enthalpy confusion

[rose786]

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So my book lists 2 different equations for calculating change in enthalpy.

Equation 1: change in enthalpy = (products) - (reactants)
Equation 2: change in enthalpy=(sum of bond energies of bonds broken) - (bonds formed)

So the 2nd one is essentially the opposite of the first one. In practice problems, I can never understand on what basis you decide which equation to use? 😕

 

[TheWiredNerv]

they are equal to the same thing.

 

[rose786]

wouldn't "bonds broken" = reactants? and "bonds formed" = products

 

[rose786]

oh look, my own personal thread to run around in! weeeeeeeeeeeeeeeeeee!!!





*sigh*

 

[Loop]

You are right, they are opposite they way I read it. Equation 2 obviously being backwards however. Typo on your part? or maybe theirs?

 

[TheWiredNerv]

So my book lists 2 different equations for calculating change in enthalpy.

Equation 1: change in enthalpy = (products) - (reactants)
Equation 2: change in enthalpy=(sum of bond energies of bonds broken) - (bonds formed)

So the 2nd one is essentially the opposite of the first one. In practice problems, I can never understand on what basis you decide which equation to use? 😕

They are both right. One deals with heat of a reaction and one deals with bond energy.

Equation 1 means the sum of the enthalpies of the products - sum of enthalpies of reactants. This is the enthalpy change of the reaction.

Equation 2 deals strictly with how much energy is required to break a bond so the answer must be positive because you must have input of energy to break a bond. This is why the equation is different than the enthalpy change of a reaction.

 

[tamkhan]

They are both right. One deals with heat of a reaction and one deals with bond energy.

Equation 1 means the sum of the enthalpies of the products - sum of enthalpies of reactants. This is the enthalpy change of the reaction.

Equation 2 deals strictly with how much energy is required to break a bond so the answer must be positive because you must have input of energy to break a bond. This is why the equation is different than the enthalpy change of a reaction.

Just to add to this:

1- When you see a question that provides values for the "Enthalpies of formation" of products and reactants, you use the first equation, whereby
Enthalpy of reaction = (Sum of the enthalpy of formation of products) - (Sum of the enthalpy for formation of reactants)

2- Wheny you see a question that provides values for "Bond energies" of bonds, such as C-H, C-C, H-H etc., you use the second equation, whereby
Bond broken = endothermic
Bond formed = exothermic

You will have a balanced equation; use that to figure out which bonds are broken and which bonds are formed. Just remember, the bonds in the reactants must first be broken before the bonds in the products are formed.🙂

Been more than two years since I touched thermo; I hope I'm right on this one. 🙂

 

[TheWiredNerv]

Just to add to this:

1- When you see a question that provides values for the "Enthalpies of formation" of products and reactants, you use the first equation, whereby
Enthalpy of reaction = (Sum of the enthalpy of formation of products) - (Sum of the enthalpy for formation of reactants)

2- Wheny you see a question that provides values for "Bond energies" of bonds, such as C-H, C-C, H-H etc., you use the second equation, whereby
Bond broken = endothermic
Bond formed = exothermic

You will have a balanced equation; use that to figure out which bonds are broken and which bonds are formed. Just remember, the bonds in the reactants must first be broken before the bonds in the products are formed.🙂

Been more than two years since I touched thermo; I hope I'm right on this one. 🙂

Looks good to me!

 

[rose786]

thanks guys

 

[illbirz]

Just to add to this:

1- When you see a question that provides values for the "Enthalpies of formation" of products and reactants, you use the first equation, whereby
Enthalpy of reaction = (Sum of the enthalpy of formation of products) - (Sum of the enthalpy for formation of reactants)

2- Wheny you see a question that provides values for "Bond energies" of bonds, such as C-H, C-C, H-H etc., you use the second equation, whereby
Bond broken = endothermic
Bond formed = exothermic

You will have a balanced equation; use that to figure out which bonds are broken and which bonds are formed. Just remember, the bonds in the reactants must first be broken before the bonds in the products are formed.🙂

Been more than two years since I touched thermo; I hope I'm right on this one. 🙂

Doesnt bond breakage result in release of energy which in exothermic, and since using energy to form bonds wont that make it endothermic?😕

 

[tamkhan]

Doesnt bond breakage result in release of energy which in exothermic, and since using energy to form bonds wont that make it endothermic?😕

Other way around.
To break bonds, you need energy.
When new bonds form, energy is released.

 

[illbirz]

Other way around.
To break bonds, you need energy.
When new bonds form, energy is released.

WORD! i think i was mixing biology and chemistry together. Cause i was thinking more along the lines of anabolism where u need energy to build, and catabolism where energy is given off. BUT, now i got it. IN CHEMISTRY: bond formation = E is released, and in bond breakage, E is needed. IN BIOLOGY: catabolism, where molecules are broken down energy is released and as far as anabolism goes energy is needed to construct molecules.

 

[TheWiredNerv]

WORD! i think i was mixing biology and chemistry together. Cause i was thinking more along the lines of anabolism where u need energy to build, and catabolism where energy is given off. BUT, now i got it. IN CHEMISTRY: bond formation = E is released, and in bond breakage, E is needed. IN BIOLOGY: catabolism, where molecules are broken down energy is released and as far as anabolism goes energy is needed to construct molecules.

None of that, IN CHEMISTRY or IN BIOLOGY stuff. The body is a biochemical system.
In biology and chemistry, you need energy input to break a bond always.
Just because you are studying biology doesn't mean it can break the rules of the universe. Instead of memorizing this stuff, think about it and learn the concept. If you can reason your way through science then your TS will go up. If you memorize things then you will probably get those "out of no where" questions wrong because you don't know how to think through them.

 

[illbirz]

I agree with not memorizing, the sad thing is i thought i had comprehended these two factors. But if Bond breaking requires energy, then how can i correlate that to anabolic processes which require energy. Cause i know anabolic processes construct larger molecules from smaller ones. Help me...plz 🙁

 

[TheWiredNerv]

Biologist tend to forget about the specific chemistry involved. For instance, it requires energy to break the phosphate bond in ATP but the energy available after breaking that bond can do work.

 

[illbirz]

awwha! I had a feeling you say that, which reassures me of my reasoning. Thank you, very much.

 

[bodiddely]

Anabolism decreases entropy in the system you are considering (the body), which requires energy.

 

[Loop]

You are right, they are opposite they way I read it. Equation 2 obviously being backwards however. Typo on your part? or maybe theirs?

Oops...sorry

Sad that I knew they were the same and had even written both down earlier that day while reviewing stuff I had learned. The 8 hours in the library a day are seriously eroding my common sense. Without thinking about it enough and seeing them together they sure did look to be to opposite at the time. But separately I understand them fine.

 

[TheWiredNerv]

Anabolism decreases entropy in the system you are considering (the body), which requires energy.

Yes, so you need a more negative enthalpy to accomplish work in the body (negative delta G)

 

[rose786]

Oops...sorry

Sad that I knew they were the same and had even written both down earlier that day while reviewing stuff I had learned. The 8 hours in the library a day are seriously eroding my common sense. Without thinking about it enough and seeing them together they sure did look to be to opposite at the time. But separately I understand them fine.

I hear you on the brain fatigue. My mind has started decomposing.

 

[userah]

omg i just wanted to say thank you to this thread. I have been racking my brain trying to figure this out and now it all makes sense. I was just about to post a thread when i found this one. thank you thank you thank you =D