arghh what's the difference between these 2 problems?

[Kneecoal]

ok so - bear with me.

topscore 2: GC #50 - i asked about this question yesterday but didn't get the answer i was looking for:

using the given bond energies, estimate the standard heat of combustion per mole of H2 gas when combined with oxygen.

ok, fine. i plugged in the numbers. but i assumed that it would be products -reactants. they did reactants minus products. why??

so then i come across this destroyer question:

consider the reaction, and calculate the enthalpy change: 3COCl2(g) + B2O3(s) --> 2BCl + 3CO2(g)

fine, plug in the numbers. because in topscore yesterday they told me reactants minus products, i did that here. i got the right number, but the wrong sign.

am i wrong to assume that both problems are talking about a delta H? where am i going wrong here?

again, thanks for all your help everyone. only a few more days of me

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[joonkimdds]

I don't know topscore question(I paid 2 hrs ago, I guess they don't wanna send me the key 🙁 )
It's products minus reactants.
If you can post the entire question for me, I might be able to help u.

 

[bennijai]

Ok just follow this guideline. It works all the time. The key is to find if the energy is bond energy or heat.

For bond energies it is sum of reactants - sum of products
For change in enthalpies it is the sum of products - sum of reactants

Hope it helps.

 

[Kneecoal]

the topscore question is:

using the given bond energies, estimate the standard heat of combustion per mole of H2 gas when combined with oxygen.

so it's: 2H2 + O2 --> 2H2O

i would've done [2H2O - (2H2 + O2)]/2 (over 2 because here's 2 moles of H2 and they're looking for one mole)

but they did [(2H2 + O2) - 2H2O]/2

 

[Kneecoal]

Ok just follow this guideline. It works all the time. The key is to find if the energy is bond energy or heat.

For bond energies it is sum of reactants - sum of products
For change in enthalpies it is the sum of products - sum of reactants

Hope it helps.

ok, i'll keep that in mind - thanks

 

[Pat Kelly]

If you form bonds, your dH is - (exothermic), similarly if you break bonds your dH would be + (endothermic). This explains why it's the other way around

 

[joonkimdds]

If it asked to find bond energies of heat of combustion, I think that would've been more clear.

It sounds as if they are asking you to find the "heat" of combustion.

 

[nze82]

ok so - bear with me.

topscore 2: GC #50 - i asked about this question yesterday but didn't get the answer i was looking for:

using the given bond energies, estimate the standard heat of combustion per mole of H2 gas when combined with oxygen.

ok, fine. i plugged in the numbers. but i assumed that it would be products -reactants. they did reactants minus products. why??

so then i come across this destroyer question:

consider the reaction, and calculate the enthalpy change: 3COCl2(g) + B2O3(s) --> 2BCl + 3CO2(g)

fine, plug in the numbers. because in topscore yesterday they told me reactants minus products, i did that here. i got the right number, but the wrong sign.

am i wrong to assume that both problems are talking about a delta H? where am i going wrong here?

again, thanks for all your help everyone. only a few more days of me

The first problem is giving you the bond energy for individual bonds within each compound. In this case:
Delta H = Sum (Bonds Broken<Reactants>) - Sum (Bonds formed<Products>)
Why?
The net amount of energy change (Delta H) is the difference between the energy that it takes to break the bonds between reactants and the amount of energy that is released when new product bonds are formed.
Why - bonds formed?
Well, when bonds are formed, energy is released, and energy release is always designated with a negative sign.

The 2nd question is quite different however. They're not giving you the bond energy for individual bonds; They're giving you the heat of formation of each species (products and reactants). In this case:

Delta H = Sum (Heat of formation of Products) - Sum (Heat of formation of reactants)

Since, these two questions are giving you different information (Bond energy vs. heat of formation) you must use two different formulas to solve them.

 

[joonkimdds]

Don't you guys think the question should have been more clear about what it was asking? It said use bond energy but it said find the heat of combustion.

 

[nze82]

Don't you guys think the question should have been more clear about what it was asking? It said use bond energy but it said find the heat of combustion.

No! The question is perfectly clear about what is given and what you should solve for.
Heat of Combustion = Delta H