Lecture 5. Chemical kinetic. Rate of reaction Prepared by PhD Falfushynska H.

Type of chemical reactions after numbers of stages Simple one elemental step Complex Several steps А В Bilateral : А В : В Parallel or competitive reactions : ВА С : А →В→С Consecutive : А →В→С : А D Conjugation : А D С В Е В Е

Nuclear chain reaction A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. A possible nuclear fission chain reaction. 1. A uranium-235 atom absorbs a neutron and fissions into two new atoms, releasing three new neutrons and a large amount of binding energy. 2. One of those neutrons is absorbed by an atom of uranium-238, and does not continue the reaction. However, one neutron does collide with an atom of uranium-235, which then fissions and releases two neutrons and more binding energy. 3. Both of those neutrons collide with uranium-235 atoms, each of which fissions and releases a few neutrons, which can then continue the reaction.

Rice-Hertzfeld Free Radical Chain Reaction Mechanism Overall Reaction: Proposed Mechanism:

Classification of chemical reactions : homogeneous reaction (occur in a single phase (gaseous, liquid, or solid) : N 2 (g) + H 2 (g) → NH 3 (g) : heterogeneous reaction (occur in different phase) : Mg (s) + HCl (aq) → MgCl 2 (aq) + H 2 (g) topochemical reactions (occurs at the boundary of solid phases СuO + C Cu + CO

The balanced chemical equation provides information about the beginning and end of reaction. The reaction mechanism gives the path of the reaction. Mechanisms provide a very detailed picture of which bonds are broken and formed during the course of a reaction. Elementary Steps Elementary step: any process that occurs in a single step. Reaction Mechanisms

Elementary Steps Molecularity: the number of molecules present in an elementary step. –Unimolecular: one molecule in the elementary step, –Bimolecular: two molecules in the elementary step, and –Termolecular: three molecules in the elementary step. It is not common to see termolecular processes (statistically improbable). Reaction Mechanisms

Rate Laws for Elementary Steps The rate law of an elementary step is determined by its molecularity: –Unimolecular processes are first order, –Bimolecular processes are second order, and –Termolecular processes are third order. Rate Laws for Multistep Mechanisms Rate-determining step is the slowest of the elementary steps. Reaction Mechanisms

Rate Laws for Elementary Steps Reaction Mechanisms

Mechanisms with an Initial Fast Step 2NO(g) + Br 2 (g)  2NOBr(g) The experimentally determined rate law can be: d[NOBr]/dt = k obs [NO] 2 [Br 2 ] (or) = k obs’ [NO][Br 2 ] Consider the following mechanism Reaction Mechanisms

General Mechanism Overall Reaction: Proposed Mechanism: Where: D = observable product M = intermediate

Rate of reaction should be determine by change of concentration and pressure against time measure  P over time time Br 2 + HCOOH → 2Br - + 2H + + CO 2 2H 2 O 2 → 2H 2 O + O 2

14 Measurement of Reaction Rates Chemical Method The concentration of a reactant or product as a function of time Reaction vessels At constant T At intervals Slows down or stop the reaction Rapidly analyze chemical compositions of the mixture Cooling the sample removing a catalyst Diluting the mixture Adding a species

Reaction Rate and Stoichiometry For the reaction C 4 H 9 Cl(aq) + H 2 O(l)  C 4 H 9 OH(aq) + HCl(aq) we know In general for aA + bB  cC + dD Reaction Rates

C 4 H 9 Cl(aq) + H 2 O(l)  C 4 H 9 OH(aq) + HCl(aq)

Rate law

Rate of heterogeneous reaction depends on surface and concentrations of reagents in gas and sollution depends on surface and concentrations of reagents in gas and sollution : V = kSС СаО (т) + СО 2(г) = СаСО 3(т) V = kS пит. (СаО) С (СО 2 )

Background on Rates & Mechanisms Main Factors which influence reaction rate: – Concentrations of Reactants - Rates usually increase as reactant concentrations increase. – Reaction Temperature - An increase in temperature increases the rate of a reaction. – Presence of a Catalyst (not all rxns have catalysts) A catalyst is a substance which increases the rate of a reaction without being consumed in the overall reaction. The concentration of the catalyst or its surface area (if insoluble) are variables which influence the rate. Some catalysts are incredibly complex - like enzymes; and others are quite simple: H + + H 2 O + CH 2 = CH ) CH 3 -CH 2 -OH + H + – Type of Reactants – “Surface Area of Insoluble Reactant”

Exponents in the Rate Law For a general reaction with rate law we say the reaction is mth order in reactant 1 and nth order in reactant 2. The overall order of reaction is m + n + …. A reaction can be zeroth order if m, n, … are zero. Note the values of the exponents (orders) have to be determined experimentally. They are not simply related to stoichiometry. Concentration and Rate

First Order Reactions (to one component) The Change of Concentration with Time Isomeric Transformation of Methyl Isonitrile to Acetonitrile

23 First-Order Reactions Half-life: First-order reaction The time needed for [A] to drop to half its value Independent of [A] o A useful indication for the chemical reaction rate

Second Order Reactions The Change of Concentration with Time

Pseudo-Order Reaction Law

Integrated Rate Law － zero order

Summary of Rate Laws to One-Component First-OrderSecond-OrderZeroth-Order differential rate law (-dC/dt) kCkC 2 k Equation C = C o ·e -kt ln C = -kt + ln C o 1/C = kt + 1/C o C = -kt + C o Linear Equationln C vs. t1/C vs. tC vs. t Linear Plot Half-Lifeln(2)/k1/kC o C o /2k Units on ktime -1 M -1 time -1 M time -1 m = -k b = ln C o m = k b = 1/C o m = -k b = C o

IV.Determination of Order  Order - from units of k: If you are given the units of the rate constant for a reaction, then you will know the overall order (slide 14). Not too common.  Order by Method #1 - from altering M: Measure initial rates keeping one reactant constant and change the concentration of another; observe the rates; calculate order as illustrated in the next few slides.  Order by Method #2 - from integrated rate expression: Use calculus & integrate the rate expression between the limits of time = 0 & time = t. By plotting out the variables of these integrated rate expressions you can determine the order. This will be shown in the lecture, and you will be doing this in the kinetics lab.

Application in pharmacy Evaluation of drug stability of liquid dosage forms A typical plot of log rate constant as a function of pH for a drug (codeine sulfate) which undergoes both acid and base catalysis. Modifi ed from M.F. Powell, J.Pharm. Sci.75, 901