SAT Subject Test Chemistry,10 edition

In general, for the reaction

Rate is expressed in units of concentration per unit time, most often moles per liter per second
(mol/L • s), which is the same as molarity per second (molarity/s).

DON’T MIX THESE UP ON TEST DAY

Keep    in  mind    the difference  between the rate    law and the equilibrium expression. The

exponents   in  the rate    law,    unlike  those   in  the equilibrium expression, do  not necessarily

have    to  do  with    the balanced    chemical    equation!

RATE LAW

For nearly all forward, irreversible reactions, the rate is proportional to the product of the
concentrations of the reactants, each raised to some power. For the general reaction:

aA  +   bB  →   cC  +   dD

the rate is proportional to [A]x [B]y, that is:

rate    =   k[A]x[B]y.

This expression is the rate law for the general reaction above, where k is known as the rate constant,
and is different for different reactions and may also change depending on the reaction conditions
(more on this below). Multiplying the units of k by the concentration factors raised to the
appropriate powers gives the rate in units of concentration/time. (The unit of k, therefore, depends
on the values of x and y.) The exponents x and y are called the orders of reaction; x is the order of the
reaction with respect to A and y is the order with respect to B. These exponents may be integers,