Chapter 13 Chemical Kinetics
13.1 Catching Lizards (563)
- Lizards are ectotherms their body temperature depends on their surroundings. That is why, to catch a lizard, many pour
ice water on them, so their internal temperature can become colder and the chemical reactions that are highly sensitive to
temperature slow down and they become immobilized, b/c the reactions that keep them active are slowed down.
13.2 The Rate of a Chemical Reaction (564)
- The rate of a chemical reaction is a measure of how fast the reaction occurs.
- When we measure the rate at which something occurs, the measurements are a change in some quantity per unit time.
- To measure the concentration of a reactant over a period of reactions we can use: Rate = - [reactant]t.
- To measure the concentration of a product when it is double the moles of the reactant, we use: Rate = [product]t.
- The increase in [HI] occurs at exactly twice the rate of the decrease in
[H2] b/c of the stoichiometry of the reaction.
- General definition of reaction rates:
Measuring Reaction Rates (567)
- A technique known as polarimetry is when the products of a reaction rotate polarized light counter clockwise, and
measuring the degree of polarization of light passing through a reacting solution.
- In a spectrometer, if the sample contains the reacting mixture, the intensity of light absorption will decrease.
- Reaction in which the # of moles of gaseous reactants & products changes as the reaction proceeds can be readily
monitored by measuring changes in pressure.
- In some reactions that occur slowly enough that samples, or aliquots, can be periodically withdrawn from the reaction
vessel & analyzed to determine the progress of the reaction. Instruments such as gas chromatography, mass spectrometry,
and wet chemical techniques such as titration, can be used to measure the relative amounts of reactants or products in the
13.3 The rate Law: The Effect of Concentration on
Reaction Rate (569)
- Rate of reaction often depends on the concentration
of one or more of the reactants.
- As long as the rate of the reverse reaction (in which
the products return to reactants) is negligibly slow,
we can write a relationship called the rate law
b/w the rate of the reaction & the concentration of
the reactant as follows:
Where k is a constant of proportionality called the
rate constant and n is the reaction order. The value
of n (usually an integer) determines how the rate
depends on the concentration of the reactant.
- If n=0, the reaction is zero order and the rate is independent of the concentration of A.
- If n=1, the reaction is first order and the rate is directly proportional to the concentration of A.
- If n=2, the reaction is second order and the rate is proportion to the square of
the concentration of A. - Zero-order reactions occur under conditions where the amounts of reactant usually
available for reaction is unaffected by changes in the overall quantity of reactant.
Determining the Order of a Reaction (570)
- The order of a reaction can be determined only by experiment.
- The method of initial rates is a common way to determine reaction order in this method; the initial rate is measured by
doing the reaction several times w/ different initial reactant concentrations to see the effect of concentration on the rate.
First Order (n=1)
To determine the order (n) if you are unsure, you can:
Units for rate constant of zero-order reaction: M s -1
Units for rate constant of first-order reaction: s -1
Units for rate constant of second-order reaction: M s
Reaction Order for Multiple Reactants (571)
- The overall order is the sum of the exponents (m+n).
- The rate law for any reaction must always be
determined by experiment.
**Example on p.572 is really helpful**
13.4 The Integrated Rate Law: The Dependence of Concentration on Time (573)
- The rate laws we have examined so far shows the relationship b/w the rate of a reaction and the concentration of a
reactant. But we often want to know the relationship b/w the concentration of reactant and time.
- The integrated rate law for a chemical reaction is a relationship b/w the concentrations of reactants & time.
Ex: A products The integrated rate law for this reaction depends on the order of the reaction.
Rate = k[A] and since Rate = -[A]t, we can substitute that instead of Rate and come up with:
For a first-order integrate rate law
Second-order integrated rate law: Zero-order integrated
rate law:- The integrated rate law for all the above three orders, have the y=mx + b form and therefore can be graphed and rate law
constant can be calculated like that as well.
The Half-Life of a Reaction (578)
- The half-life (t ) of a reaction is the time required for the concentration of a reactant to fall to of its initial value.
- The life expression which defines the dependence of life on the rate constant and the initial concentration is
different for different reaction orders.
- For a first-order reaction: t1/20.693k. For a first-order reaction,
t1/2 independent of the initial concentration. A constant half-life is
unique to first-order reactions, making the concept of half-life