This is done in a calorimeter lab. So suppose, the calorimeter is not such a good insulator and actually ends up releasing some of the heat into surroundings. I'm guessing it'd effect the final temperature of the water, and overall effect the enthalpy.. but does it decrease or increase?
-
You are measuring the amount of heat per unit mass or moles. So if you loose some heat, your derived value of enthalpy will decrease from the heat loss.