Chapter 11 Rate of Reaction

This Chapter  Chemical kinetics is the study of reactions rates Meaning of Reaction Rate 11.2 & Concentration and Time Models for Reaction Rate Temperature Catalysis 11.7 – Reaction Mechanisms

Learning Objectives (11.4, ) 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 Theory Three 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(E a ) (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 E a = 0, then e -Ea/RT = e 0 = 1 If E a = RT, then e -Ea/RT = e -1 = 0.37 If E a = 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 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 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 Hoff Nobel Prize “1 st Nobel Prize in chemistry ever awarded” Chemical kinetics, physically proved the Arrhenius equation, van’t Hoff factor Arrhenius Nobel Prize Father of physical chemistry, ions, Arrhenius definition of acids, greenhouse effect. Ostwald Nobel Prize Catalysis, chemical equilibrium, reaction velocities, law of dilution. Coined the term, “mole” in 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 temperature If 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 -E a /R (R = 8.31 J/mol K) If you multiply slope by R you can determine E a. *Very similar to Clausius-Clapeyron*

Arrhenius Equation: Graphical

Arrhenius Equation: Two Point Equation Relating k and T

Effect of Catalyst A 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 $100 Grubbs catalyst – 2 g $750

Enzyme Catalysis Not uncommon for the rate constant, k, to increase by a factor of …. 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.