CHEMICAL KINETICS

• Chemical kinetics


  The branch of chemistry that deals with the study of the speed or the rate at which chemical reactions occur
• Chemical reactions vary in the speed at which they occur
• The conversion of 11-cis-retinal to rhodopsin occurs in 10^-12 to 10^-6 seconds
• The conversion of graphite to diamond takes thousands, if not millions, of years to complete
• Instantaneous rate


  The rate at any given point in the reaction curve, obtained by drawing a straight line tangential to that point
• The instantaneous rate is faster at the beginning of the reaction due to the abundance of reactants
• As more products are formed, the reactants become fewer, making them harder to "meet" and causing a decrease in reaction rate
• Rate expression


  Equation that describes the rate of formation of products or disappearance of reactants
• Writing rate expressions


  1. Δ[C]/Δt = rate of formation of product C

  2. Δ[D]/Δt = rate of formation of product D

  3. -Δ[A]/Δt = rate of disappearance of reactant A

  4. -Δ[B]/Δt = rate of disappearance of reactant B
• Negative sign in rate expressions

  Signifies that the species is being used up as the reaction proceeds, not that the rate is negative
• The rate of formation of products is equal to the rate of disappearance of reactants
• Writing rate law expression


  1. rate = k[A]^x[B]^y

  2. Where k is the rate constant, and x and y are the reaction orders with respect to A and B
• Rate laws


  • Always expressed in terms of the concentration of the reactants only
  • Reaction orders x and y are not necessarily equal to the numerical coefficients in the balanced chemical equation
• Method of initial rates

  Use the initial concentration of reactants and the initial rate of the reaction to determine the reaction orders
• Integrated rate laws


  Use the concentration of reactants at different time points to determine the reaction orders
• Rate law


  Can be expressed as rate = k[A]^x[B]^y, where x and y are known as the reaction orders with respect to A and B, respectively
• Rate laws are always expressed in terms of the concentration of the reactants only
• Reaction orders x and y are not necessarily equal to the numerical coefficient of the balanced chemical reaction, and hence must be determined experimentally
• Method of initial rates

  A method to calculate reaction orders from experimental data, using the initial concentration of all the reactants and the initial rate of the reaction
• Method of initial rates

  1. Use the initial rate, as the reaction proceeds the concentration of the reactant decreases and can be difficult to measure

  2. The reverse reaction from product to reform the reaction may also occur, complicating the calculation of reaction orders
• Sample problem


  • Reaction of peroxydisulfate ion with iodide ion
  • S2O8^2- (aq) + 3I- (aq) → 2SO4^2- (aq) + I3- (aq)
• Reaction order


  The exponent of a reactant concentration in the rate law expression
• Determining reaction order


  1. Get the ratio of any two rate law expressions using the data provided

  2. Choose two runs wherein the concentration of only one reactant varies, and is constant for the rest of the other reactants
• The rate constant k only varies if the temperature varies
• Overall reaction order


  The sum of all the reaction orders in the rate law expression
• Molecularity


  The number of reactant particles involved in the chemical reaction which affects the reaction rate
• Integrated rate laws


  Equations that relate the concentrations of reactants to the elapsed time of the reaction
• Summary of the kinetics of a zeroth-, first-, and second-order reaction

  • Zeroth order: [A] = -kt + [A]0
  • First order: ln([A]t/[A]0) = -kt
  • Second order: 1/[A]t - 1/[A]0 = kt
• Sample problem


  • Decomposition reaction of ethane (C2H6) to methyl radicals: C2H6(g) → 2CH3(g)
• Correct rate law expression

  rate = k[C2H6]
• Rate law expressions are expressed in terms of the reactants only
• The curve with the highest r^2 will be the order of the reaction