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Topic: How do I know when to take the absolute values in thermodynamic problems?  (Read 12633 times)

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jtlbb2

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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 - ?

Offline plu

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Re:How do I know when to take the absolute values in thermodynamic problems?
« Reply #1 on: February 12, 2006, 11:18:03 AM »
From my experience with the few chemistry courses I've taken at the undergrad level, when asked for amount of heat "absorbed or evolved", the answer should always be a positive number.  So, in your case, since the reaction is exothermic (dH = -238 kJ), it would "evolve 238 kJ of heat".  These textbooks and their silly wording  :P

Offline cofi

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Re:How do I know when to take the absolute values in thermodynamic problems?
« Reply #2 on: February 12, 2006, 02:00:33 PM »
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...
« Last Edit: February 12, 2006, 02:00:50 PM by cofi »

jtlbb2

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Re:How do I know when to take the absolute values in thermodynamic problems?
« Reply #3 on: February 13, 2006, 06:09:12 AM »
Ok, I think I get what you guys are saying.  If it is asking for the amount, or quantity, then the value will be positive.  If it is simply asking for the change, then it will be either positive or negative.  Thanks.

Offline Yggdrasil

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Re:How do I know when to take the absolute values in thermodynamic problems?
« Reply #4 on: February 13, 2006, 11:08:54 PM »
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).
« Last Edit: February 13, 2006, 11:10:08 PM by Yggdrasil »

jtlbb2

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Re:How do I know when to take the absolute values in thermodynamic problems?
« Reply #5 on: February 16, 2006, 08:09:11 AM »
Yggdrasil,

I don't know if I am understanding you correctly.  To me, it sounds like you're saying that the problem I posted ends up with a positive value for its answer because it is an exothermic reaction.  And it would be a negative answer if this had been an endothermic reaction.

If this is what you're saying, then I don't think it will work out all the time.  For example, check this problem out:

"Calcium carbide, CaC2, is manufactured by the reaction of CaO with carbon at a high temperature.  (Calcium  carbide is then used to make acetylene.)

CaO(s) + 3 C(s) ---> CaC2(s) + CO(g)  [Delta]H = +464.8kJ

Is this reaction endothermic or exothermic?  If 10.0g of CaO is allowed to react with an excess of carbon, what quantity of heat is absorbed or evolved by the reaction?"

I'm not going to work this problem; however, this is an example of an endothermic reaction, and the answer to this problem is a postive value.  If I am understanding your explanation correctly, you're saying that a problem like this one should have a negative value.

If that's not what you meant, can you explain it in a different way?

Offline arnyk

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Re:How do I know when to take the absolute values in thermodynamic problems?
« Reply #6 on: February 16, 2006, 11:19:27 AM »
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).  

jtlbb2

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Re:How do I know when to take the absolute values in thermodynamic problems?
« Reply #7 on: February 16, 2006, 05:58:29 PM »
My problem was never determining whether or not a reaction was exothermic or endothermic.  It was why do some of the answers to the problems have positive values when they mathematically work out to be negative, and vice-versa.

Offline Borek

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Re:How do I know when to take the absolute values in thermodynamic problems?
« Reply #8 on: February 16, 2006, 06:52:50 PM »
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.
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