Reactions in bomb calorimeters occurs at constant volume while the coffee cup calorimeter has reactions occurring at constant pressure. If one was to perform identical experiments in both of these instruments (one at constant volume and the other at constant pressure) the two values would differ very so slightly (excluding the precision of both methods). For example, burning 1 mole of octane at constant volume would liberate 1223 kcal of energy, while burning the same amount of octane at constant pressure would liberate 1220 kcal of heat energy. Why the difference (about 0.2%) between the two values?
How is qv~qp?