Figure 2-128. The heat (Q) released in an exothermic reaction is presented with a minus sign. The heat (Q) consumed in an endothermic reaction is presented with a plus sign. 2.12.2. Rates of Chemical Reactions. Guldberg and Waage's Law of Mass Action. Rate Constant. Concentration and Temperature as Key Factors Influencing Reaction Rate.
Different reactions occur at different rates. Neutralization of a strong alkali (such as KOH) with a strong acid (such as HCl) occurs within the time of mixing. Likewise, precipitation of BaSO4
on addition of Na2
to a solution of BaCl2
is instantaneous. The oxidation of sucrose (sugar) with KMnO4
in the presence of NaOH, however, is noticeably slower, taking minutes as shown in this video
. The time frame for some chemical transformations such as the rusting of iron in the air and the decomposition of hydrogen peroxide in dilute aqueous solutions can be months and even years. The rate of a chemical reaction is determined by measuring the decrease in reactant concentration or the increase in product concentration in a unit of time. A widely used reaction rate unit is M/sec, where M is molarity.
The branch of chemistry that deals with rates of chemical transformations is called chemical kinetics
. Studying reaction rates is beneficial for a number of reasons. One is that the information obtained in such studies often reveals important details about how a particular reaction occurs. These details, including the dependence of reaction rate on various factors help us develop means to speed up important chemical transformations that are too slow and slow down those that are too fast. It is not surprising that all industrially important chemical processes have been most scrupulously studied by methods of chemical kinetics.
The rate of a chemical reaction can be influenced by a number of factors, including reagent concentration, temperature, pressure, light, and the presence or absence of a catalyst. Watch this educational and entertaining video
for a discussion of how various factors affect reaction rates. The first and foremost factor that obviously should and does influence reaction rates is reagent concentration. In order for molecules to react with one another, they must collide. The probability of collision between molecules is higher at higher concentrations. Pioneered by two Norwegian scientists, Cato M. Guldberg (1836-1902) and Peter Waage (1833-1900), the law of mass action
states that the rate of a chemical reaction is proportional to the product of concentrations of the reagents. For a general chemical reaction, a A
+ b B
are the reagents and a
are the coefficients),
the rate V
is expressed as:
V = k [A]m [B]n
] and [B
] are molar concentrations of the reagents A
are the powers the concentrations are raised to; and
is the coefficient of proportionality that is called the rate constant
Some important comments are due. The powers n
are known as the orders
of the reaction. We say that the reaction is nth
order in reagent A
order in reagent B
. The sum (n
) is called the overall reaction order
. Alas, it is not uncommon to see in some textbooks the statement that the powers n
are equal to the coefficients a
, respectively, of the balanced equation a A
+ b B
→ products. Although in some instances n
, this is not always the case. The values of the powers n
can be determined only experimentally. Reactions are known where the order in a reagent involved is fractional. Some other reactions display zero
order in one of the reagents. The physical meaning of zero order in a reagent is that the reaction rate is independent of the concentration of that reagent (any number
to the power of zero is 1). The experimental determination of the overall reaction order and the order in each of the reagents often sheds light on reaction mechanism, the sequence of elementary steps involved in a chemical transformation.
The rate constant k
depend on reagent concentrations. As is clear from the expression for V
is the rate of the reaction when the concentration of both reagents A
equals 1 mol/L (1 raised to any power is 1). To compare intrinsic rates of two or more different chemical reactions, their k
values should be used as the reference, not rates V
, which are often measured at randomly selected concentrations of reacting substances.
While being independent of reagent concentration, the rate constant k
of a chemical reaction is temperature-dependent. Molecules move and collide nonstop. Not every collision, however, results in chemical change. The colliding particles must possess certain energy, known as activation energy
, in order for the reaction to happen. At higher temperatures, molecules move faster, thereby colliding more frequently and with higher energy, which results in faster reaction.
The dependence of k
and, consequently, V
on temperature is expressed by the Arrhenius equation
. Although the Arrhenius equation is beyond the scope of this course, it is useful to remember the simple rule of thumb, sometimes referred to as van't Hoff's rule: For every 10 o
C rise in temperature, the reaction rate increases by a factor of approximately 2 to 4.