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Chemical Reactions

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... the concentration of a reactant frequently increases the rate of a reaction. The Collision Theory explains this as being due to increased collision frequency ... – PowerPoint PPT presentation

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Title: Chemical Reactions


1
Chemical Reactions
  • How Fast? Why Use Catalysts?

2
Rate of Reaction
  • Under a given set of conditions, each chemical
    reaction requires a characteristic amount of time
    to produce a given amount of product
  • Rate of the reaction - the amount of a
    participant undergoing change per unit time, such
    as lbs/hour, tons/day, etc.

3
Collision Theory
  • For reaction to occur, particles of the reactants
    must collide with sufficient energy to form a
    transition state in which all the reacting atoms
    are in a proper orientation
  • Transition State - unstable state produced by the
    collision of reacting molecules - all reacting
    atoms are held together by weak bonds

4
Transition State
  • 2 HI(g) H2(g) I2(g) ?H 3 kcal
  • Reactants Products
  • 2 HI(g) H - H H2(g) I2(g) ?H 3
    kcal Reactants ?????? Products
    I - I
  • Transition State

5
Energy of Activation, Ea
  • The minimum energy required to produce the
    transition state is called the energy of
    activation (Ea) for the reaction
  • Following is a graph, called an energy profile,
    for the decomposition of HI into H2 and I2, where
    ?H 3 kcal and Ea 41 kcal

6
Energy Profile of Decomposition of HI
2 HI(g) H2(g) I2(g) ?H 3 kcal and Ea
41 kcal
7
Significance of Energy of Activation
As Ea increases, the fraction of collisions that
can produce products decreases. Therefore, the
higher the energy of activation,
the slower the rate of the reaction.
8
Orientations of Transition State
Orientation 1 is a proper orientation - capable
of producing products
Orientation 2 is an improper orientation - not
capable of producing products
9
Factors Which Influence the Rates of Chemical
Reactions
  • 1. Nature of the Structure and Bonding of the
    Reactants
  • a. Different reactions involving substances
    with different bonding require different energies
    of activation
  • b. Different reactions involving substances
    with different structures will have different
    proper orientations

10
1. Structure and Bonding of the Reactants
  • C6H14 Br2 C6H13Br HBr (Slow) Hexane
  • C6H12 Br2 C6H12Br2 (Fast) Hexene
  • Difference in rates due to difference in bonding
    (changes Ea) and difference in structure (changes
    the geometry of the proper orientation)

11
2. Surface Area
  • An increase in the surface area of the more
    condensed reactant causes an increase in the rate
    of a heterogeneous reaction (a reaction involving
    reactants in more than one phase such as a
    gas and a solid)
  • Lycopodium powder burns explosively when
    dispersed in air

12
3. Concentrations of Reactants
  • Increasing the concentration of a reactant
    frequently increases the rate of a reaction
  • The Collision Theory explains this as being due
    to increased collision frequency

13
4. Temperature
  • Increasing temperature increases the rates of all
    reactions
  • A 10 oC increase in temperature causes the rates
    of many reactions to double or triple

14
Continued...
  • Due, in part, to increased collision frequency as
    the molecules move faster at the higher
    temperature
  • Due mostly to the increase in the fraction of
    molecules possessing the energy of activation

15
Temperature and Energy of Activation
16
5. Catalyst
  • A catalyst is a substance whose presence alters
    the rate of a reaction without undergoing
    permanent change itself
  • Adding KI(s) to 2 H2O2(aq) 2 H2O(l) O2(g)
    ?H -50 kcal increases rate, but all KI(s) can
    be recovered unchanged after reaction - KI is a
    catalyst

17
Reaction Without Catalyst
2 H2O2(aq) 2 H2O(l) O2(g) ?H -50 kcal
High Ea since two large molecules must collide
18
Reaction with Catalyst
H2O2 I- IO- H2O H2O2
IO- H2O I- O2 2 H2O2
2 H2O O2 Fast, Ea 25 kcal
Collision of peroxide and iodide has lower Ea,
faster rate.
19
Heterogeneous Catalysis
  • Heterogeneous catalyst is in a different state
    from the reactants usually a solid while
    reactants are gases or liquids
  • Provides a surface to which reactants are
    attracted which, in turn, weakens bonds in
    reactants, lowers Ea, and increases rate of
    reaction
  • MnO2(s) catalyzes peroxide reaction
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