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
