large, it may take a long time before a significant relative decrease in [A]
occurs. Consequently, we have an approximate steady state with respect to
the concentration of B. Of course, this does not last indefinitely, since
ultimately all A will be consumed. This means that it may be of importance
to distinguish between an equilibrium state and an (approximate) steady
state, since predictions about what would occur in the long run are clearly
different for the two cases.
A true steady state can be attained if, for example, the system is
confined in a reaction vessel where a solution of A is continuously added
to the system while some of the product is continuously removed at the
same volume flow rate. Such steady states are by no means exceptional
and occur often in living cells or chemical reactors. A steady state then
lasts as long as the reaction conditions, including rates of inflow of
reactant(s) and outflow of product(s), are kept constant. Also for other
rate processes, e.g. involving mass or heat transfer, steady states are often
achieved.
Reaction Heat. A reaction can only proceed ifDG<0. In relation
to Eq. (4.6), it was mentioned that for most reactionsDHis larger than
TDS. This implies that during the reaction heat is produced (the amount of
reaction heat can be measured by calorimetry). The reaction then is said to
beexothermicandenthalpy driven. There are alsoendothermicreactions,
where heat is consumed; in other words,DH>0. BecauseDGmust be
negative for the reaction to proceed, this implies thatTDS>DH, and the
reaction is said to beentropy driven.
4.3 RATE THEORIES
Virtually no food is in thermodynamic equilibrium. We all know that food is
a source of energy and that, for example, one gram of carbohydrate yields
on oxidation in the body about 17 kJ (about 4 kcal). Assuming the
elementary reaction to be
CH 2 OþO 2 ?CO 2 þH 2 Owe calculate the molar bond energy difference at about
30617 ¼500 kJ?mol^1 & 200 RT. Inserting this value asDGin Eq. (4.6),
we come up with the immense figure of 10^87 for the equilibrium constant.
Now this calculation is not too precise, and several refinements must be
made, but that does not materially alter the result. So the driving force for
oxidation by the O 2 in air of, say, plain sugar is very large; nevertheless plain
sugar appears to be stable almost indefinitely.