[blz]

Let's consider a balloon filled with H₂ and O₂. If a lighter is put under the ballon, 2H₂ + O₂    2H₂O

First, we're breaking:
1) Two H-H bonds (2 * 436.4 kJ/mol = Investing 872.8 kJ/mol)
2) One O=O bond (double bond) (1 * 489.7 kJ/mol = Investing 498.7 kJ/mol)
in total, 872.8  + 498.7  = 1371.5 kJ/mol

Then, four O-H bonds are formed:
4 * 460 kJ/mol = releasing 1840 kJ/mol.

My question is:
These molecules will release 1840 kJ/mol, and the lighter "provided"  them with only 1371 kJ/ mol. Where does the extra 469 kJ/mol come from?   

[billnotgatez]

I would like to add this aside
From WIKI
http://en.wikipedia.org/wiki/Hydrogen_safety#Ignition

    "Hydrogen-air mixtures can ignite with very low energy input, 1/10 that required igniting a gasoline-air mixture. For reference, an invisible spark or a static spark from a person can cause ignition."
    "Although the autoignition temperature of hydrogen is higher than those for most hydrocarbons, hydrogen's lower ignition energy makes the ignition of hydrogen–air mixtures more likely. The minimum energy for spark ignition at atmospheric pressure is about 0.02 millijoules."

and
http://en.wikipedia.org/wiki/Minimum_ignition_energy

Minimum ignition energy (MIE) is the minimum amount of energy required to ignite a combustible vapor, gas or dust cloud, for example by means of an electrostatic discharge. Ignition of a fuel/air mixture is possible only when the rate of liberation of heat near the ignition zone is greater than the heat loss by conduction. Heat loss due to radiation is not considered, as it is assumed to be negligible, and the ignition process is assumed to be steady and one-dimensional[clarification needed].

[blz]

If I understood correctly, that means you can start this process with 0.02 milijoules, which only enhances my problem - where did the 1371 kJ required to break the bonds come from?

[Borek]

You don't have to react a whole mole at once, all you have to do is to start the process.

Think about starting a fire. You start with a single match, which is enough to ignite a splinter, which gives enough heat to ignite a lightly thicker twig, which will ignite a branch and so on. In the end you can burn a thick log. Amount of heat required to ignite a log is well beyond capabilities of a single match.

Think about 20 μJ as if it was a match.

[blz]

Thanks, However more energy is released by the balloon system than the balloon system received. How does that happen?

[billnotgatez]

Does this help?
From WIKI
http://en.wikipedia.org/wiki/Exothermic_reaction

An exothermic reaction is a chemical or physical reaction that releases heat. It gives out energy to its surroundings. The energy needed for the reaction to occur is less than the total energy released