Block 3 - Kinetics Flashcards
kinetics
study of chemical reactions and the factors that affect them
use of kinetics
understand reactions mechanism (what happens at molecular level) and control it
reaction rate
- no. times reaction happens per second (always positive)
- how fast reactant decreases
- how fast product increases
methods of measuring reaction rate experimentally
- pH changes
- colour changes (could be affected by conjugation)
- mass of solid
- capture gas escaping (easiest way)
instantaneous rate
+-1/a.d[A]/dt
instantaneous rate on graph
conc. vs. time graph:
- tangent
average rate
total change in conc. over total time
factors affecting rate
- nature of reaction
- temp.
- action of light (may depend on wavelength)
- state (surface area)
- conc.
- catalyst
rate laws can allow
prediction of reaction rate from conc. of reactant [A]
- rate calculated from rate law and rate constant
order in/with respect to A
- zero order: A: increase/decrease, rate: no change
- first order: A and rate change by same amount
- second order: A: doubled, rate: 4x
determining rate law experimentally
- change conc. of one reactant while keeping everything else constant
- investigate how change affects (initial) reaction rate to find order in that reactant
- repeat for other reactants
- calculate rate constant using one of the experiments
(realistically slightly different values for each–experimental error– but could average or use differences to determine confidence)
rate constant
contains info about nature of reaction and temperature
integrated rate law
instead of carrying out multiple experiments, measure conc. at various times after reaction starts to determine slope of graph
integrated first order rate law
- exponential decrease
- straight line in ln(conc.) vs. time graph
integrated second order rate law scenarios
- single second order reactant
- two first order reactants
single second order reactant
straight line in 1/[R] vs. time graph
two first order reactants
since initial conc. may be different and they may change at different rates, make conc. of one reactant very large => changes negligible => first order problem
- PSEUDO-FIRST ORDER
half-life
amount of time (usually in sec) taken for amount of reactant to decrease to half (t1/2)
radioactive decay is a
first order process
collision theory
reactions happen when molecules effectively collide
- more collisions = more chances of reaction (high conc)
effective collisions criteria
- enough energy to surpass Ea (high Ea = slow reaction): temp. matters
- correctly aligned/have correct orientation: nature of reaction, temp. doesn’t matter
absolute temp. scale
0 K = -273.15ºC
if E of reactants at r.t higher than Ea?
still need enough reactants and correct orientation for reaction to occur but will likely cause faster reaction if it works
unimolecular reaction
single molecule reaction
- molecules collide = energy transplanted = too much energy = get rid of by falling apart
elementary reactions
- single collision
- rate dependent on rate of collisions (conc. and temp.)
elementary reaction naming
unimolecular, bimolecular, termolecular
overall reaction rate depends on
slowest step (rate determining step)
determining the slow step
- given
- graph (highest Ea = slow step)
- chemical intuition (slow to produce radicals)
fast pre-equilibrium
use equilibrium constant equation of first fast step to replace intermediate in rate law deducted from second slow step
if experimental rate law correlates with that of a potential mechanism
good evidence but would need to confirm by directly detecting intermediate by some sensitive means
catalysts
substance that changes rate of reaction without being used up overall
- usually react in early step and regenerated/reformed in later step
- do not appear in overall balanced chemical equation
- changes elementary steps in reaction
catalysts method of action
- creates new pathway where Ea is lower
- molecules aligned by catalyst
catalysts in competing reactions
product distribution depends on relative Ea of competing reactions and a catalyst can provide a pathway for one reaction but not the other which alters the outcome of reactions (different major products)
types of catalysts
homogenous, heterogenous
homogenous catalysts
catalyst in same phase as reactants
- usually appears in rate law
- liquid catalyst to soln. or gas catalyst with gaseous reactants
heterogenous catalysts
catalyst in different phase as reactants
- solid cat. + gas react.
- solid cat. + solution
- immiscible liquid phases
