Ask A Scientist Chemistry Archive

name         Brandon B.
status       student
age          18

Question -   Why are decomposition reactions always endothermic?
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Brandon,

They're not always endothermic. Consider the decomposition of
nitroglycerin -- it's quite exothermic.

Bond breaking processes are always endoenergetic. Similarly, bond-making
processes are always exoenergetic. Note the change in word choice:
"Endoenergetic" and "exoenergetic" are preferred to "endothermic" and
"exothermic." Not all reactions are "thermic" -- some produce things like
light and electricity etc.

Regards,
ProfHoff 587
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I do not think that all decomposition reactions are endothermic. The
oxidation of hydrocarbons is exothermic and there are anaerobic reactions
that are also. For example if you put your hand in a pile of grass that has
been piled up for several days in warm weather, the temperature inside the
pile of grass can become quite hot.

Vince Calder
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I am not quite sure what you mean by "decomposition reactions".  If you mean
reactions in which gas is given off, as frequently happens in the detonation
of explosives, there is no intrinsic reason for the reaction to absorb heat
from the surroundings.  However, it is possible for the reactions to be
spontaneous (favorable) even if they are endothermic by virtue of great
entropy increase resulting from the release of gases.  Gases have much more
freedom than solids or liquids, so that there can be an entropy increase
even without releasing heat.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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Dear Brandon,

What do you mean by decomposition? If you mean changing a molecule
into elements, decomposition is not always endothermic.

Decomposition of NO to N2 and O2 is exothermic (-90.25kJ/mole)
N2O4 is too (-9.16 kJ/mole) as is N2O (-82.05 kJ/mole).
On the other hand the decomposition of gas-phase NH3 is indeed
endothermic (+46.11 kJ/mole). The decomposition of ozone (O3)
to oxygen (O2) is exothermic (-142.7 kJ/mole)...I could go on...

There is one kind of decomposition reaction which is always
endothermic; the decomposition of a gas-phase diatomic molecule (or for
that matter a polyatomic molecule) into gas-phase atoms. One must put
energy into a stable molecule to get it to break apart into atoms.
If the atoms do not form molecules, the reaction will definitely
require energy overall. The energy required is roughly (but not exactly)
equal to delta H, so the reaction will be endothermic (the correction is
small). So, for example the decomposition of ozone (O3) to ATOMIC oxygen 
(O) is
endothermic (+754.81 kJ/mole). However, if the product atoms form molecules,
energy will be liberated to the environment as the bonds are formed. If
the energy liberated by bond formation is greater than the energy
required by bond breakage, the overall reaction will be endothermic.

In sum: the sign of delta H depends on what the compound is and what the
decomposition products are. If the reactant is a gas-phase molecule
and the products are gas-phase atoms, delta H will always be positive.
If anything is a solid, the crystallization enthalpy has to be taken
into account, and if anything is in solution solvation effects can
be a determining factor in the sign of delta H.

best, Prof. Topper
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