HARD chemistry CW - help needed!

[Chocolatechick]

DD has been stuck for this for a while and i'm totally useless at anything like this, she is much more intelligent than me not wanting to feel totally helpless, i mentioned to her that there are some people on here that may help. She writes:

'For: methanol, ethanol, propanol, butanol, pentanol and hexanol, I need to find the theoretical energy transfer if they were burned in air (oxygen).

This requires balancing the equations which I have already done but when it comes to the actual calculations, my results have no pattern and aren't all negative numbers (as they should be as they are exothermic.) I know they are wrong but I can't see where I am making the mistakes. If someone could explain, do one example for me, I would be so grateful, here's what you need to know:

The equation: Alcohol + Oxygen ---> Carbon dioxide + Water
Bond values (Kj/mol)
H-C - 413
C-C - 347
O-C - 336
O=O - 498
C=O - 805
O-H - 464

Thanks to anyone who can help as I always get the wrong answer (no pattern and some are positive - not exothermic.) '

Extra Info:
Balanced eg: 2CH3OH + 3O2 ---> 2CO2 + 4H20

for all equations:
O2 = double bond
CO2 = C=0 double bond x2
All other compounds/elements contain single bonds

Other balanced equations, if required:

Ethanol: C2H5OH + 3O2 ---> 2CO2 + 3H20

Propanol: 2C3H7OH + 9O2 ---> 6CO2 + 8H2O

Butanol: C4H9OH + 6O2 ---> 4CO2 + 5 H2O

Pentanol: 2C5H11OH + 15O2 ---> 10CO2 +12 H2O

Hexanol: C6H13OH + 9O2 ---> 6CO2 + 7H2O

DRAW OUT THE STRUCTURE OF THE ALCOHOL SHOWING ALL THE BONDS

DO THE SAME FOR

O C H2O

CALCULATE THE TOTAL ENERGY OF ALL THE BONDS BROKEN ON THE LEFT SIDE OF EQUATION

THIS IS +

CALCULATE THE TOTAL ENERGY OF ALL THE BONDS
FORMED ON RIGHT HAND SIDE OF EQUATION

THIS IS -

ADD THE 2 TOGETHER THE FINAL ANSWER SHOULD BE -

AND IS THE THERORETICAL VALUE OF THE ENERGY PRODUCED

DON'T FORGET TO COUNT ALL THE BONDS

IN THE BALANCED EQUATION

Are you drawing out the reactions so that you can see clearly the bonds that are being broken and formed? Are you remembering to multiply out your values as shown when you have balanced your equations?

If you show us a worked example it would be easier for us to see where you are going wrong.

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ETHANOL 5 X 413 + 1 X 347 + 1 X 336 + 1 X 464 + 3 X 805 - 4 X 805 - 6 X 464

SNORKLE

BUT THAT IS 2 MOLECULES OF METHANOL

HALF ANS FOR 1

Message withdrawn

5 X 413 + 1 X 347 + 1 X 336 + 1 X 464 + 3 X 498 - 4 X 805 - 6 X 464

SORRY

Message withdrawn

• 1298 kJ per mol

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er, why are you doing your dd's coursework?

If she can't do it, the teacher needs to address the problem, not you!

Actually perhaps we shouldn't have helped

Perhaps she should go and tell the teacher that she needs help

Message withdrawn

not doing it, i would if i could believe me! teacher at school isn't helping at all and so just thought id ask. she writes:

' sorry i am finding the theoretical energy transfer so i have to do products - reactants (or so i have been told, is this right?) for methanol i did:

2(413x3)+2(464) + 3(498) --> 2(805 x 2) + 4(928)
which gives 4900 ---> 6932
=2062 kJ/mol (WRONG!)

other answers are:
eth-1619
prop-1062
but-3911
pent-1702
and i gave up at this point. it had already taken me years to balance them.'

YOU HAVE FORGOTTEN THE C - O IN METHANOL

AND THINK OTHERS ARE WRONG TOO

'oooo epipthany! i havent used c-o in any. i was doing c-h and o-h but no c-o! i will retry all like this. hopefully then i will get some sort of structure to my answers.'

'sorry to be a pain but i just calculated:

Methanol = 672+2478+928=4078
o2 = 498 x 3 =1494

co2 + H2o= 3220 + 3712 =6932

so i get 6932-5572
which is positive. = 1360

i think its time to give up!'

if you work out the example that i worked out do you get the same answer ?????????

' just worked out ethanol and got the same answer as you! happyness :D'

Deffo brill

follow the same pattern

and then tell me the answers and will tell you if ok

-3832 for propanol?

YOU HAVE DOEN IT FOR 2 MOLECULES

SO HALF IT FOR 1