Balancing Chemical Equations and Calculating Heats of Reaction: Two Often Overlooked Helps for Chemical Process Developers.

Many years ago, when I was a graduate student with Professor R.B. Woodward, at a group party at Harvard University, my wife asked Woodward, who was arguably the finest synthetic organic chemist in the world, what kept him modest. Although the question was expressed with a fair measure of pique, Dr. Woodward did not seem flustered.  He replied something to the effect that Nature itself did this, since even the most accomplished chemist more often proposes what turns out to be unsuccessful than what is successful.

How true. How true. But even so in our false sophistication we often fail to take simple precautions, which would easily avoid silly errors. I would like to speak of two of these:

·        failing to balance the chemical equation, and

·        failing  to calculate the enthalpy change in a reaction using simple bond energies

Balancing the Chemical Equation

Inorganic chemists rarely show this delinquency but organic chemists, because they are so accustomed to writing a starting material, a reagent and a product as part of a proposed series of reactions, almost never balance equations.  The result is that more often than you would expect even experienced lab workers do not get the stoichiometry correct and add either an excess or a deficiency of a reagent.  This is particularly true of oxidation reactions.  The second difficulty that results is that they cannot see the importance of the coproduct, which is formed along with their desired product, because the coproduct only ecomes import when one tries to balance and so they cannot see the possibilities and the complications that may arise from its presence. 

Calculating the Enthalpy of the Reaction

We organic chemists often seem to have gotten it into our heads that so long as we can draw a self-consistent series of arrows, showing the movement of electron pairs, then a reaction has a reasonable possibility to proceed. Usually we are protected from error by the fact that the transformation we are contemplating is completely analogous to a known reaction.  Nevertheless, it is a simple matter using bond energy tables to calculate the net enthalpy change of the reaction we are hoping will occur.  The result is that it will become more apparent to us whether a desired reaction is just weakly favored (so that steric hindrance, inadequate solvation etc. can inhibit it), disfavored or so strongly favored that we need to be concerned about the exothermicitry of the process and take appropriate precautions.

To be sure, it is negative free energy not a negative enthalpy, which is necessary to have a favorable equilibrium. but it is the less common situation when the entropy of the reaction makes the difference in driving the reaction and when it does, this is almost always when gases are involved or when the reaction is a fragmentation.

To make simpler the calculation of Enthalpy of Reaction, I have gathered together typical bond energies for the covalency between different atoms listed them below.  These should be treated as median or average values.  You may be able by inspection of the substrate or the reagent you intend to employ to recognize bonds, which can be expected to be stronger or weaker than these representative values.

Bond	Value	Bond	Value	Bond	Value
F-C	108	O-H	110-111	N≡C	212.6
F-F	37	O-C	85-91	N≡N	225.8
F-H	135	O=C	173-181	N-Si	76.5
F-O	45	O-O	35	P-P	41
F-C	108-116	O-N	53	P-C	65
F-Si	193	O=N	145	P-H	76
Cl-Cl	58	O-Cl	52	Si-Si	81
Cl-Br		O-Br	48	Si-C	77
Cl-I		O-P	91	Si-H	94
Cl-S		O=P	119-130	C-H	98.7
Cl-N	46	O-Si	111	C-H (vinyl)	108
Cl-P		O-I	56	C-H(acetylene)	128
Cl-C	81	O=S	132	C-C	82.6
Cl-Si	113	S-H	88	C=C	145.8
Cl-H	103.2	S-S	54	C≡C	199.6
Br-Br	46.1	S-C	60
Br-I		S-Si	70
Br-P		S=C	128
Br-S		N-N	39
Br-N		N=N	100
Br-Si	97	N-H	93.4
Br-C	66	N-C	72.8
I-I	36.1	N=C	147