[sinthreck]

Heat is released when hydrogen burns in air according to the following reaction:

H₂(g) + O₂(g)  2 H₂O

Which of the following is responsible for the the heat?

(A) Breaking H-H bonds releases energy
(B) Breaking O-O bonds releases energy
(C) Forming O-H bonds releases energy
(D) Breaking both H-H and O-O bonds releases energy
(E) Breaking both H-H and O-O bonds and forming O-H bonds releases energy

My understanding is that breaking bonds requires energy, hence are endothermic. Intuitively, this means sense as if you want to break something, you need energy to do so. Also, melting ice is endothermic.

Forming bonds therefore must release energy - although, I'm not exactly sure why.

Therefore the answer is (C).

I'm a little confused though, when I think about biology. Breaking down ATP releases energy. This makes me think I could be wrong!

[Borek]

Breaking down ATP releases energy.

No. That's a common mistake. Overall reaction gives energy, but breaking down ATP requires energy. Your first thought was correct.

[sinthreck]

hrm, I did a bit more research on it and found the following:

Often the reaction is written as:

ATP  ADP + Pi

This gives the impression that only bond breaking is occuring. In reality the reaction is this:

ATP + HOH  ADP + Pi.

You break:
P-O (in ATP) and O-H (water)

and form:
P-O (in Pi) and O-H (ADP).

because the bonds you form are slighly stronger than the bonds you break, the overall reaction is exothermic.

Apparently, the difference in strength is not all that great (~ 30kj/mol) and as such it is stupid to call them "high energy bonds"

Is everything I found true?

[nigel433]

The proper definition of "a high energy bond" is a bond which requires
high energy to break. Therefore the bond in ATP which ignorant
biologists refer to as "high energy" is in fact a (comparatively)
low energy bond.

I have never yet met a biologist who remembered enough school
chemistry to understand where he was going wrong.

Perhaps they remember vaguely that a molecule contains vibrational
energy and think this is what bonds are also about - instead of
thinking of chemical potential energy.

Potential energy is NEVER localised in matter. Even the simple example of a ball
held up from the earth's surface and then dropped illustrates this.
Because the ball accelerates more obviously than the earth, the books say the
BALL has potential energy, "mgh". But if a force external to the earth/ball
system prevented the ball from moving, the earth would eventually fall towards
the ball and realise that potential energy in its (the EARTH's) kinetic
energy of motion.

Unfortunately, everyday language sends up the garden-path. Who hasn't said

"That hamburger contains 500 calories"   ?