Chapter 9.3-9.5

9.3 Molecularity of a Reaction

  • Rxns rarely take place as shown by the balanced equation.

  • Most rxns take place through multiple steps

  • Overall rxn = sum of several steps; the sequence of steps by which a rxn occurs is called the mechanism of the rxn.

    • Description how molecules arroach each other, how bonds are formed or broken etc. is the reaction mechanism

  • Molecularity - number of particles in a single elementary step

    • no fractional number of particles only whole numbers (realistic numbers)

    • 1, 2, or 3

  • Elementary step - a single step in the mechanism

  • Rate-determining step - the lowest step in the mechanism

  • Intermediate - the species which appears in one elementary step and is used up in another step

    • Does not appear in the overall rxn

    • Created in the rxn and used in the rxn

Experimentally found to be second order k1 << k2 Step 1 & Step 2 = Elementary steps NO3 - Intermediate Step 1 = rate-determining step

  • rate = k1[NO2]²

Unimolecular Reactions

  • Only one particle interacts; involves one reactant molecule in elementary step

  • Examples: isomerization, decomposition, ring opening, racemization

  • Follows first-order rate law

  • The reactant molecule collides with another molecule and becomes energetically excited at the expense of the other

Bimolecular Reactions

  • any elementary step that involves two reactant molecules in elementary step

Termolecular Reactions

  • involving three reactant molecules in elementary step

  • probability of this quite small

  • Ex: Nitric oxide and halogens reacting

  • M - collider molecule, usually inert gas

9.4 More Complex Reactions

Reversible Reactions

  • All rxns are reversible to an extent.

  • For the rate of the rxn equation

  • Principle of microscopic reversibility: at equilibrium, the rates of the forward and reverse processes are equal for every elementary reaction occuring

  • A→B is balanced by B→A

  • Equilibrium cannot be maintained by a cyclic process

  • The transition state for the forward and reverse reaction is identical

Consecutive Reactions

  • The product from the first step becomes the reactant for the second step

  • Steady state: assume the same amount of B being produced is equivalent to the amount of B being used up

    • Applicable if the k2 » k1

      • b/c b is being produced slowly and immediately used up when produced

  • [A]0 = [A] + [B] + [C]

  • [C] = [A]0(1-e-k1t)

  • [A] = [A]0e-k1t

  • [B] = k1/k2 [A]0e-k1t

Chain Reactions

9.5 The Effect of Temperature on Reaction Rate

  • Four types of temperature dependence for rate constants

    • a) Rate increases with increasing temperature (most typical)

    • b) Rate initially increase with temperatures, reaches maximum, then decreases (ex: enzymatic catalysis)

    • c) Rate decreases with increasing temperature (ex: some exothermic rxns)

    • d) Rate increases steadily with temperature, then increase very rapidly (ex: chain reaction)

The Arrhenius Equation

  • Describes the effect of temperature on the rate constant

  • Ea - activation energy: minimum amount of energy in order to initiate a chemical rxn

  • Ae - frequency factor (pre-exponential factor) : frequency of collisions between the reactant molecules

  • The power of e is raised to —> The fraction of molecular collisions having the enrgy equal to or higher than activation energy