Exp. 8 Kinetics: Iodine Clock Reaction

• Zeroth-order reaction

  • Heterogenous catalysis
  • Clock reaction
  • $NO_2 +$$CO -> NO +$$CO_2$
  • NO2 = second order reaction
  • CO = zeroth-order
• First-order reaction

  • SN1, E1 in organic chemistry
  • Exponential decay, Radioactive decay
  • $H_2O_2 -> H_2O +$$1/2 O_2$
  • H2O2 = second order
• Second-order reaction

  • SN2, E2
  • $NO_2 +$$CO -> NO +$$CO_2$
  • NO2 = second order
  • CO = zeroth order
• Differential rate law

  Zeroth-order reaction

  $r=$$-d[A]/dt=$$k$

  First-order reaction

  $r=$$-d[A]/dt=$$k[A]$

  Second-order reaction

  $r=$$-d[A]/dt=$$k[A]^2$
• Integrated rate law

  Zeroth-order reaction

  $[A]_t=$$[A]_0-kt$

  First-order reaction

  $ln[A]_t=$$ln[A]_0-kt$

  $[A]_t=$$[A]_0exp(-kt)$

  Second-order reaction

  $1/[A]_t=$$1/[A]_0+$$kt$
• Pseudo-order reaction

  For a reaction with more than one reactant, the concentration of one reactant is usually lower than that of other reactants.
• Pseudo-order reaction

  The order of reaction depends on the reaction with the lowest concentration.
• Clock reactions

  In chemical kinetics, concentrations of chemical species usually change smoothly with respect to time
• The sudden change in color due to a pH indicator marks the endpoint.
• If the release of the titrant is intrinsic to the reaction (in situ constant production of titrant by another reaction), the sharp visual change will spontaneously be visible after an “induction period” or “clock time.”
• The induction period is reproducible if temperature and composition of the reaction mixture is controlled.
• Clock reaction

  A) fast

  B) slow

  C) aA

  D) bB

  E) cC

  F) dD

  G) k1

  H) dD

  I) eE

  J) k2

  K) bB

  L) fF

  M) aA

  N) eE

  O) cC

  P) fF

  Q) step 1

  R) step 2

  
• The reagents are A and E (as reactants) with B and D (as catalysts). Only relatively small amount of A is used ([A] << [E]), so that A is the limiting reactant.
• The total reaction tells us that the reactants molecules (A and E) are consumed while the products (C and F) are formed.
• An expression relating the rate of a reaction to the rate constant (k) and concentrations of the reactants is called rate law.
• Clock reaction rate law

  A) k2

  B) D

  C) x

  D) E

  E) y

  
• Rate of reaction is the rate of the slowest step
• Rate depends on concentration of reactants.
• Rate of reaction is positive, need minus for the change in reactant concentration.
• For rate of law it depends on D (not reactant)
• Overal reaction order is equal to the sum of reaction order.
• The reaction order is constant.
• Clock reaction color change:

  Colorless --> Dark blue
• In the Iodine Clock Reaction, the term "clock" refers to the time it takes for the iodine to change color
• The function of $I_2$ in the reaction is Catalyst.
• When the iodine clock reaction occurs the Iodine is produced.
• The Iodine Clock reaction is zero-order reaction.