Kinetics How fast does a reaction (event) occur? Reaction rates are controlled by: Nature of reactants Ability of reactants to meet Concentration of reactants Temperature Presence of a catalyst Rate of pay = €10/hour UNITS: mol/L x 1/s =mol.L -1.s -1 or M.s -1

Kinetics Change of reaction rate with timeConcentration and rate A + B  products In general it is found that: rate  [A] m [B] n The values of the exponents, m and n, must be determined empirically (by experiment). We can replace  by = if we introduce a rate constant, k. Rate = k [A] m [B] n This expression is the rate law

Rate Laws Example: H 2 SeO 3 + 6I - + 4H +  Se + 2I H 2 O Rate = k[H 2 SeO 3 ] x [I - ] y [H + ] z Experimentally found that x=1, y=3, z=2 Rate = k[H 2 SeO 3 ][I - ] 3 [H + ] 2 At 0  C, k=5.0 x 10 5 L 5 mol -5 s -1 (units of rate constant are such that the rate has units of mol.L -1.s -1 ) Notice that exponents in rate law frequently are unrelated to reaction stoichiometry. Sometimes they are the same, but we cannot predict this without experimental data! Exponents in the rate law are used to describe the order of the reaction with respect to each reactant. The overall order of a reaction is the sum of the orders with respect to each reactant (6 th order in example above).

Determining exponents in a rate law One way to do this is to study how changes in initial concentrations affect the initial rate of the reaction Initial Concs [A] [B] Initial rate (mol L -1 s -1 ) A + B  products Rate = k [A] m [B] n 1-3: [B] is constant. Rate changes due only to [A] m must be 1 3-5:[A] is constant. When [B] is doubled, rate increases by factor of 4 (=2 2 ). When [B] is tripled, rate increases by factor of 9 (=3 2 ). n must be 2

Concentration and Time-1 st order reactions Rate = k[A] Integrated rate law We can show that A plot of ln[A] t versus t is a straight line y = mx + c with slope -k and y intercept ln[A] 0.

Concentration and Time-1 st order reactions Half-life: time required for half of initial concentration of reactant to disappear. Set [A] t = ½[A] 0 t 1/2 = ln2/k A plot of ln[A]t versus t is a straight line with slope -k and y intercept ln[A] 0.

Concentration and Time-2nd order reactions Simplest 2 nd order: 2A  B Rate = k[A] 2 Integrated rate law Half-life t 1/2 = 1/k[A] 0 Half-life depends on initial concentration

Temperature dependence of reaction rates Activation Energy In order to form products, bonds must be broken in the reactants. Bond breakage requires energy. The Arrhenius equation relates the activation energy to the rate constant

Activation Energy Consider the reaction between Cl and NOCl: –If the Cl collides with the Cl of NOCl then the products are Cl 2 and NO. –If the Cl collided with the O of NOCl then no products are formed.

Arrhenius Arrhenius discovered most reaction-rate data obeyed the equation k is the rate constant, E a is the activation energy, R is the gas constant (8.314 J/mol-K) and T is the temperature in K. A is called the frequency factor. A is a measure of the probability of a favorable collision. Both A and E a are specific to a given reaction.

Catalysis A catalyst provides a reaction with an alternate pathway that has a lower energy of activation. A catalyst is not consumed in a reaction. Enzymes are biological catalysts.

Nerve Agents-Inhibition of Acetylcholinesterase

Ozone depletion

Radio-activity Unstable atomic nuclei may decay by emitting particles that are detected with special counters. Alpha, beta, and gamma emission are common types of radioactivity. In beta decay the emitted particles are electrons; in alpha decay they are helium nuclei, and in gamma decay they are high energy photons. Counters can be sensitive to either alpha, beta, or gamma-ray particles. The rubidium isotope 37 Rb 87 decays by beta emission to 38 Sr 87, a stable strontium nucleus: 37 Rb 87  38 Sr 87 + . From the following experimental data, calculate (a) the rate constant and (b) the half-life of the rubidium isotope. From a 1.00 g sample of RbCl which is 27.85% 37 Rb 87, an activity of 478 beta counts per second was found. The molecular weight of RbCl is g mole -1.

Summary Of Decay Types

94 Pu Pu 239 is used for nuclear weapons and for energy Radiological Properties of Important Plutonium Isotopes Pu-238Pu-239Pu-240Pu-241Pu-242 Half-life(in years) , ,000 Specific activity(curies/gram) Principal decay modealpha betaalpha some sponta neous fission (a) Decay energy(MeV) Radiological hazards  