Title: Chemical Reactions
1Chemical Reactions
- How Fast? Why Use Catalysts?
2Rate 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.
3Collision 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
4Transition 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
5Energy 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
6Energy Profile of Decomposition of HI
2 HI(g) H2(g) I2(g) ?H 3 kcal and Ea
41 kcal
7Significance 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.
8Orientations of Transition State
Orientation 1 is a proper orientation - capable
of producing products
Orientation 2 is an improper orientation - not
capable of producing products
9Factors 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
101. 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)
112. 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
123. 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
134. 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
14Continued...
- 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
15Temperature and Energy of Activation
165. 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
17Reaction Without Catalyst
2 H2O2(aq) 2 H2O(l) O2(g) ?H -50 kcal
High Ea since two large molecules must collide
18Reaction 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.
19Heterogeneous 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