Calculate ΔE for the combustion of a gas that releases 235 kJ of energy to its surroundings and does 92.5 kJ of work on its surroundings.
ΔE = q+w, where ΔE is the change in energy, q is heat and w is work
Mayya's Trick: Think of the system as lazy and greedy. If the work is done on the system, it is positive. If the system has to do work, it is negative.
If heat is added to the system, it is positive. If heat is taken away from the system, it is negative.
Here, the system is the gas. Gas does 92.5 kJ of work on its surroundings, which means work will be negative.
It releases 235 kJ of energy to its surroundings which would be negative also.
ΔE = -235kJ + (-92.5 kJ ) = -328 kJ