pressure, and V is the volume of the system. The
change in enthalpy equals the energy flow as heat at a
constant pressure. Used to predict the heat flow in
chemical reactions.
enthalpy of activation (standard enthalpy of acti-
vation), ∆t-H° (SI unit: kJ mol–1) The standard
enthalpy difference between the TRANSITION STATE
and the GROUND STATEof the reactants at the same
temperature and pressure. It is related to the tempera-
ture coefficient of the rate constant according to the
equation:
H= RT^2 (∂lnk/∂T)p– RT= Ea– RT
= –R(∂ln(k/T) / ∂(1/T))pwhere Eais the ENERGY OF ACTIVATION, providing that
the rate constants for reactions other than first-order
reactions are expressed in temperature-independent
concentration units (e.g., mol dm–3, measured at a
fixed temperature and pressure). If lnkis expressed as
lnk= (a/T) + b + c lnT+ dTthen
H= –aR+ (c–1)RT+ dRT^2If enthalpy of activation and ENTROPY OF ACTIVATION
are assumed to be temperature independent, then
H= –aRIf the concentration units are mol dm–3, the true and
apparent enthalpies of activation differ by (n–1)/
(αRT^2 ), where nis the order of reaction and αthe
thermal expansivity.
See alsoENTROPY OF ACTIVATION; GIBBS ENERGY
OF ACTIVATION.
enthalpy of formation(Hf) The change in the
enthalpy that occurs during a chemical reaction.
entropy The amount of energy in a closed system
that is not available for doing work; disorder and ran-
domness in a system. The higher the entropy, the less
energy is available for work. The second law of ther-
modynamics states that the entropy of the universe will
always increase.
entropy of activation(standard entropy of activa-
tion), ∆t-So(SI unit: J mol–1K–1) The standard entropy
difference between the TRANSITION STATE and the
GROUND STATEof the reactants, at the same tempera-
ture and pressure.
It is related to the GIBBS ENERGY OF ACTIVATION
and ENTHALPY OF ACTIVATIONby the equations∆t-S= (∆t-H– ∆t-G)/T
= ∆t-H/T – Rln(kB/h) + Rln(k/T)or, if lnkis expressed as lnk= a/T+ b+ clnT+ dT,∆-tS= R[b–ln (kB/h) + (c–1)(1 + lnT) + 2 dT]provided that rate constants for reactions other than
first-order reactions are expressed in temperature-
independent concentration units (e.g., mol dm–3, mea-
sured at a fixed temperature and pressure). The
numerical value of ∆t-Sdepends on the standard state
(and therefore on the concentration units selected). If
entropy of activation and ENTHALPY OF ACTIVATION
are assumed to be temperature-independent,∆t-S= R[b–ln(kB/h)]Strictly speaking, the quantity defined is the entropy of
activation at constant pressure, from which the entropy
of activation at constant volume can be deduced.
The information represented by the entropy of acti-
vation can alternatively be conveyed by the pre-expo-
nential factor A (seeENERGY OF ACTIVATION).environment The total living and nonliving condi-
tions of an organism’s internal and external surround-
ings that affect an organism’s complete life span.enzyme A macromolecule that functions as a BIOCAT-
ALYSTby increasing the reaction rate, frequently con-
taining or requiring one or more metal ions. In general,
an enzyme catalyzes only one reaction type (reaction
specificity) and operates on only a narrow range of
SUBSTRATEs (substrate specificity). Substrate molecules
are attacked at the same site (regiospecificity), and only
one, or preferentially one of the ENANTIOMERs of CHI-
RAL substrate or of RACEMIC mixtures, is attacked
(enantiospecificity).
See alsoCOENZYME.98 enthalpy of activation