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HARD chemistry CW - help needed!

27 replies

Chocolatechick · 27/03/2007 19:43

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.) '

OP posts:
RTKangaMummy · 27/03/2007 19:50

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

Chocolatechick · 27/03/2007 19:45

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

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