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Chapter 14: Rates of Reaction

Chapter 14: Rates of Reaction. Chemistry 1062: Principles of Chemistry II Andy Aspaas, Instructor. Chemical kinetics. Study of the rate or speed of reactions Rate may be affected by any of the following: Concentration of reactants Presence or concentration of a catalyst

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Chapter 14: Rates of Reaction

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  1. Chapter 14: Rates of Reaction Chemistry 1062: Principles of Chemistry II Andy Aspaas, Instructor

  2. Chemical kinetics • Study of the rate or speed of reactions • Rate may be affected by any of the following: • Concentration of reactants • Presence or concentration of a catalyst • Temperature at which reaction occurs • Surface area of solid reactant or catalyst

  3. Reaction rate • Reaction rate can be defined in terms of appearance of product or disappearance of reactant • Rates are given as a change in molar concentration in a certain time interval, unit mol/(L·s) • For the reaction A + 2B  C, the rate of the reaction may be expressed 3 ways

  4. Dependence of rate on concentration • Reaction rate usually depends on the concentrations of reactants and catalysts • Rate law shows this dependence • For the reaction a A + b B  d D + e E (reactants A and B form D and E with catalyst C) Rate = k [A]m[B]n[C]p where m, n, and p are exponents, usually integers • m, n, and p must be detemined experimentally! C

  5. Reaction order • The reaction order can be given with respect to a certain reactant, or overall • For a certain reactant, it’s the exponent in the rate law • Overall, it’s the sum of the exponents • Ex. 2NO(g) + 2H2(g)  N2(g) + 2H2O(g) Rate = k [NO]2[H2] • The reaction is second order in NO, first order in H2, and third order overall.

  6. Determining the rate law • The method of initial rates is often used to determine the rate law and order of a reaction • Several experiments are run, varying the concentration of individual reactants and catalysts • The exponents in the rate law can be determined algebraically • The rate constant is determined by substituting the concentrations of any experiment into the rate law

  7. Method of initial rates OH– I–(aq) + ClO–(aq)  IO–(aq) + Cl–(aq) What is the rate law, and what is the rate constant, k? Concentrations are in mol/L, rates are in mol/(L·s)

  8. Change of concentration writh time • First order rate law for aA  products: • Second-order rate law for aA  products:

  9. Change of concentration with time • Zero-order rate law for aA  products: • Half life (t1/2): time at which [A]t = -(1/2)[A]0 (Reactant concentration is at 1/2 its initial value Radioactive decay, etc.

  10. Graphing kinetic data • While the method of initial rates is a quick way of determining reaction order, graphing the data is more effective • Concentration of a reactant is measured in several time intervals throughout the reaction • Integrated rate laws can be rearranged if necessary to y = mx + b format for graphing, where m is the slope and b is the y-intercept

  11. Determination of reactant order by graphing • Graph 3 times for the 3 rate laws, and determine which has a straight line • Zero order: [A] vs. t is linear, slope = -k • 1st order: ln[A] vs. t is linear, slope = -k • 2nd order: 1/[A] vs. t is linear, slope = k

  12. Rate dependence on temperature • Collision theory: rate constant of a reaction is a factor of molecular collision frequency, activation energy, and the fraction of collisions which occur with a constructive orientation • Activation energy: minimum molecular energy required in order for a collision to produce a reaction • Transition-state theory: reactions must pass through an activated complex, an unstable grouping of atoms that has an equal chance of breaking into reactants or products

  13. Potential energy diagrams • Plot of potential energy (kJ/mol) vs. the course of a reaction (reactants becoming products by passing through an activated complex) • NO + Cl2 NOCl2‡  NOCl + Cl = + = ∆ = +

  14. Arrhenius equation • Rate constant of a chemical reaction is related to the activation energy and temperature • A is a constant, based on collision frequency, and proper orientation, etc.

  15. Elementary reactions • A chemical reaction may consist of several steps in order to get from reactants to products • Elementary reaction: a single molecular event, ex. the collision of molecules, or the separation of a molecule • Reaction intermediate: species produced during a reaction that does not appear in the net equation (cancels out when elementary reactions are added) • The order of a rate law for an elementary reaction can be predicted, but without knowledge of the mechanism, the rate law for an overall reaction cannot be predicted

  16. Catalysis • A catalyst increases the rate of a reaction but is not consumed • Must be re-generated stiochiometrically in an elementary reaction • Catalysts do not appear as a reactant or product in the overall reaction (shown above arrow) • Work by reducing activation energy

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