Transcript Factors Affecting Rate - Ms. Witt's Site

Factors Affecting Rate
• Collision model: in order to react, reactant
molecules must collide
• Chemical reactions speed up when
temperature increases, but not as much as
one would think
• Only a small fraction of collisions actually
react……..
Activation Energy
• Threshold energy that must be overcome in
order for chemicals to react
• According to the collision model, energy comes
from kinetic energy stored in reactants’ bonds.
Kinetic energy becomes potential energy during
intermediates.
• Collisions between reactant molecules must
have enough energy to allow products to form
Reaction Rate
• Temperature will exponentially increase the number of collisions with
enough activation energy to react
# of collisions with AE = (total # collisions)e-Ea/RT
Ea = activation energy
R = constant (8.3145)
T = kelvin
e-Ea/RT = fraction of collisions with energy Ea or greater at temperature, T.
Still Not Enough
• Even though the reactants collide with enough
energy, they still often will not react…
– Molecular orientations can play a roll in reactions
Successful Collisions:
1. Needs enough energy to meet or exceed the
activation energy
2. Orientations must allow for new bonds to form
NEW RATE CONST. (Arrhenius) EQUATION:
K = Ae-Ea/RT
A = frequency factor
e-Ea/RT = fraction of collisions with enough energy
Natural Log
ln(k) = -(Ea/R) (1/T) + ln(A)
Y = m
x + B
Plotting ln(k) versus 1/T will give a line
K(s-1)
T(°C)
2.0 X 10-5
20
7.3 X 10-5
30
10-4
40
9.1 X 10-4
50
2.9 X 10-3
60
2.7 X
Example
2N2O5(g) -> 4NO2(g) + O2(g)
Calculate the value for Ea
for this reaction.
Answer: Answer: 1.0 X 105
J/mol
Rearranging the Equation
ln(k2/k1) = (Ea/R)[(1/T1)-(1/T2)]
Example
CH4(g) + 2S2(g) -> CS2(g) + 2H2S(g)
• At 550°C the rate constant for this reaction is
1.1 L/mol S, and at 625°C the rate constant is
6.4 mol/L s. Using these values calculate Ea for
this reaction.
Answer: 1.4 X 105 J/mol
Example #2
H2(g) + I2(g) -> 2HI(g)
If the activation energy for the reaction
is 167 kJ/mol and the rate constant at
302°C is 2.45 X 10-4 L/mol, what is the
rate constant for the reaction at
205°C?
Answer: 2.06 X 10-7 L/mol s
Example #3
A second order reaction has rate constants of
8.9 X 10-3 L/mol and 7.1 X 10-2 L/mol at 3°C
and 35°C respectively. Calculate the value of
the activation energy for the reaction.
Answer: 46 kJ/mol
Reaction Rate: Catalysis
• Sometimes, raising the
temperature is not feasible
(temp. regulation or $)
• Catalysts: substances that
speed up reactions without
being used up
• New pathway formed with a
LOWER ACTIVATION
ENERGY
Types of Catalyst
• Homogeneous Catalysts: Present in the
same phase as the reacting molecules
• Heterogeneous Catalysts: In a different
phase, usually a solid
Heterogeneous Catalysis
• Usually involves gases being absorbed
on a solid’s surface
• Adsorption: on surface
• Absorption: penetrates surface
• Example: hydrogenation - conversion of
CC double bonds to CC single bonds
through the addition of hydrogen
Homogeneous Catalysis
• Same phase (usually gases or liquids)
• Example: ozone gases, freons, etc.