Calculating the total internal energy of a system (thermochemistry)

system is changed in a two step process:

Step 1: The system absorbs 70 J of heat while 30 J of work is done on it.
Step 2: The system releases 30 J of heat to the surroundings while performing 70 J of work.

What is ΔU for the system overall?

I'm thinking that because heat is absorbed and work is done on the system in the first part, that is 100J. The second part is -100J because work is done by the system and heat is released.

Do these add together to make 0J?


Thanks for any help =)

well you were close for step 1 you were right it is 100J and for the second part, when a system releases heat it is negative BUT the system does 70J of work so thats positive so:

step 1: 70J + 30J= 100J
step 2: -30J + 70J= 40J

Delta U= 100-40= 60J of energy

hope that helps