Identify each of the following energy exchanges 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.)

ΔE negative,ΔE positive,ΔE positive

Explanation:

Internal energy change of a system is positive when the system acquires energy from outside and internal energy change is negative when energy is being liberated from the system.

a Sweat evaporates from skin,cooling the skin ΔE is negative as heat energy is liberated through evaporation of sweat.

b A balloon expand against an external pressure ΔE  positive because external energy is required to expand the balloon.

c An aqueous chemical reaction mixture is warmed with an external flame ΔE is positive because heat energy is provided by the external flame to warm the aqueous chemical reaction.