Lecture_notes_14.1-14.4

Chemical kinetics

is the study of the factors

that affect the rates of chemical reactions, such

as temperature, time of reaction, concentration

of reactants.

reaction rate

The speed of a chemical reaction is called its

Kinetics

the study of how fast

reactions take place

• Some reactions are fast

(photosynthesis)

• Some reactions are slow (conversion of

diamond to graphite)

a. Reactant Concentration

Generally, the greater the concentration of

reactant molecules, the faster the reaction.

– This increases the frequency of reactant molecule

contact.

– Concentration of gases depends on the partial

pressure of the gas.

• Higher pressure = higher concentration.

Temperature

Increasing temperature increases the reaction

rate for most reactions.

– In general, for every 10 °C rise in temperature, the

speed of the reaction doubles.

• Svante Arrhenius discovered a mathematical

relationship between the absolute temperature

and the speed of a reaction. This relationship will

be examined later.

c. Structure and Orientation of the Colliding

Particles

Nature of the reactants refers to what kind of reactant

molecules there are and what physical condition they are in.

– Small molecules tend to react faster than large molecules.

– Gases tend to react faster than liquids, which react faster

than solids.

– Powdered solids are more reactive than “blocks.”

• More surface area for contact with other reactants

Rate

is how much a

quantity changes in a

given period of time

Units are M/s

average rate

the change in measured concentrations in any

particular time period.

• Linear approximation of a curve

– The larger the time interval, the more the average rate deviates from the

instantaneous rate

instantaneous rate

the change in concentration at any one

particular time.

• Slope at one point of a curve

– The instantaneous rate is determined by taking the slope of a line tangent

to the curve at that particular point

Reaction Rate and Stoichiometry

In most reactions, the coefficients of the balanced equation are

not all the same.

H2(g) + I2(g) 2 HI(g)

• For these reactions, the change in the number of molecules of

one substance is a multiple of the change in the number of

molecules of another.

– For the above reaction, for every 1 mol of H2 used, 1 mol of I2 will

also be used and 2 mol of HI made.

– Therefore, the rate of change will be different.

• To be consistent, the change in the concentration of each substance is

multiplied by 1/coefficient.

The Rate Law: The Effect of Concentration

on Reaction Rate

The rate law of a reaction is the mathematical relationship

between the rate of the reaction and the concentrations of

the reactants .

• Rate = k[A]n

• The rate of a reaction is directly proportional to the

concentration of each reactant raised to a power.

• For the reaction aA + bB → products, the rate law would

have the form given below.

– n and m are called the orders for each reactant.

– k is called the rate constant.

Rate = k[A]m[B]n

Reaction Order

• The exponent on each reactant in the rate law is called

the order with respect to that reactant.

• The sum of the exponents on the reactants is called the

overall order of the reaction.

• The rate law for the reaction 2 NO(g) + O2(g) → 2 NO2(g)

is as follows:

– Rate = k[NO]2[O2]

– The reaction is

• second order with respect to [NO];

• first order with respect to [O2].

– The reaction is third order overall.

– The exponents on each reactant are determined experimentally. The

coefficients of the reactants in the balanced equation DO NOT

automatically become the exponents.

Zero Order

Rate = k[A]n

• If a reaction is zero order, the rate of the

reaction does not change.

– Doubling [A] will have no effect on the

reaction rate.

• Rate law:

– Rate = k[A]0 = k

• Rate constant (k) units:

– Rate = M/sec, k = M/sec (M sec–1)

First Order:

Rate = k[A]n

• If a reaction is first order, the rate is directly

proportional to the reactant concentration.

– Doubling [A] will double the rate of the reaction.

• Rate law:

– Rate = k[A]1 or Rate = k[A]

• Rate constant (k) units:

– When Rate = M/sec, k = sec–1

Second Order

Rate = k[A]n

• If a reaction is second order, the rate is directly

proportional to the square of the reactant

concentration.

– Doubling [A] will quadruple the rate of the

reaction.

• Rate law:

– Rate = k[A]2

• Rate constant (k):

– When Rate = M/sec, k = M−1 · sec−1

Determining the Rate Law When There Are

Multiple Reactants

(Method of Initial Rates)

Changing the concentration of each reactant will

affect the overall rate of the reaction.

• By changing the initial concentration of one

reactant at a time, the effect of each reactant’s

concentration on the rate can be determined.

• In examining results, differences in reaction rate

are compared that differ only in the

concentration of one reactant.