"Depends. Does your reaction *only* involve the breaking of chemical bonds? e.g. Cl2 => 2Cl• ? That's always endothermic. Does it involve both breaking of reactant bonds and formation of new bonds in the products? e.g. Cl2 + H2 => 2HCl. It could be endothermic or exothermic, depending on whether the bonds were stronger in the reactants or products.
(Note to the pedantic. If you care about the difference between energy and enthalpy, replace "energy" with "enthalpy" throughout the following.)
Breaking bonds is uphill, unfavourable, endothermic, requires an input of energy. Obviously, it must be, because bonds exist, right? If breaking bonds released energy and were a favourable process, there would be no such thing as molecules, all matter would just fall apart into constituent atoms, break all the bonds, and release all the energy.
Consider the simplest molecule, dihydrogen, H2. Look up its bond strength: 432 kJ/mol. That means you need to put 432kJ/mol of energy INto the system to separate the H atoms from each other and break the bond. Energy is required for a bond to break. Bond breaking: endothermic.
Forming bonds is downhill, favourable, exothermic, releases an output of energy. Obviously, it must be, because bonds exist, right? If forming bonds were an unfavourable process that required an INput of energy, there would be no such thing as molecules, all matter would exist as atoms, because making bonds and forming molecules would move everything to a less stable, unfavoured, higher energy state.
Consider the simplest molecule, dihydrogen, H2. Its bond strength is 432 kJ/mol. That means that two H atoms will spontaneously come together and form a new bond, moving to a lower energy state by releasing that energy, releasing 432kJ/mol OUT of the system as the H atoms combine. Energy is released when a bond is formed. Bond making: exothermic.
If you do both, the net energy change for the overall reaction depends on which are stronger: stronger reactant bonds than products bonds means more energy in to break than out to make, and the reaction overall absorbs energy and is endothermic; stronger product bonds than reactant bonds means less energy in to break than out to make, and the reaction overall releases energy and is exothermic.
But, but, but. I thought catabolism released energy! I thought catabolism broke bonds and made smaller molecules, and breaking bonds releases energy!
You need to balance your reactions -- in any biochemical process, you also FORM new bonds in the products, and if you release energy it's because the bonds you made were stronger (released more energy) than the bonds you broke (absorbed less energy).
Example: reaction of α-ketoglutarate in the citric acid cycle is often written as breaking a C-C bond: –[O2C(CH2)3C(O)CO2]– (αKG) ==> –[O2C(CH2)3C(O)SCoA] + CO2 (succinylCoA + carbon dioxide). Break a bond, energy comes out.
But you formed a C-S bond from the CoA as well. And the C-O bonds in CO2 are far far stronger than the C-O bonds in a carboxylate group of the glutarate. And you lost two electrons, which form a new C-H bond in NADH. The whole balanced reaction is:
–[O2C(CH2)3C(O)CO2]– + –[SCoA] + H+ + [NAD]+ ==>
–[O2C(CH2)3C(O)SCoA] + NADH + CO2.
break: C-C and a C-O (from αKG carboxylate group)
make: C-S (in succinylCoA), C=O (in CO2), C-H (in NADH).
Even though it's catabolism, breaking the organic species into smaller bits via bond breaking, you form more new bonds that you lost, you form stronger bonds than you lost, and that's the reason it's exothermic and releases energy -- the new bonds you made release form energy than the old bonds you broke.
Summary:
Breaking bonds is always endothermic. An overall chemical reaction might be endo or exo, depending on if the reactant bonds or product bonds are stronger."
