Hi guys, I need a little bit of help.  I am taking chemistry 102 at the college level, and we're doing problems involving such things as specific heat capactiy, enthalpy, changes of states, etc.  But I have noticed that when I work some of the problems straight forwardly, I get the wrong answer even though it is the "right" number, because I am getting the +/- signs wrong.
 
For example one of the homework problems is:

"Nitrogen monoxide, a gas recently found to be involved in a wide range of biological processes, reacts with oxygen to give brown NO₂ gas.

2 NO(g) + O₂(g) ---> NO₂(g) (Delta)H= -114.1 kJ

Is this reaction endothermic or exothermic?  If 1.25g of NO is converted completely to NO₂, what quantity of heat is absorbed or evolved?"

Okay, this is what I do to work out this problem:

1.25g NO * 1mol NO/30.00g NO * 2mol NO₂/2mol NO * -114.1 kJ/2mol NO₂ = -238 kJ.

So for my answer, I get -238.  But for some reaon, in this problem, the answer is supposed to be +238 kJ.  I don't understand why this is.  In fact, in other problems I have similar to this one, the answer really is negative, even though it's the exact same type of problem.

Can anyone help me understand how I am supposed to know when my answer is going to be + or - ?

they ask for *amount* of heat...
+/- is just a sign that says if the heat was evolved or absorbed...
the amount of something is always a positive number... you can't buy -25kg of apples...

For example one of the homework problems is:

"Nitrogen monoxide, a gas recently found to be involved in a wide range of biological processes, reacts with oxygen to give brown NO₂ gas.

2 NO(g) + O₂(g) ---> NO₂(g) (Delta)H= -114.1 kJ

Is this reaction endothermic or exothermic?  If 1.25g of NO is converted completely to NO₂, what quantity of heat is absorbed or evolved?"

Okay, this is what I do to work out this problem:

1.25g NO * 1mol NO/30.00g NO * 2mol NO₂/2mol NO * -114.1 kJ/2mol NO₂ = -238 kJ.

So for my answer, I get -238.  But for some reaon, in this problem, the answer is supposed to be +238 kJ.  I don't understand why this is.  In fact, in other problems I have similar to this one, the answer really is negative, even though it's the exact same type of problem.

Can anyone help me understand how I am supposed to know when my answer is going to be + or - ?

The sign of Delta H does NOT refer to the amount of heat evolved or absorbed in a reaction.  Specifically, delta H refers to the change in potential energy of the chemical species invovled.  In this reaction, delta H is negative, so the products are at a lower energy than the reactants, and energy is released in the reaction (q > 0).  Similarly for an endothermic reaction (delta H > 0), the reactants are at a higher energy than the products, so heat must be absorbed from the surroundings (q < 0).

A positive number indicates that there is more energy in the system than it began with.  Therefore, it must've absorbed the energy from elsewhere, making it endothermic.  

A negative number means that there is less energy than it began with, meaning it must've lost some energy to the environment.  Therefore it is exothermic.

Think about it from the point of view of the products, not the environment.  If the products lost energy (negative), that means the environment (you) gained energy (as heat = exothermic).  If the products gained energy (positive), that means the environment (you) have lost energy (*cold* = endothermic).  

Sign is always a matter of convention used - energy loss of the system if energy gaing of the surroundings and vice versa. It all depends on the point of view. No idea how it looks in the books you are using but I recall books used 20 years ago here - and way too often they were inconsistent; although usually it was obvious from the context what author means.