A student is using a coffee-cup calorimeter to determine the enthalpy change of the endothermic reaction of two aqueous solutions. After both solutions are added to the cup, the student neglects to put the lid on the cup. This would cause the magnitude of the calculated ΔH° value to be: the answer is: too small, since the solution will absorb heat from the room. But why? Wouldn't depend on if the reaction releases or absorbs heat. Wouldn't it be too large because heat escapes the cup? I'm so confused

See the answer below

Explanation:

First, it should be understood that an endothermic reaction is one that absorbs energy in the form of heat from the surrounding. The products of endothermic reactions usually have higher energy than their reactants. Hence, the ΔH° which is referred to as the enthalpy change is usually positive.

Forgetting to cover the coffee-cup calorimeter means some of the heat energy absorbed by the reactants would be exchanged back to the surroundings - a loss. It also means that the enthalpy change would be smaller compared to if the cup had been covered because some of the heat has been lost to the surrounding.