_Lecture 1 - Chemical Kinetics

CHEP 424: CHEMICAL REACTION ENGINEERING LECTURE 1: INTRODUCTION TO CHEMICAL KINETICS

Engr. Michael Allan Ramos Department of Chemical Engineering Technological Institute of the Philippines

Lecture Outline Classifications of Reactions

Rate of Reaction

Rate Law Equation

Factors Affecting Rate of Reaction •  Nature of Reactants •  Presence of Catalyst •  Concentration of Reactants •  Temperature

Classifications of Reactions

• Homogenous or Heterogeneous

• Batch or Continuous Flow

•  Irreversible or Reversible

• Simple or Complex

• Elementary or Non-Elementary

• Catalyzed or Non-catalayzed

Factors Affecting Rate of Reaction Definition of Rate of Reaction

•  Consider the following reactions:

1.  A + B à C

2.  2A + B à 3C + D

Factors Affecting Rate of Reaction Speed of Chemical Reactions

Magnitude of Rates of Different Reactions [moles/m3.s]

Factors Affecting Rate of Reaction 1) Nature of Reactants

•  based on degree of dispersion of reactants

Reactivity: Gases > Liquids > Solids

Factors Affecting Rate of Reaction 2) Presence of a Catalyst

•  Catalysts lowers the activation energy (Ea) needed for the reaction to proceed by providing alternative pathway.

Ratecatalyzed > Rateuncatalyzed

Factors Affecting Rate of Reaction The Rate Law Equation

•  Consider a single phase reaction: a A + b B à c C + d D

Law of Mass Action states: “The rate of a chemical reaction is directly proportional to the active

masses of the reactants each raised to a certain power .”

- rA = -rB = +rC = +rD = k CAp CB

q

Where: p = order of reaction w.r.t. A q = order of reaction w.r.t. B p + q = over-all order of reaction

Temperature Dependency Term

Concentration Dependency Term

Factors Affecting Rate of Reaction 3) Concentrations Dependency

Consider a reaction: A + B à P The rate controlling mechanism involves collision or interaction of a single molecule of A with a single molecule of B.

Rate of Reaction α Number of Collisions

Factors Affecting Rate of Reaction 4) Temperature Dependency

•  Rate of chemical reaction is observed to increase with increasing temperature. (usually 10 deg C doubles the rate of reaction)

Arrhenius Equation:

k: rate constant k0: frequency / pre-exponential factor Ea: activation energy R: Universal Gas Constant T: Temperature in Kelvin

Svante Arrhenius

Factors Affecting Rate of Reaction Collision Theory

•  Rate of reaction is governed by the number of energetic collisions between reactants. Assumes the intermediate breaks rapidly.

k = k0 T1/2 e-E/RT

Transition-state Theory •  Rate of reaction is governed by the rate of decomposition of the intermediate.

Assumes the rate of formation of the intermediate is very rapid.

k = k0 T e-E/RT

Factors Affecting Rate of Reaction General Equation:

k = k0 T m e-E/RT

For Arrhenius Law: m = 0 Collision Theory: m = 0.5 Transition-state Theory: m = 1.0

Factors Affecting Rate of Reaction

Temperature Sensitivity of Reaction Rate

k = k0 T m e-E/RT

Factors Affecting Rate of Reaction Temperature Sensitivity of Reaction Rate

Temperature Rise Needed to Double the Rate of Reaction for Activation Energies and Average Temperatures Shown:

Factors Affecting Rate of Reaction 1.  Homogeneous Reactions

rate = f ( T, P, Composition)

2.  Heterogeneous Reactions

rate = f ( Heat Transfer, Mass Transfer)

Factors Affecting Rate of Reaction Heterogeneous Reactions

•  More than one phase is involved •  Materials may have to move from one phase to the other •  Distribution of temperature results into different rates of reaction at

various points. Examples: Burning of coal

Exothermic reaction in a porous catalyst pellet

- END - For next meeting: Problem Solving involving Chemical Kinetics

SAMPLE PROBLEMS

PROBLEM 1 • Given the reaction, what is the relation between the rates

of formation and disappearance of the three reaction components?

2 NO2 + ½ O2 à N2O5

PROBLEM 2 What will be the unit of the rate constant for a reaction with the following over-all order if the rate of reaction is in moles per cubic meter per second? a)  Zero Order b)  First Order c)  Second Order

PROBLEM 3 For a gas reaction at 400 K the rate is reported as a.  What are the units of the rate constant? b.  What is the value of the rate constant for this reaction if

the rate equation is expressed as

PROBLEM 4 The pyrolysis of ethane proceeds with an activation energy of about 300 kJ/mol. How much faster is the decomposition at 650°C than at 500°C?

PROBLEM 5 The maximum allowable temperature for a reactor is 800 K. At present our operating set point is 780 K, the 20-K margin of safety to account for fluctuating feed, sluggish controls, etc. Now, with a more sophisticated control system we would be able to raise our set point to 792 K with the same margin of safety that we now have. By how much can the reaction rate, hence, production rate, be raised by this change if the reaction taking place in the reactor has an activation energy of 175 kJ/mol?

PROBLEM 6 On doubling the concentration of reactant, the rate of reaction triples. Find the reaction order.

PROBLEM 7 Calculate the activation energy for the decomposition of benzene diazonium chloride to give chlorobenzene and nitrogen. Use the following information for this first-order reaction: