Presentation on theme: "The activation energy of combined reactions"— Presentation transcript:
1The activation energy of combined reactions Consider that each of the rate constants of the following reactionsA A → A* A (E1)A A* → A A (E1’)A* → P (E2)has an Arrhenius-like temperature dependence, one getsThus the composite rate constant also has an Arrhenius-like form with activation energy,E = E1 + E2 – E1’Whether or not the composite rate constant will increase with temperature depends on the value of E,if E > 0, k will increase with the increase of temperature
3Theoretical problem 22.20The reaction mechanismA2 ↔ A + A (fast)A + B → P (slow)Involves an intermediate A. Deduce the rate law for the reaction.Solution:
4Chain reactionsChain reactions: a reaction intermediate produced in one step generates an intermediate in a subsequent step, then that intermediate generates another intermediate, and so on.Chain carriers: the intermediates in a chain reaction. It could be radicals (species with unpaired electrons), ions, etc.Initiation step:Propagation steps:Termination steps:
523.1 The rate laws of chain reactions Consider the thermal decomposition of acetaldehydeCH3CHO(g) → CH4(g) + CO(g)v = k[CH3CHO]3/2it indeed goes through the following steps:1. Initiation: CH3CHO → . CH CHO kiv = ki [CH3CHO]2. Propagation: CH3CHO + . CH3 → CH4 + CH3CO kpPropagation: CH3CO. → .CH CO k’p3. Termination: .CH CH3 → CH3CH3 ktThe net rates of change of the intermediates are:
6Applying the steady state approximation: Sum of the above two equations equals:thus the steady state concentration of [.CH3] is:The rate of formation of CH4 can now be expressed asthe above result is in agreement with the three-halves order observed experimentally.
7Example: The hydrogen-bromine reaction has a complicated rate law rather than the second order reaction as anticipated.H2(g) + Br2(g) → 2HBr(g)YieldThe following mechanism has been proposed to account for the above rate law.1. Initiation: Br M → Br Br. + M ki2. Propagation: Br H2 → HBr + H kp1H Br2 → HBr Br kp23. Retardation: H HBr → H Br kr4. Termination: Br Br. + M → Br M* ktderive the rate law based on the above mechanism.
8The net rates of formation of the two intermediates are The steady-state concentrations of the above two intermediates can be obtained by solving the following two equations:substitute the above results to the rate law of [HBr]
9Effects of HBr, H2, and Br2 on the reaction rate based on the equation continuedThe above results has the same form as the empirical rate law, and the two empirical rate constants can be identified asEffects of HBr, H2, and Br2 on the reaction rate based on the equation
10Self-test 23.1 Deduce the rate law for the production of HBr when the initiation step is the photolysis, or light-induced decomposition, of Br2 into two bromine atoms, Br.. Let the photolysis rate be v = Iabs, where Iabs is the intensity of absorbed radiation.Hint: the initiation rate of Br. ?
11Exercises 23.1b: On the basis of the following proposed mechanism, account for the experimental fact that the rate law for the decomposition2N2O5(g) → 4NO2(g) + O2(g)is v = k[N2O5].N2O5 ↔ NO2 + NO k1, k1’NO NO3 → NO2 + O2 + NO k2NO + N2O5 → NO2 + NO2 + NO2 k3
12ExplosionsThermal explosion: a very rapid reaction arising from a rapid increase of reaction rate with increasing temperature.Chain-branching explosion: occurs when the number of chain centres grows exponentially.An example of both types of explosion is the following reaction2H2(g) O2(g) → 2H2O(g)1. Initiation: H2 → H H.2. Propagation H OH → H H2O kp3. Branching: O H → .O OH kb1.O H2 → .OH H Kb24. Termination H. + Wall → ½ H2 kt1H. + O M → HO M* kt2
14Analyzing the reaction of hydrogen and oxygen (see preceding slide), show that an explosion occurs when the rate of chain branching exceeds that of chain termination.Method: 1. Set up the corresponding rate laws for the reactionintermediates and then apply the steady-state approximation.2. Identify the rapid increase in the concentration of H.atoms.Applying the steady-state approximation to .OH and .O gives
15Therefore,we write kbranch = 2kb1[O2] and kterm = kt1 + kt2[O2][M], thenAt low O2 concentrations, termination dominates branching, so kterm > kbranch. Then this solution correspondsto steady combustion of hydrogen.At high O2 concentrations, branching dominates termination, kbranch > kterm. ThenThis is an explosive increase in the concentration of radicals!!!
16Self-test 23.2 Calculate the variation in radical composition when rates of branching and termination are equal.Solution:kbranch = 2kb1[O2] and kterm = kt1 + kt2[O2][M],The integrated solution is [H.] = vinit t