question regarding enthaply change

[ndn diva]

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In the TPR book, the enthalpy of rxn is found by taking the enthalphy of products - enthalpy of reactants. However...on Kaplan fl5 solution for #33, the equation if reversed, reactants - products. Which one is it?

 

[willthatsall]

Pretty sure it's products minus reactants.

 

[ExplodingBoy]

yeah, usually whenever you are finding the difference in something, it's products minus reactants.


The only thing i can think of is if the reaction they give you is the reverse of what you have the original delta H's for?

 

[liverotcod]

Delta anything is equal to final state minus initial state.

 

[mdsquared]

It's always going to be products minus reactants, reactants - products is probably a mistake. If you want to reverse the sign of delta H you have to reverse the equation. For example, A + B yields C + D ; delta H = 100 KJ
if you reverse this reaction, C + D yields A + B ; delta H = -100 KJ.
When you're making the products 100 KJ of heat is absorbed (endothermic), when youre going from products to reactants 100 KJ of heat is released (exothermic).

 

[ComicBookHero20]

it proll was about determining bond energy of a reaction--trust me on this, this is how it works for genchem bond energy (i know bc i beat the curve on a test bc i remember this from ap chem)

For EVERYTHING its products - reactants

if ur talking about BOND ENERGY its reactants - products (because it takes energy to break a bond so this tells you how much energy is released in a reaction)

 

[ndn diva]

it proll was about determining bond energy of a reaction--trust me on this, this is how it works for genchem bond energy (i know bc i beat the curve on a test bc i remember this from ap chem)

For EVERYTHING its products - reactants

if ur talking about BOND ENERGY its reactants - products (because it takes energy to break a bond so this tells you how much energy is released in a reaction)

Thanks comicbookhero20...so if it is talking about bond energy, then I need to take reactants-products...ok, that is where I was getting confused.

 

[willthatsall]

Thanks comicbookhero20...so if it is talking about bond energy, then I need to take reactants-products...ok, that is where I was getting confused.

It might be easier to think about bonds broken as positive energy (takes energy to break bonds) and bonds formed as negative energy (releases energy when bonds are formed). Just add together the positives and negatives and that is your answer.