Why Q = -DH.
Throughout the explanation, imagine a man with hot hands and a woman with cold hands.
1st Point of View: Man is reaction; woman is environment. Man releases or loses heat; he is
an exothermic reaction. DH for man is (-). Woman feels heat; she absorbs it, and her temperature increases. Her Q = (+) . Recall that Q = mc DT, where DT = Tf - Ti.
Note DH = -Q.
2nd Point of View: Woman is reaction; man is environment. Woman takes heat from man. She is an endothermic reaction. She steals or absorbs heat from the man. DH for woman is (+).
Man feels a cold hand; he loses heat to the woman, so his temperature drops. His Q = (-).
Again DH = -Q; the signs are again opposite of one another.
Suppose that NaOH(aq) is neutralizing HCl(aq). Both are dilute solutions in water. When they react they release heat into the surrounding water, just like the man. Their DH =(-). But you record an increase in temperature because you are inserting a thermometer into the water. The water is like the woman in the previous analogy. So for water, Q = (+).
If NaNO3 dissolves in water, the sodium nitrate absorbs or steals heat from the water. The DH for the reacting nitrate is (+), just like the woman's when she is regarded as the reactant. The water is now like the man; it experiences a drop in temperature and its Q is (-).