Chapter 11Rate of Reaction

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

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

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)

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

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

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.

Collision Theory

Collision Theory

collision theory orientation

collision theory video game analogy

Energy Diagram-Endothermic

Energy Diagram-Exothermic

Factors Affecting Reaction Rates

Reaction rates are affected by:–Temperature

–Concentration

–Particle Size

–Catalysts/Inhibitors

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

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”.

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.

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%.

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*

Arrhenius Equation: Graphical

Arrhenius Equation: Two Point Equation Relating k and T

Effect of CatalystA catalyst is a substance that increases the rate of reaction without being consumed

Heterogeneous Catalysis

A heterogeneous catalyst is one that is in a different phase from the reaction mixture.

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

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.