[WashableMarker]

Okay, so I had a test on organic chemistry today, and one question involved coming up with the equation for the complete combustion of hexane, and then calculating the energy change and coming up with a thermochemical equation for one mole of hexane. I spent 15 minutes trying to get it to come out exothermic like I knew it should, but I just COULDN'T get it. It's bothering me now, so hopefully someone can spot some kind of obvious mistake I'm making... here's what I had:

Equation:
C₆H₁₄ + 9.5 O₂ ----> 6 CO₂ + 7 H₂O

Bonds Broken:
14 H-C bonds
= 14 * 413kJ
= 5782kJ

5 C-C bonds
= 5 * 348kJ
= 1740kJ

9.5 O=O bonds
= 9.5 * 498kJ
= 4731kJ


Bonds Formed:
12 C=O bonds [2 for each mol of carbon dioxide, correct?]
= 12 * 804kJ
= 9648kJ

14 H-O bonds [2 for each mol of water]
= 14 * 464kJ
= 6496kJ

Total energy released in reaction = 12253kJ
Total energy in bonds formed = 16144kJ
For a difference of 3891kJ

Therefore, thermochemical equation is:

C₆H₁₄ + 9.5 O₂ + 3891kJ ----> 6 CO₂ + 7 H₂O

---------------------------------------------

It just doesn't make SENSE to me... I mean, I understand the calculations and what they're saying, but come on! We're blowing up HEXANE! This is bothering me like you wouldn't believe.

[arnyk]

Remember that triangle...fuel + oxygen + heat.

[WashableMarker]

so you're saying it IS endothermic then, and that bit of heat was a bit of an "activation energy" of sorts, which happened to be bigger than the final BOOM! of energy?

[arnyk]

No wait, that's right.

total energy input = 12 253kJ
total energy output = 16 144kJ

net energy change = 12 253kJ - 16 144kJ
                           = -3891kJ

The thermochemical equation for the reaction is:

C₆H₁₄ + 9.5 O₂ ---> 6 CO₂ + 7 H₂O + 3891kJ

The reaction is exothermic.

[xiankai]

the enthalpy of a reaction, ^H (where ^ is a triangle), is the net energy amount that has been interchanged IN the reactants.

so if the enthalpy is negative, it means the reactants lose energy, hence energy is given out. if it was positive, it would mean it took in energy, making it endothermic

remember, to calculate ^H u need to find energy gained - energy lost, so that energy gain will be positive, and energy loss will be negative.

[jdurg]

Also, remember that the formation of a bond releases energy and the breaking of a bond requires energy.  So bond breaking is endothermic and bond forming is exothermic.

[Dude]

Remember that all models in science or engineering are balances.  Your convention of putting energy on the left side of the equation does NOT mean that you need to add that much energy for the reaction to occur.  It is simply an energy balance that accounts for the energy released in going from reactant to products (i.e. X = X).