Lesson 10: Rate Law

• Rate Law

  aA + bB → products (at a constant temperature)

  rate ∝ [A]m[B]n

  rate = k[A]m[B]n (where k is the rate constant and m & n are the rate law exponents)
• Reaction B happens faster because the initial concentrations of reactants are greater which leads to more collisions
• Situation B will be faster because there are more particles (more concentrated) so more collisions might happen
• Rate law exponents

  • Must be determined experimentally and are usually 1 or 2 but can be 0 or 3 or fractions
• Rate is proportional to concentration of reactants
• Keep all variables constant except for the one you're testing for
• Terminology

  • Reactant
  • Exponent (m or n)
  • Overall reaction (m + n)
  • 0 order (m = 1, n = 0)
  • 1st order (m = 1, n = 1)
• 1st order

  Only one reactant concentration change will affect the reaction rate
• Higher order means more effect on the reaction
• The Rate Constant

  Small k = slow reaction, Large k = fast reaction
• As time increases reaction rate will decrease because the concentration of reactants is decreasing, but k is always constant
• Initial rates method

  Vary the initial concentrations of reactants and measure the initial rates to determine the rate law
• 2 N2O5 (g) → 4 NO2 (g) + O2 (g)

  • Experiment 1: [N2O5] = 0.010 mol/L, rate = 4.8 x 10-6 mol/L·s

  Experiment 2: [N2O5] = 0.020 mol/L, rate = 9.6 x 10-6 mol/L·s

  Experiment 3: [N2O5] = 0.030 mol/L, rate = 1.5 x 10-5 mol/L·s
• We use rate law for airbags because we need to make sure we have enough concentration of particles so when the gases mix, it stops you in a certain amount of time to not get hurt
• Base^exponent = value

  log base(value) = x

  Or

  X^y  =  Z

  y = log Z / log X