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Lecture 3 kinetics of homogeneous reactions
- 1. 1
- 2. 22 CHE - 338 Chemical Reaction Engineering I
- 3. Chemical Reaction Engineering I Chemical Reaction Engineering (CRE) is the field that studies the rates and mechanisms of chemical reactions and the design of the reactors in which they take place. Reactor Raw Materia l Product
- 4. Chemical Reaction Engineering 4 Mole Balance Rate Laws Stoichiometry These topics build upon one another.
- 5. CRE Algorithm 5 Rate Laws Stoichiometry Isothermal Design Heat Effects
- 6. 6 Mole Balance Rate Laws Be careful not to cut corners on any of the CRE building blocks while learning this material!
- 7. 7 Mole Balance Rate Laws Stoichiometry Isothermal Design Heat Effects Otherwise, your Algorithm becomes unstable.
- 8. Reaction Rate A + B R 8
- 9. 9 Types of Chemical Reactions Multiple Reaction: A R Single Reaction A R S Series A R S Parallel A R A S Competitive Side by side
- 10. 10 Types of Chemical Reactions A + B R R + B S Side by side A + B R H2 + Br2 2HB r
- 11. 11 Types of Chemical Reactions A *R S Nonchan Reaction A *R + A *T +S S Initiation Propagation TerminationChain Reaction
- 12. Molecularity No. of colliding molecular entities that are involved in a single reaction step A + B C Uni/bi/termolecular reaction 12
- 13. Order of Reaction aA + bB C http://www.chemguide.co.uk/physical/basicrates/orders.html 13
- 14. Order of Reaction In chemical kinetics, the order of reaction with respect to a given substance (such as reactant, catalyst or product) is defined as the index, or exponent, to which its concentration term in the rate equation is raised. r = [A]x [B]y [A], [B], are concentrations, x for substance A & y for substance B, the reaction orders/ partial reaction orders). Overall reaction order is x + y + .... 14
- 15. Representation of an Elementary Reaction 2A 2R A R k1 -rA = rR = k1C2 A k1 -rA = rR = k1CA 15
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- 28. Representation of an Elementary Reaction B +2D 3T k1 -rB = kBCBC2 D -rD = kDCBC2 D rT = kTCBC2 D -rB = -(1/2)rD = (1/3)rT kB = (1/2)kD = (1/3)kT 28
- 29. Representation of a non Elementary Reaction 29 N2 + 3H2 2NH3
- 30. Ideal Reactors 30 Batch Reactor Uniform Composition in reactor Composition changes with time
- 31. Ideal Reactors – Steady state 31 Plug Flow Reactor Fluid passes through the reactor with no mixing of earlier and later entering fluid, and with no overtaking. it is as if the fluid moved in single file through the reactor.
- 32. Ideal Reactors – Steady state 32 Mixed Flow •Uniformly mixed •Same composition, in reactor and at
- 33. 33 Rate-Temperature Dependency ri = f1 (temp). f2(comp) k = koe-E/RT ln k/ko = lne-E/RT k1 k2 T1 T2 Arrhenius law
- 34. 34 Rate-Temperature Dependency 1/T k ln k α -E/RT Low E High E k1 k2 T1T2 463K 376K 2000K 1000K 1000K 87K
- 35. Activation Energy and Temperature Dependency From Arrhenius' law a plot of In k vs 1/T gives a straight line, with large slope for large E and small slope for small E. Reactions with high activation energies are very temperature-sensitive; reactions with low activation energies are relatively temperature- insensitive. Any given reaction is much more temperature-sensitive at a low temperature than at a high temperature. From the Arrhenius law, the value of the frequency factor k, does not affect the temperature sensitivity. 35
- 36. Engr. Muhammad Sajid, UOG 36
- 37. 37 Problems
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- 39. Engr. Muhammad Sajid, UOG 39 Year No. of Days T 1/T 1/Tx10^(3) ln(1/day) 1976 87 22 295.15 0.00338 8 3.39 - 4.46591 1977 8523.4 296.55 0.00337 2 3.37 - 4.44265 1982 7426.3 299.45 0.00333 9 3.34 - 4.30407 1984 7824.3 297.45 0.00336 2 3.36 - 4.35671 1985 9021.1 294.25 0.00339 8 3.40 - 4.49981 -
- 40. 40 -4.55 -4.5 -4.45 -4.4 -4.35 -4.3 -4.25 3.33 3.34 3.35 3.36 3.37 3.38 3.39 3.40 3.41 ln(Rate)=ln(1/days) 1/T Series1 Linear (Series1) Slope = -E/R - E = slope x R
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Editor's Notes
- Such reactions in which the rate equation corresponds to a stoichiometric equation are called elementary reactions. When there is no direct correspondence between stoichiometry and rate, then we have nonelementary reactions.
- A nonelementary reaction is one whose stoichiometry does not match its kinetics. For example, Stoichiometry: N2 + 3H2 =2NH3 Rate: This nonmatch shows that we must try to develop a multistep reaction model to explain the kinetics.
- where k, is called the frequency or pre-exponential factor and E is called the activation energy of the reaction." This expression fits experiment well over wide temperature ranges and is strongly suggested from various standpoints as being a very good approximation to the true temperature dependency. At the same concentration, but at two different temperatures, Arrhenius' law indicates that
- delT= 87 doubling the rate