5f Flashcards
what happens on the surface of things to be so important
- heterogeneous catalysis
- corrosion
- advesion
whats adsorption
studying the surface of things, when a gas condenses on the surface of a solid
what does theta,, egg with line like - in the middle,, mean
theta means the covarage
what is the theta equation
its the number of sites on a surface that are filled / the total number of sites
when the coverage aka theta value is 1,, what does this mean
it means u have a monolayer formeddddd
the normal theta values are between what
the normal theta levels are between 0 and 1
when theta aka coverage is above 1,, what does that mean
it means we have a multilayer
aka the adsorbate starts to adsorb on top of itself
what are the two types of adsorption
u have:
- chemisorption
- physiosorption
describe chemisorption
- formation of a chemical bond aka e- are exchanged
- very specific,, certain things adsorb on certain substrates but not all of them
- the enthalpy of adsorption is more than 50kjmol-1
- needs activation // activation occurs,, aka the energy barrier needs to be overcome in order for the reaction to occur. aka for N2 to adsorb onto Fe u need to break the N2 bond.
- only forms one monolayer
makes sense tbhhh
- it has an Ea meaning it needs energy so the chance of it forming a multilayer is low,, and bc its specific ,, it would adsorb onto itself, just certain substartes,, and be it has an Ea that means it involves the transfer of e- meaning its the chemisorption adsorption. and that means the energy need for it to occur is high meaning the enthalpy of adsorption is usually above 50kjmol-1
descrive physiorption
- forms VDW bonding,, no e- exchange
- non specific ,, the adsorbate can adsorb on any substrate. needs to occur at low temp tho
- change in enthalpy of adsorption is ,ess than 50kjmol-1
- not activated
- can form multilayers
- makes sense tho: low temp so not specific and no Ea meaning obvs it can form multilayers, and bc its only VDW the enthalpy of adsorption is lower than 50kjmol-1
hydrogen equation that shows chemio and physio adsorption
H2(g) <–> H2(a) <–> 2H(a)
the first is physiosorption bc ur adsorbing but not transfering any e- etc.
the 2nd is chemio bc ur exchanging e- and adsorbing.
describe the graph for potential energy against distance from surface
u have 2 anharmonic oscillator type graphs
u have one closer to the lhs and one to the rhs and slightly raised up
the lhs one is the chemiosorption one,, where u have 2H.
the rhs one is the physiosorption one where u have H2
th troughs of the graphs are the adsorbed bits
the distance from the physiosorption trough to the crossover point is the Ea needed for adosprtion,, aka the energy needed to break that bond.
whats the langmuir adsorption isotherm based on
4 assumption: u keep temp constant but very the pressure
- fixed number of identical adsorption sites
- all sites are independent,, the enthalpy of adsorption of one site is not affected from neighbouring sites
- adsorption only ocurs when a molecule impacts an empty space,, it bounces off filled sites and cannot enter an empty site, it needs to land in an empty site
- the system is at equilibrium.
what the rate equation for desorption
its A(a) —> A(g)
where the arrow is kd
but ofc the conc of adsorbant adsorbed is the same as the coverage so can be written as theta
so the rate equation of desorption is
rate = kd theta.
wrate equation for adsorption
A(g) —> A(a)
where the arrow is ka
we need the A(g) conc ,, aka the copnc of the adsorbant as a gas,, also known as the pressure of the adsorbant.
but when smt adsorbs it needs an empty site,, and the rate also depends on its ability to have an empty site
so we add * : this is the empty spaces
so the equation is
rate = ka [*] P
aka the rate constant,, the conc of empty sites and pressure
how ever is theta is coverage and it can be from 1-0,, then the number of empty spaces = 1 - theta = [*]
so the new rate equation is
rate = ka P (1 - theta)
the more sites u have filled,, the waht rate
the slower the rate of adsorbance
bc it needs to find an empty site,, or it will just keep on bouncing off.
at equilibrium whats the diff in rate of adsorbance and desorption
the rate is the same
aka ka P (1 - theta) = kd theta
and then we try to get theta alone so we have
KP - KP.theta = theta
were the K is a new constant,, bc we had to do ka/ kd
then you have:
theta = KP / ( 1 + KP)
the langmuir adsorption isotherm
what does the langmuir adsorption isotherm tell us
how the coverage of a surface changes as u change pressure
whilst keeping temp the same
but u very the pressure
what happens when u plot the langmuir adsorption isotherm of theta on the y axis and Pressure on the x axis
u have a curved graph that kinda looks like an ‘r’
u have the limiting value of 1 for theta,, where the r shape doesnt increase over this.
but we want a staright line ands not a curve so that we can find the y intercept and slope
so we rearrange the equation:
P/theta = 1/k + p
y = c + mx
where pressure is on the y axis,, P is on the x axis
u get a straight line with 1/k as the intercept and we get a slope of 1, this tells us that the line obeys the langmuir adsorption isotherm
what do we want to know
we want to know the monolayer coverage
aka how much it takes to cover the whole surface
how can we figure out the volume of smt we need to cover the whole surface:
aka if an oxide adsorbs 100cm3 of N2 at 4 trr and 125cm3 at 10 torr, find the monolayer capacity of the oxide
we only have 2 points so theres no point in plotting the data.
theta = coverage = the fraction of surface occupied
= V/Vm = volume adsorbed / monolayer volume
or mass adsorbed / monolayer mass
and from the LAI we know theta = KP/1 + KP aka its unitless
so V/Vm = KP / 1+ KP —>
V+ VkP = VmKP
then u substitute the values u have into it
100 + 100K4 = Vm K 4
125 + 125K10 = Vm K10
then its a simultaneous equation type thing where we need to match smt,, so we multiply the first equation by 2.5 so that the 4 and 10 cancel out
then we substract once they match to get K
k = 125/ (10 x 25) = 1/2
then sub this into an original equation to find Vm
aka the monolayer volume!!!!
