Chap. 13
Chemical Kinetics

    Rate or speed a chemical reaction occurs.

    Kinetic refers to the reaction rate. Does the rxn proceed rapidly enough? Is the rxn product favored and will we get the desired products?

Average rate of reaction

    Measured by the decrease in concentration of a reactant or an increase in concentration of a product over a time interval.

 

Reactants ---> Products
A B

    Average Rate = - [A] / time

    Or

    Average Rate = [B] / time

 

    Change in concentration([ ] means Molarity) / time interval

    Rate is always a + quantity

 

plot of concentration vs time
from experimentation

    1. Instantaneous rate: at a specific time this is the negative of the slope of a tangent line.

    2. Initial rate: at time = 0 this is the negative slope of the tangent line. Instantaneous rate at time zero.

 

Factors affecting reaction rates

    1. The nature of the reacting species

    a. Aqueous

 

    b. Type of reaction requiring a specific orientation

 

 

    2. Have different phases or layers where the rxn occurs at the boundary of the subdivision. Directly related to surface area of the contact site.

 

    3. Temperature of the reactants: Reaction rate is temperature dependent, so a Temperature is given.

    increase in temperature, increase in rate!!

 

 

    4. Collison theory: The rate is proportional to the number of molecular collisions per unit time. Related to the concentration of the reactants at a fixed temperature.

 

 

    5. Calalyst: a substance that increases the rate of a chemical reaction without itself being consumed.

    ratecatalysed> rateuncatalyzed

 

    6. Energy of activation: Ea minimum amount of energy required to initiate a chemical reaction forming an activated complex (transition state).

    Arrhenius equation: p. 532

 

 

Rate Law or rate equation

    A mathmatical equation determined experimental for the effect of concentrations of reactants on reaction rates at constant temperature.

 

 

aA + bB --> cC + dD

    Rate = k [A]a[B]b

 

    k = rate constant for a particular rxn at a specific temperature. Need to be able to calculate the units!!! Greater the k, faster the reaction

    [ ] = Molarity of each reactant

    a, b, etc = order of reaction with respect to each are exp. determined

Order of reaction

    0 is called zero order

    1 is called first order

    2 is called second order

    3 is called third order

 

    Overall order = sum of the order of rxn with respect to each reactant NOT RELATED TO COEFFICIENTS

NO + O3 NO2 + O2 at 25oC

    1. What is the general rate equation?

Given

    exp [NO] [O3] Rate = M/s

 

    1. 1.00 3.00 .660

    2. 1.00 6.00 1.32

    3. 1.00 9.00 1.98

    4. 2.00 9.00 7.92

     Give the rate law with the orders of rxn. What is the value of the rate constant?

CH3CHO CH4 + CO at 100oC

    Exp. [CH3CHO] rate = M/s

    1. 1.75 x 10-3 2.06 x 10-11

    2. 3.50 x 10-3 8.24 x 10-11

    3. 7.00 x 10-3 3.30 x 10-10

    Give the general rate law with the orders of reaction.

    Calculate the rate constant.

ICL + H2 HCl + I2 at 225 K

    Exp. [ICl] [H2] Rate = M/s

    1 1.5x10-3 1.5x10-3 3.7x10-7

      2. 3.0x10-3 1.5x10-3 7.4x10-7

    3. 3.0x10-3 4.5x10-3 2.2x10-6

    General rate law,Rate constant

      Rate if you have 4.5x10-3 ICl 7.5x10-3 H2?

others

Integrated rate equation

    Related to the overall order of the rxn.

    Four equations

    Half-life: t1/2 time required for the concentration of a reactant to decrease to half of the original.

1st order overall

    Integrated rate equation, p. 520

 

    ln ([A] / [Ao]) = -kt

    half-life, p. 525

    t1/2 = 0.693 / k

 

2nd order overall

    Have either one reactant that is second order or two reactants, each first order, with the same concentration. P. 527

 

    1/ [A] 1/ [Ao] = kt

    t1/2 = 1/ k[Ao]

problems