe is positive or negative for the system.
a. sweat evaporates from skin, cooling the skin. (the evaporating sweat is the system.)
b. a balloon expands against an external pressure. (the contents of the balloon is the system.)
c. an aqueous chemical reaction mixture is warmed with an external flame. (the reaction mixture is the system.)

Answer 1

A. Heat. Positive as the sweat gains heat to evaporate.

B. Work. Negative as the balloon does work to expand.

C. Heat. Positive as heat is transferred to the mixture to warm it up.

Answer 2

Heat is released in an exothermic reaction and heat is absorbed in an endothermic reaction.  

Heat is negative for an exothermic reaction and it is positive for an exothermic reaction.  

Work done by a system is positive and work done on the system is negative.  

a. The evaporating sweat is the system. Here the sweat absorbs heat to make the phase transition from liquid water to vapor. The reaction is endothermic and the heat is positive.

b. The balloon (system) does work to expand against an external pressure. As the work is done by the system it is positive.

c. As the chemical reaction is warmed it has absorbed heat. The reaction is endothermic and the heat is positive.