113 Rates and equilibria

Chemical equilibriaare ratios relating the forward and backward direction of a reaction
to each other. This ratio is represented by the letterKin the following equation:

K = products / reactants

113.1 Rate of reaction

113.1.1 Definitions

Rate of reactionis the speed at which a chemical reaction takes place, expressed as moles
per unit time and unit volume. The raterof a general reactionaA+bB+...−>pP+qQ+...
is defined by:

r=−a^1 dCdtA=−^1 bdCdtB=...=^1 pdCdtP=....

from the expression above, it is clear that the usual convention is that reaction rate is taken
as products formation rate.

Rate is a function of the concentration of reactants and products, temperature, pressure
and presence of a catalyst.

113.1.2 Rate expression

A common expression for reaction rate is thepower law:

r=kCαACBβ....

kis called thekinetic constant, andα,β, etc. are called thereaction order with respect
to the reactant A, B(or thepartial order of A, Betc.), respectively. The sum of all the
orders is theglobal orderof the rate expression. Therefore the rate expressionr=kCACB
is asecond-orderandfirst-orderin A and B.

Thereactionordersarethesameasthestoichiometriccoefficientinthecaseofanelementary
reactiononly; in most cases they must be determined experimentally and are valid in the
window of experimental conditions.

Elementary data on reaction orders can be obtained by changing the concentration C of one
of the reactant, say A, and measuring the initial rate r. Plotting a rate vs concentration
log-log graph, one obtains a straight line whose slope is the partial order in—say—A by
virtue of the relationlogr=logkCαA =logk+αlogCA