I teach Biochemistry and know that heating water can break weak bonds like hydrogen bond or ionic interaction, but not disulfide bond (without reducing agent). The bond energy of hydrogen bond in water is about 12–30 kJ/mol, while the disulfide bond is 251 kJ/mol. I initially thought the heated water would have the energy much lower than 251 kJ/mol, but higher than 30 kJ/mol. But when I looked at the energy storage in 1 L of water at 90 degrees Celsius from here, it was calculated to 294 kJ. Now I wonder whether I can compare this energy to the bond energy. What do I miss here?
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3$\begingroup$ In thinking that the water will cool to near zero, releasing the energy so that the disulfide bond can be broken. Also, the sulfides will reform the bond since it is the most stable bond available. $\endgroup$– LDC3Jul 24, 2015 at 4:12
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$\begingroup$ could it be because 294KJ/L $\endgroup$– CognisMantisJul 26, 2015 at 3:51
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Chemical reactions do not simply occur because there is enough energy around to make something happen. They also require a mechanism that enables something to occur. It isn't enough that the product will release energy on some reaction happening.
Some reactions will happen spontaneously with enough input of thermal energy to start them off (burning coal, for example). Others, like breaking disulphide bonds) will not. Even when there is a plausible mechanism for a reaction, you have to add enough energy to make the reaction happen which may be much more energy than you expect. Most reactions have an "activation energy" to get past the barriers specific mechanisms holding back the reaction.
answered Jun 11, 2016 at 13:40