kinetics Flashcards
factors affecting ROR
concentration, temperature, catalysts
ROR definition
change in []/ time taken for change
mol dm-3 s-1
what is k
rate constant
what is order of reaction to respect reactants and overall for k[H2[NO]^2
first order with respect to H2
second order with respect to NO
third overall (first plus second)
state the units for k in H2 + I2 –> 2HI
rate = k [H2][I2]
moldm-3s-1 / moldm-3 x moldm-3 = k
k = s-1/moldm-3
k = s-1mol-1dm3
order or reaction meaning
the power to which concentration of a reactant is raised in the rate equation
reaction rate and conc
zero order = straight horizontal line, reaction rate doesn’t change with conc
first = diagonal, reaction rate is proportional
second = curve upwards, reaction rate is exponential
concentration and time
zero = straight line first = smaller curve downwards second = big curve downwards
halflife
the time taken for the concentration of a limiting reactant to fall to half of its initial value
calculate k constant when H2O2 is 0.02, I- is 0.01, ROR is 3.5 x 10^-6 and rate equation is k[H2O2][I-]
rate = k[H2O2][I-]
3.5x10^-6/ (0.02)(0.01) = k
k = 1.72 x 10^-2 moldm-3s-1
formula of half life
t1/2 = 0.693/k
calculate the rate constant for which the half life is 17 mins
t1/2 = 17 x 60 = 1020 seconds 1020 = 0.693/k k = 6.79 x 10^-4
effect of temp on rate constant
when temp increases, rate of reaction increases. Rate constant is proportional the number of molecules that exceed Ea so also increases
whar is the rate-determining step
slowest step in the reaction mechanism
rate of reaction depends of rate determining step so only reactants in this step is included in rate equation
is it catalyst or intermediate
when something is a reactant and then becomes a product again = catalyst (used and then regenerated)
when something is a product and then becomes a reactant later = intermediate
homogeneous catalysts
when the catayst is in the same phase as the reaction mixture
hetergeneous catalysts
when the catalyst is in a different phase as the reaction mixture.
homogenous reaction condiditons
SEP for catalyst lies between SEP of reactants
adsorption
reactant molecules form bonds with atoms on the catalyst surface
adsorption process
- reactants diffuse to the catalyst surface
- the reactant molecules are chemically adsorbed to surface and old bonds break
- adsorbed reactant atoms react on the surface, new bonds form
- deadsorption where the bonds between product and catalyst break
- product diffuses away
the iodine - peroxodisulfat reaction
S2O82- + 2I- –> 2SO42- + I2
redox reactions
- 2Fe3+ + 2I- –> 2Fe2+ + I2
- 2Fe2+ + S2O82- –> 2Fe 3+ + 2SO42-
can occur in any order.
so both Fe3+ and Fe2+ are used and then produced so they are catalysts.
two bumps on energy level profile. Catalysts used in one step and then produced in another step
nitrogen oxides
SO2 + 1/2 O2 –> SO3
1. SO2 + NO2 –> SO3 + NO
2. NO + 1/2 O2 –> NO2
used in one step and then produced in another step
