Reaction Kinetics and Thermodynamics - PowerPoint PPT Presentation

1 / 14
About This Presentation
Title:

Reaction Kinetics and Thermodynamics

Description:

note that the equilibrium condition is governed by thermodynamics, and a ... Alcoholysis of Acetic Anhydride: Reactions in Non-Ideal Solutions. CHEE 323. 2.13 ... – PowerPoint PPT presentation

Number of Views:107
Avg rating:3.0/5.0
Slides: 15
Provided by: par55
Category:

less

Transcript and Presenter's Notes

Title: Reaction Kinetics and Thermodynamics


1
Reaction Kinetics and Thermodynamics
  • We define a catalyst as a substance that
    increases the rate of approach to equilibrium of
    a reaction without being substantially consumed
    in the reaction
  • note that the equilibrium condition is governed
    by thermodynamics, and a catalyst does not alter
    the equilibrium state, but the rate at which this
    state is reached.
  • What bearing does thermodynamics have on reaction
    kinetics?
  • Ultimate yield
  • Restriction of reaction orders
  • Influence of activity on the reaction rate

2
Ultimate Reaction Yield
  • The equilibrium composition of a system is
    dictated by thermodynamics.
  • Reactions serve to minimize the Gibbs Free energy
    of the system.
  • The state to which reaction kinetics lead is
    always the equilibrium state.
  • Consider the gas phase isomerization of 2-butene
  • The thermodynamic properties of the components
    are
  • 2-Butene (673K)
  • cis trans
  • DHf kJ/mole -13.8 -17.2
  • DSf kJ/Kmole 0.331 0.325
  • DGf kJ/mole 91.7 89.1
  • What is the final composition of the system?

3
Reaction Rates Concentration Dependence
  • In simple reactions of perfect gases, it is found
    from experiment that volume concentration is the
    key variable.
  • reaction velocity is not a function of alternate
    variables such as chemical potential, or mole
    fraction.
  • For the forward reaction of a simple system of
    near perfect gases
  • it is often found experimentally that the rate is
    proportional to small powers of concentration
  • where,
  • k is independent of concentration
  • a and b are not necessarily equal to a, b,
    respectively
  • A simple interpretation of this result is
    generated by collision theory, assuming that
    reactions occur by molecular collisions whose
    frequency increases with the spatial density of
    reactants.

4
Thermodynamic Restrictions on Reaction Order
  • For many reactions, the equilibrium distribution
    of products is not displaced predominately in one
    direction or the other. One example is the
    decomposition of hydrogen iodide vapour
  • Experimental work shows the rate of HI
    decomposition may be expressed in the form
  • where k and k are constants.
  • For given concentrations, only the net rate of
    decomposition can be measured. The forward and
    reverse rates have meaning only by
    interpretation.

5
Thermodynamic Restrictions on Reaction Order
  • Thermodynamics requires
  • the reaction rate be positive in the direction
    that decreases the free energy of the system
  • at equilibrium, the rate must reduce to zero
  • As the decomposition of hydrogen iodide reaches
    an equilibrium condition, -dHI/dt must approach
    zero,
  • or
  • which is the correct form of the equilibrium
    constant for this system.
  • The ratio k/k of the experimental velocity
    constants (Kistiakowsky, 1928) equals the
    measured equilibrium constant (Bodenstein, 1899)
  • thermodynamic conditions are satisfied by this
    rate expression

6
Thermodynamic Restrictions on Reaction Order
  • If we consider a generic, elementary gas-phase
    reaction
  • we have at equilibrium
  • If we measure the formation of C from A,B at low
    concentrations of the product, we are effectively
    measuring the forward reaction rate. Suppose we
    can express the formation of C as
  • (commonly, a,b1, g0)
  • If we wish to represent the reaction velocity
    over all concentrations of A,B and C, we must
    consider the reverse reaction, which yields

k k
7
Thermodynamic Restrictions on Reaction Order
  • Having determined the reaction orders a,b,g by
    experiment, thermodynamics restricts the values
    of a,b,g.
  • At equilibrium the reaction rate must reduce to
    zero, therefore
  • or,
  • The equilibrium relationship derived from the
    kinetic expression is
  • Eq. A
  • while that known from the stoichiometry of the
    reaction is
  • Eq. B

8
Thermodynamic Restrictions on Reaction Order
  • For the kinetic rate expression to be consistent
    with thermodynamics (Eq. A equivalent to Eq. B)
    the parameters a,b,g must comply with
  • Eq. C
  • where n is any positive value.
  • Suppose, for example, the reaction is
  • If by experiment we determine the forward rate of
    reaction to be,
  • then permissible expressions for the reverse
    reaction include,

9
Thermodynamic Restrictions on Reaction Order
  • Consider the following base-catalyzed addition of
    water to acetophenone to generate the
    corresponding hydrate.
  • Can you describe the equilibrium state of this
    reaction?
  • The mechanism is straightforward
  • Can you derive a rate expression from this
    mechanism that is consistent with the
    thermodynamic expression?

10
Reactions in Non-Ideal Solutions
  • The use of volume concentrations in describing
    reaction kinetics has is origins in experimental
    research near perfect gas mixtures.
  • In liquid phase reactions, we know that the
    equilibrium relationship for a reaction such as
  • in solution
  • must be expressed as
  • Given that this is the ultimate limit of a
    kinetic rate expression, the reaction rate should
    (strictly speaking) depend on activities rather
    than concentrations.
  • which, provided the reaction orders satisfy Eq.
    C, will generate the appropriate equilibrium
    expression.

11
Reactions in Non-Ideal Solutions
  • Treatment of reaction kinetics with simplified
    expressions derived from gas behaviour, such as,
  • is done routinely. However, the kinetic rate
    constants prove to be functions of
    concentration when extended over a wide range.
    This is particularly true in reactions involving
    ions and/or ionic intermediates.
  • Roughly speaking, the reaction velocity may be
    regarded as being largely determined by the
    collision frequency (volume concentration), but
    non-ideality resulting from complex molecular
    interactions requires the application of activity
    coefficients or an analogous treatment.

12
Reactions in Non-Ideal Solutions
  • The influence of solution non-ideality on
    reaction rates is frequently observed in the
    dependence of reaction velocity on solvent.
  • Alkylation of triethylamine
  • Alcoholysis of Acetic Anhydride

13
Summary - Kinetics and Thermodynamics
  • The common use of volume concentrations in
    reaction kinetics is derived from experimental
    research on perfect gas mixtures.
  • Thermodynamics requires any kinetic rate
    expression to
  • be positive in the direction of decreasing Gibbs
    Energy
  • reduce to zero at an equilibrium condition
  • represent the equilibrium condition accurately
  • Reactions in solutions are, in a strict sense,
    poorly represented by rate equations that make no
    reference to component activities
  • In some cases (pH dependent reactions, ionic
    equilibria) it may be necessary to adopt an
    activity coefficient approach
  • Beware that reactions in solution are usually
    solvent dependant, and rate constants derived
    from data in one solvent may not accurately
    represent the system in another.

14
Food for Thought
  • Working as an Engineering Chemist in an isoprene
    polymerization facility, you meet a salesperson
    that wishes to sell your company a new catalyst
    technology. According to the sales literature,
    the new organometallic complex polymerizes
    isoprene to produce cis-poly(isoprene) in 99
    yield versus the thermodynamically more stable
    trans-poly(isoprene).
  • Argue whether this is possible using a free
    energy diagram to illustrate your point of view.
Write a Comment
User Comments (0)
About PowerShow.com