Could someone please explain...?

Question

Here is the problem
Calculate delta E (change in internal energy) and determine whether the process is endothermic or exothermic for the following cases:
a a system absorbs 85 kj of heat from it's surroundings while doing 29 kj of work on the surroundings.
B. Q equals 1.50 kj and w equals -.657kj
c the system releases 57.5 kj of heat while doing 13.5 kj of work on the surroundings.
answers.
a 56kj endothermic
b. .84kj endo
c. -71 exothermic

What I don't understand is why the formula changes. I thought it is always supposed to be change in E = q + w but for a it is q-w, b it is q+w, and c it is -(q+w). Could someone please explain? It's probably something simple I just am missing. Thanks a lot.

Answer 1

The formula is always E = q + w It is just that you have to recognize when q and w are positive or negative. If heat is leaving the system, the sign is - (just like a bank account) and when heat is entering the system, the sign is + When work is being done by the system, the sign is - and when work is being done on the system, the sign is +

Lets see the first one

85 kJ going in so this is + doing 29 kJ of work so this is -

E = q + w = (+85) + (-29) = +56 kJ A positive sign means heat going into the system (into = endo) so endothermic. try the others and be careful about the signs of q and m.

Answer 2

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