Download Chemistry 1011

Chemistry 1011
TOPIC
Rate of Reaction
TEXT REFERENCE
Masterton and Hurley Chapter 11
Chemistry 1011 Slot 5
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11.6 Reaction Rate and
Temperature
YOU ARE EXPECTED TO BE ABLE TO:
• Use the kinetic theory of gases to explain
the effect of temperature on reaction rate.
• Use the Arrhenius equation to determine the
activation energy for a reaction given
appropriate rate data.
Chemistry 1011 Slot 5
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Reaction Rate and Temperature
• Most reaction rates increase with
temperature
• A 10o rise in temperature will double the
rate of reaction
• Food cooks faster in a pressure cooker
• Food spoils faster outside a refrigerator
Chemistry 1011 Slot 5
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Using the Kinetic Theory of
Gases
• At any given temperature, only a fraction of
molecules possess enough energy to react
• Raising the temperature raises the average
energy of the molecules. This substantially
increases the number of molecules
possessing the energy necessary to react
Chemistry 1011 Slot 5
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Temperature effect on energy
Chemistry 1011 Slot 5
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The Arrhenius Equation
• Remember that
k = p x Z x e-Ea/RT
– Assume that p is not temperature dependent
– Z is only somewhat temperature dependent
• then
k = Ae-Ea/RT
• A is a constant
(R is the gas constant)
(T is the absolute temperature in Kelvin)
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The Arrhenius Equation
k = Ae-Ea/RT
• Taking logs:
ln k = ln A - Ea/RT
• This is the Arrhenius Equation
• It represents a straight line with slope -Ea/R
Chemistry 1011 Slot 5
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Arrhenius Equation Plot
ln k = ln A - Ea/RT
Slope = - Ea/R
= -1.61 x 104
Ea = -R x slope
= - (8.31J/mol)(-1.61 x 104K)
Ea = 134kJ/mol
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Calculating Ea from two Data Points
• Given two rate measurements at two
different temperatures:
ln k2 = ln A - Ea/RT2
ln k1 = ln A - Ea/RT1
• Subtract the second from the first
ln k2 - ln k1 = -Ea/R[1/T2 - 1/T1]
ln k2/ k1 = Ea T2 - T1
R T2 T1
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Example 11.6
• Rate constant doubles when temperature
increases from 15oC to 25oC
– Calculate Ea
– Calculate k at 100oC
• Use “two point” method to find Ea
• Use ln k2/ k1 = Ea T2 - T1
R T2 T1
to find k2
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