# How to determine whether the enthalpy of solution is positive or negative by calorimetry?

How can we determine if the enthalpy of solution of a compound is positive or negative?

In order to make a clear idea about what I mean, take the following problem:

A 3.5 g sample of $\ce{NH4NO3}$ was added to 80. mL of water in a coffee cup calorimeter of negligible heat capacity. As a result, the temperature of solution decreased from $21.6 \ ^\circ\text{C}$ to $18.1\ ^\circ\text{C}$. Calculate the enthalpy change of the solution ($\Delta H_{\text{sol}}$ ). (specific heat of the solution = $4.18 \frac{\text{J}}{\text{g}\cdot ^\circ\text{C}}$; density of water $1.00 \frac{\text{g}}{\text{mL}}$)

Answer: $+28 \frac{\text{kJ}}{\text{mol}}$

Why it is positive despite there is a decrease in temperature?

• Possible duplicate of chemistry.stackexchange.com/questions/27159/… Apr 12 '15 at 16:16
• This question is not actually about enthalpy of formation (not a duplicate). It is about enthalpy of solution and the calorimetry experiment one might do to determine said enthalpy. I have edited the question to make it clearer. Apr 12 '15 at 16:59