At constant pressure, which of these systems do work on the surroundings? Check all that apply. a. 2A(g)+3B(g) --> 4C(g) b. A(s)+B(g) --> 2C(g) c. A(g)+B(g) --> 3C(g) d. A(s)+2B(g) --> C(g)

Answer 1

At constant pressure, work (w) is equal to the pressure times the change in volume, and if the change in volume is positive, w will be negative, and that means work was done BY the system ON the surroundings.

w = -P∆V

So, for this problem we want to see which reactions have a POSITIVE change in volume:'

a). 2A(g) + 3B(g) ==> 4C(g) has 5 mols on left and 4 mols on right = reduction in volume = no

b). A(s) + B(g) ==> 2C(g) has 1 mol gas on left and 2 on right = increase in volume = yes

c). 2C(g) + B(g) ==> 3C(g) has 3 mol gas on left and right = no change in volume = no

d). A(s) + 2B(g) ==> C(g) has 2 mol gas on left and 1 on right = reduction in volume = no

So, the only system that does work on the surroundings would be system (b).