Enthalpty stoichiometry problem?? PLEASE help??!!

Question

The value of H for the reaction below is -72 kJ. _______kJ of heat are released when 1.0 mol of HBr is formed in this reaction.
H2 (g) + Br2(g) => 2HBr (g)

I tried to solve it..
1.0 mol HBr (-72 kJ/ 2 mol HBr) = -36 kJ

My possible answers only include a positive 36.
Am I wrong?? If not, why on earth do I keep getting the wrong signs on the H when I do these calculations? I want to make sure I get the answers right every time. Any explanation as to why I am getting the wrong signs would be greatly appreciated!!!!!!!!

Answer 1

You have the correct answer, sort of.  Think of enthalpy as the potential energy stored inside of the compounds in a chemical reaction. The production of HBr from H₂ and Br₂ results in a reduction, or loss, of this chemical potential energy. Where, you ask, does this potential energy go? It is released as heat energy to the environment. The negative value that you calculated is a result of your focus on the chemicals and the chemical reaction (and it is perfectly valid to do so), and shows that energy flows out of the system. The sentence within your problem is asking for the magnitude of the heat energy transferred, and always be a positive value. The direction of the transfer is in the word "released".

This situation is analogous to the distinction between speed and velocity. Any time you move, you have a positive speed, regardless of whether you move forward or backward. Speed is a scalar value. That is, it only has a magnitude, which can only be greater than or equal to zero. By bringing direction into play, we might think of the value as being positive or negative, but in fact the speed is always positive. Velocity, on the other hand, incorporates direction as well as magnitude. It is a vector quantity. That is, its value can be either negative or positive, depending upon whether the direction is the same as our frame of reference or opposite to it. For example, "up" is positive and "down" is negative, "to the right" is positive and "to the left" is negative. "Away from" may be positive or negative, depending on your point of view. In this case of chemistry, we treat the transfer of heat energy away from the chemical system as negative, and the transfer of heat energy into the system as positive.