Calculate the change in internal energy (∆E) for a system that is giving off 55.0 kJ of heat and is performing 855 J of work on the surroundings.

Question

A) -54.1 kJ

B) -55.9 kJ

C) -9.00 × 102 kJ

D) 8.00 × 102 kJ

E) 54.1 kJ

According to the answer key, the answer is B but I keep getting D. I don't get it. I'm using the ∆E=q+w equation.

Answer 1

In order to add/subtract numbers they must have the same units. So 855J = 0.855kJ.

Also, if work is done on the surroundings, that means the system is losing internal energy (so w=-0.855kJ).

Hopefully this helps clear things up!