factors affecting reaction rates -

• chemical kinetics - study of reaction speed
• physical state of reactants - more collisions >> faster reaction
  • solid surface area increase >> reaction rate increase
• reactant concentration - higher concentration >> faster reaction
• temperature - higher temperature >> higher kinetic molecular energy >> more collisions >> faster reaction
• catalyst - substances that increase reaction rate w/o being used up
• collisions must include enough energy and correct positioning to lead to reaction
• average rate = change in concentration / change in time
• instantaneous rate - rate at a specific moment in reaction
  • rates tend to decrease as reaction continues
  • initial rate - reaction rate when reaction first begins
• rate of reactant disappearance = rate of product appearance

rate law - shows how rate depends on concentrations

• aA + bB >> cC + dD
• rate = k [A]m[B]n
• k = rate constant, changes w/ temperature (units of rate / units of concentration2)
• exponents m, n = reaction order
• usually 0, 1, or 2
• shows how concentration affects rate (0 = no change when concentration changed)
• overall reaction order - sum of orders for each reactant
• determined experimentally, not from equation/coefficients

first-order reaction - reaction where rate depends on concentration of single reactant

• ln[A]t = -kt + ln[A]0
• y = mx + b
• only needs 3 quantities to solve for the 4th

second-order reaction - reaction where rate depends on concentrations of 2 reactants  

• 1 / [A]t = kt + 1/[A]0

For the following data and the reaction X + Y >> Z, what is the order for X and Y?  
initial X concentration initial Y concentration initial rate 0.200 0.200 7.50 0.400 0.200 30.00 0.200 0.800 30.00

• as X increases by a factor of 2, the rate increases by a factor of 4
  • 4 = 22
  • for X, reaction is second order
• as Y increases by a factor of 4, so does the rate
  • rate and Y concentration increases at same rate
  • for Y, reaction is first order

For a second order reaction, the rate constantis 25 L/mol-s at 20 C. Find the time it takes for the concentration to go from 0.025 M to 0.010 M  

• Given:
  • 1 / [A]t = kt + 1/[A]0
  • k = 25
  • [A]t = 0.010
  • [A]0 = 0.025
• 1 / 0.01 = 25t + 1 / 0.025
• 25t = 100 - 40 = 60
• t = 60/25 = 2.4 sec

For a first order reaction involving popcorn, 6 kernels pop every 5 seconds when there are 150 kernels. How long until 75 of the kernels pop?  

• Given:
  • ln[A]t = -kt + ln[A]0
  • k = rate / concentration
  • [A]t = 75
  • [A]0 = 150
• rate = 6/5
• concentration = 150
• k = 6/5 / 150 = 1 / 125
• ln (75) = -1/125 t + ln (150)
• ln (75) - ln (150) = -1 / 125 t
• t = 87 sec