© University of South Carolina Board of Trustees Chapt. 13 Sec. 6 Reaction Mechanisms

© University of South Carolina Board of Trustees Reaction Mechanism Most overall chemical reactions take place through a series of elementary steps. This series of steps constitutes the mechanism of the reaction.

© University of South Carolina Board of Trustees 2H 2 O  H 2 O 2 + H 2 H 2 O 2  H 2 + O 2

© University of South Carolina Board of Trustees Definitions Elementary step: A reaction occurring between molecules in a single encounter. Overall reaction: The stoichiometric relationship between initial reactants and final products. Intermediate: Created and destroyed during the reaction. Does not appear in the overall reaction. Mechanism: A series of elementary steps that lead to an overall reaction.

© University of South Carolina Board of Trustees Overall Rate Laws The coefficients of an overall reaction do not determine its rate law. The overall rate law can be determined from the mechanism and the elementary steps. A + B  C Rate = k[A][B]

© University of South Carolina Board of Trustees Elementary Rate Laws The molecularity of an elementary step does determine its rate law. ●Unimolecular –one reactant molecule –1 st order rate law ●Bimolecular –two reactant molecules –1 st order in each reactant - 2 nd order overall A + B  C Rate = k[A][B] A  C Rate = k[A]

© University of South Carolina Board of Trustees Student Example The mechanism of a reaction is 2NO  N 2 O 2 N 2 O 2 + O 2  2NO 2 a) What is the overall reaction? b) Identify an intermediate in the reaction. c)What is the rate law for each step in the mechanism?

© University of South Carolina Board of Trustees Mechanism  Overall Rate Law I Rate-Limiting Step: The slowest step in a mechanism. ●The rates of steps after the rate-limiting step are unimportant. The rate of the overall reaction is determined by the rate of the rate-limiting step.

Determining Rate Limiting Step © University of South Carolina Board of Trustees For the reaction 2NO 2 + O 3  N 2 O 5 + O 2 the experimentally determined rate law is rate = k[NO 2 ][O 3 ]. Identify the rate limiting step in the proposed two-step mechanism: NO 2 + O 3  NO 3 + O 2 step 1 NO 3 + NO 2  N 2 O 5 step 2

Complex Reaction Mechanisms © University of South Carolina Board of Trustees 2NO  N 2 O 2 N 2 O 2 + O 2  2NO 2 If the first step is rapid and the second step is the rate limiting step, the rate law is: rate = k 2 [N 2 O 2 ][O 2 ] With the second step being rate limiting, the first step is able to reach equilibrium fast slow

Complex Reaction Mechanisms © University of South Carolina Board of Trustees 2NO N 2 O 2 N 2 O 2 + O 2  2NO 2 If the first step reaches equilibrium rate 1 (forward) = rate -1 (reverse): rate 1 = k 1 [NO] 2 rate -1 = k -1 [N 2 O 2 ] Because the rate at equilibrium are equal: rate 1 = rate -1 k 1 [NO] 2 = k -1 [N 2 O 2 ] [N 2 O 2 ] = (k 1 /k -1 )[NO] 2

Complex Reaction Mechanisms © University of South Carolina Board of Trustees Substituting into the expression for step 2: rate = k 2 [N 2 O 2 ][O 2 ] rate = k 2 (k 1 /k -1 )[NO] 2 [O 2 ] Combine all the rate constants: rate = k [NO] 2 [O 2 ] The reaction is second order in NO and first order in O 2.

© University of South Carolina Board of Trustees Chapt. 18 Electrochemistry Sec. 1 Assigning Oxidation States

© University of South Carolina Board of Trustees Definitions I 2Na (s) + Cl 2(g)  2Na + + 2Cl − Oxidized: Loses electrons ●Na is oxidized. ●Cl 2 is the oxidizing agent. Reduced: Gains electrons ●Cl 2 is reduced. ●Na is the reducing agent.

© University of South Carolina Board of Trustees Definitions II Redox reaction: A reaction involving the transfer of electrons (reduction + oxidation). Oxidation state: Net charge on an atom. ●Used to monitor gain/loss of electrons

© University of South Carolina Board of Trustees Calculating Oxidation State Oxidation state = Net charge on an atom ●Electrons shared between identical atoms are split evenly ●Electrons shared between different atoms are all assigned to the more electronegative atom

© University of South Carolina Board of Trustees Student Example Draw the Lewis dot structure of NH 3 and use it to find the oxidation state of each atom.

© University of South Carolina Board of Trustees Oxidation-State Short Cuts ●The oxidation state of an element is zero ●The sum of atomic oxidation states = total charge on an ion or molecule ●The oxidation state of F is always -1 (except in F 2 ) ●The oxidation state of O is always -2 (except with F or O) ●The oxidation state of H is always +1 (except with metals or in H 2 )

© University of South Carolina Board of Trustees Example: Using Short Cuts Find the oxidation state of each atom in K 2 CrO 4

© University of South Carolina Board of Trustees Student Example Assign oxidation states to each atom in a) MnO 4 2- b) NH 4 Cl