If a system has 3.00x10^2 kcal of work done to it, and releases 5.00x10^2 KJ of heat into the surroundings, what is the change in internal energy of the system. what is the KJ ?
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This is based on first law of Thermodynamics which states : DU = q + w . In this case (the convention adopted by scientists),
q is positive if heat flows into the system and w is positive is the work is done on the system,
3.00*10^2 kcal of work is done ON the system, so w is positive. Heat, however, flows FROM the system, so q is negative.
To do the problem in In kJ: DU = -5.00*10^2 kJ + (3.00*10^2 kcal)*(4.184 J/cal)
= - 500 Kj + 1255.2 KJ = + 755.2 kJ (answer)
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