At constant pressure, which of these systems do work on the surroundings? check all that apply.
a(s) + 2b(g) > c(g) 2a(g) + b(g) > 4c(g) a(g) + 2b(g) > 2c(g) a(s) + b(s) > c(g)

I believe the correct answers from the choices listed above are the second and the last option. At constant pressure, the systems 2A(g) + B(g) ---> 4C(g)  and 2C(g) A(s) + B(s) ---> C(g) produces work to the surroundings. When a gas is evolved during a chemical reaction, the gas can be imagined as displacing the atmosphere - pushing it back against the atmospheric pressure. The work done is therefore V*P where V is the volume of gas evolved, and P is the atmospheric pressure. 

Elias is almost correct. he just got it backwards.the correct answer is that it demonstrates elastic potential energy transforming into kinetic energy.