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Chapter 11Rate of Reaction
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This Chapter Chemical kinetics is the study of
reactions rates
11.1 - Meaning of Reaction Rate
11.2 & 11.3 - Concentration and Time
11.4 - Models for Reaction Rate
11.5 - Temperature
11.6 - Catalysis
11.7 – Reaction Mechanisms
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Learning Objectives (11.4, 11.5 11.6)
• Know the main tenets (points) of collision theory
• Be able to calculate activation energies• Be able to identify the factors that affect
reaction rates• Be able to interpret reaction diagrams• Arrhenius equation and calculations• Catalysis- Heterogeneous,
Homogeneous, and Enzymatic
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Collision TheoryThree conditions must be met for a successful reaction to occur…
1- Must hit each other in a proper
orientation (p = steric factor)
2- Atoms/ions/molecules must collide
(Z = collision frequency)
3- A fraction of the collisions must occur with enough energy to be greater than activation energy(Ea) (f = fraction of collisions)
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Collision Theory: Advanced We can take these factors and quantitate them.
The rate constant for a particular rxn, k, can be
written …
k = p x Z x f
p = hard to predict this value, it is less
than 1, usually much less.
Z = can be calculated precisely from kinetic
theory of gases, but we won’t
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Collision Theory: Advanced
We can calculate f (fraction of collisions)
using following equation ….
f = e -Ea/RT where e = base of natural logarithms R = gas constant T = absolute temp. in K
k = p x Z x f
k = p x Z x e -Ea/RT
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Collision Theory: Advanced
k = p x Z x e -Ea/RT
If Ea = 0, then e -Ea/RT = e0 = 1If Ea = RT, then e -Ea/RT = e-1 = 0.37If Ea = 2RT, then e -Ea/RT = e-2 = 0.14
So as the activation energy goes up, the rate constant goes down.
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Collision Theory
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Collision Theory
collision theory orientation
collision theory video game analogy
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Energy Diagram-Endothermic
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Energy Diagram-Exothermic
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Factors Affecting Reaction Rates
Reaction rates are affected by:–Temperature
–Concentration
–Particle Size
–Catalysts/Inhibitors
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Factors Affecting Reaction Rates
Lets summarize:– Temperature
(higher temperature makes rate faster)
– Concentration
(higher molarity makes rate faster)
– Particle Size
(smaller particles makes rate faster)
– Catalysts/Inhibitors
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Arrhenius Equation: Background
Equation solved and proven by Svante Arrhenius in 1889. Very influential chemist from Sweden. Besides his work in chemical kinetics and acids and bases, he presented the basic idea of the greenhouse effect in an 1896 in a paper entitled, “On the Influence of Carbon Dioxide in the Air on the Temperature of the Ground”.
Interesting fact: Arrhenius proposed that salts, strong bases, and strong acids like NaCl, NaOH, and HCl completely dissociated in water in his Ph. D. thesis. His research advisors at Uppsala in 1884 were not convinced and gave him the lowest possible passing grade, “approved without praise”.
He sent his thesis all over the world and only two young chemists gave positive responses. Jacobus van’t Hoff and Wilhelm Ostwald. These three young men were mocked by mainstream chemists and called “ionists”.
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The Ionists
Van’t HoffNobel Prize- 1901
“1st Nobel Prize in chemistry ever awarded”
Chemical kinetics, physically proved the
Arrhenius equation, van’t Hoff factor
ArrheniusNobel Prize- 1903
Father of physical chemistry, ions,
Arrhenius definition of acids, greenhouse
effect.
OstwaldNobel Prize- 1909
Catalysis, chemical equilibrium, reaction
velocities, law of dilution. Coined the term, “mole” in
1900. Avid peace advocate and philosopher.
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Arrhenius Equation: Temperature Dependence
Recall this equation k = p x Z x e -Ea/RT
-The steric factor p is presumably temperature independent-The collision number Z is relatively insensitive to temperatureIf you go from 500K to 600K, Z changes by less than 10%.
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Arrhenius Equation: Graphical
k = Ae –Ea/RT (take natural logarithm of both sides)
ln k = ln A - Ea/RT y = b + mx
So plot ln rate constant (k) on y axis vs. 1/T on x axis and your y intercept will be ln A and slope should be -Ea/R (R = 8.31 J/mol K) If you multiply slope by R you can determine Ea.
*Very similar to Clausius-Clapeyron*
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Arrhenius Equation: Graphical
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Arrhenius Equation: Two Point Equation Relating k and T
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Effect of CatalystA catalyst is a substance that increases the rate of reaction without being consumed
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Heterogeneous Catalysis
A heterogeneous catalyst is one that is in a different phase from the reaction mixture.
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Homogeneous Catalysis
A homogeneous catalyst is one that is in the same phase as the reaction mixture.
Wilkinson’s catalyst – 1 g $100Grubbs catalyst – 2 g $750
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Enzyme Catalysis
Not uncommon for the rate constant, k, to increase by a factor of 1012 …. But enzymes operate in a narrow range of temperature and higher temperatures “unfold” the enzyme making it inactive. Sometimes this is overcome by bonding enzyme to solid supports such as polymers or glass.