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Chemical Kinetics


Law of constant proportions - compounds made up of same elemental ... Metathesis - rearrangement of atoms, ions: precipitation, softening. 4. Chemical Equations ... – PowerPoint PPT presentation

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Title: Chemical Kinetics

Chemical Kinetics
  • Chapter 3

Chemical Reactions
  • Elements combine to form compounds by chemical
    reaction (atomic shuffle or rearrangement),
  • Law of conservation of mass
  • Law of constant proportions - compounds made up
    of same elemental components in same proportion
    by weight
  • Law of multiple proportions - ratios of wt of 1
    element combining with fixed amt of another is in
    small whole nos.

Types of Chemical Reactions
  • Proton transfers (acid base reactions)
    neutralization, alkalinity measurements
  • Electron transfer (oxidation/reduction)
    chlorination, ozonation, biological treatment
  • Electron sharing (covalent, ionic bonding) ion
    exchange, precipitation, softening
  • Metathesis - rearrangement of atoms, ions
    precipitation, softening

Chemical Equations
  • Used to describe chemical reactions
  • aA bB-----gt cC dD
  • products reactants
  • Balanced equation (including redox reactions)
  • Reaction quantities

Chemical Kinetics
  • Approaches in evaluating a chemicals fate and
    treatment kinetics and equilibrium
  • Kinetics deals with the rates of reactions.
  • Kinetic approach is appropriate when the reaction
    is slow relative to our time frame or when we are
    interested in the rate of change of
  • Rate Change in concentration/time

Factors that affect reaction rate
  • Concentration of reactants
  • Increasing concentration of reactants often
    increase the rate of reaction
  • Temperature
  • Increasing the temperature generally increases
    the rate of reaction
  • Catalysts
  • Catalysts increase the rate of reaction without
    themselves being consumed

The Rate Law
  • Rate law used to predict the rates of chemical
    and biological processes.
  • The rate law describes the relationship between
    the rate of a reaction and the concentration of
    the reactants.

Reaction Rate
  • Rate (R ) kAaBb
  • The rate constant, k
  • Relates rate and concentration at a particular

The reaction order
  • The rate law describing the decrease in
    concentration of chemical C with time

where C is concentration of C t is time k is
rate constant dependant on the order of the
reaction n, reaction order
Zero-order Reaction
  • If n is zero,
  • This is the rate law describing a zero-order

Unit conc/time (moles/L-s)
First-Order Reaction
  • If n1,
  • This is the rate law for a first-order reaction.
  • E.g. radioactive decay.

(Unit of time-1,e.g. s-1)
Example 13
  • How long will it take the CO concentration in a
    room to decrease 99 after the source of CO is
    removed and the windows are opened? Assumed the
    first-order rate constant for CO removal (due to
    dilution by incoming clean air) is 1.2 h-1.

Pseudo-First-Order Reactions
  • The rate law for this reaction is
  • If the concentration of A does not change
    significantly during the reaction i.e.
  • , the
    concentration of A may be assumed to be
    constant can be incorporated into the rate
    constant, k.

  • Then, the rate law becomes
  • where k is the pseudo-first-order rate constant
    and equals kA0a.
  • The rate law for disappearance of substance B
  • If b1,

Example 14
  • An engineer is modeling the transport of a
    chemical contaminant in groundwater. The
    individual has a mathematical model that only
    accepts first-order degradation rate constants
    and a handbook of "subsurface chemical
    transformation half- lives." Subsurface
    half-lives for benzene, TCE, and toluene are
    listed as 69, 231, and 12 days, respectively.
    What are the first-order rate constants for all
    three chemicals?

  • The half-life, t½, is defined as the time
    required for the concentration of a chemical to
    decrease by one half (e.g. C0.5C0.
  • For zero-order reactions 0.5C0 C0-kt½
  • For first-order reactions 0.5C0 C0e-kt

Example 15
  • After the Chernobyl nuclear accident, the
    concentration of 137Cs in milk was proportional
    to the concentration of 137Cs in the grass that
    cows consumed. The concentration in the grass
    was, in turn, proportional to the concentration
    in the soil. Assume that the only reaction by
    which 137Cs was lost from the soil was through
    radioactive decay and the half-life for this
    isotope is 30 years. Calculate the concentration
    of 137Cs in cow's milk after 5 years if the
    concentration in milk shortly after the accident
    was 12,000 Bq/L. (Note A Bequerel is a measure
    of radioactivity. One Bequerel equals one
    radioactive disintegration per second.)

Effect of Temperature on Rate Constant
  • Rate of molecular motion was a function of
  • Higher the temperature, the faster the molecules
    move, which results in an increase in the number
    of collisions per unit time. A higher temp. also
    increases the energy of the collisions.
  • As a result, a greater fraction of the collisions
    results in a chemical reaction.
  • Thus, the rate of chemical reactions that depend
    on collision of two or more molecules will be
    increased by an increase in temperature.

Arrhenius equation
  • The Arrhenius equation is used to adjust rate
    constants for changes in temperature
  • where k is the rate constant of a particular
  • A is termed as preexponential factor (same unit
    as k)
  • Ea is the activation energy (kJ/mole)
  • R is the gas constant
  • T is a temperature (K)

Rate Constant for CBOD
  • The carbonaceous biochemical oxygen demand (CBOD)
    rate constant, kL, known at a particular
    temperature, is typically converted to other
    temperatures using the following expression
  • is a dimensionless temperature coefficient.
  • In fact, equals

Example 16
  • The rate constant for CBOD at 20oC is 0.1 day-1.
    What is the rate constant at 30oC? Assume