At the beginning of the last century, ethylene, that is a colourless gas, was considered to be a che — Physical Chemistry Chemistry Question
Catalytic Hydrogenation of Ethylene
At the beginning of the last century, ethylene, that is a colourless gas, was considered to be a chemical curiosity without any practical importance. Today, large amounts of ethylene are produced: in Germany, 60 kg per capita were produced in 2001. Ethylene can be converted into ethane by various catalysts. By using a zinc oxide catalyst, the reaction is so slow that the reaction mechanism can be analyzed. The pictures below show the reaction steps of the hydrogenation of ethylene (charges and stoichiometric coefficients are neglected in all the following tasks).
Write down the correct order of the steps by numbering them consecutively.
Model Answer
No. 1 ... O ... Zn ... O ... H H
No. 5 ... O+ ... Zn ... O+ ... HH CH2 CH2
No. 3 ... O+ ... Zn ... O+ ... HH
No. 7 ... O ... Zn ... O ... CH3 CH3
No. 4 ... O+ ... Zn ... O+ ... HH CH2 CH2
No. 2 ... O ... Zn- ... O+ ... HH
No. 6 ... O+ ... Zn- ... O ... H CH2 CH3
θ(H) describes the fraction of surface sites that are occupied by hydrogen atoms, θ(C2H4) describes the fraction of surface sites that are occupied by ethylene molecules and θ(C2H5) describes the fraction of surface sites that are occupied by the adsorbed intermediate.
Which of the following rate equations is correct, if the hydrogenation of the adsorbed intermediate is the slowest step of the reaction?
(1) r = k × θ(H)
(2) r = k × θ(C2H4)
(3) r = k × θ(H) × θ(C2H4)
(4) r = k × θ(H) × θ(C2H5)
Model Answer
The hydrogenation of the adsorbed intermediate is the slowest step of the reaction. This is the reason why the concentration, or in this case the fraction of surface sites that are occupied, has to be part of the rate equation.
Answer (4) is correct.
When zinc oxide is used as a catalyst, the hydrogenation of ethylene is blocked by water.
Explain this blocking by drawing the interaction between water and the catalyst analogous to that of task 1 of this problem.
Model Answer
Four answers can be accepted:
showing some kind of π–complex, or
with a bond between the zinc and the oxygen atoms
showing a hydrogen bond
showing the formation of “zinc hydroxide” and OH–groups on the catalyst surface.
If a metal catalyzes the hydrogenation of alkenes, isomer alkenes are formed in a side reaction. When D2 reacts with 1–butene the side products 1 and 2 will form.
Complete the reaction scheme on the next page and write down the structures of the intermediates.
Model Answer
Reaction scheme with completed structures of intermediates.
The fraction of surface sites that are occupied by adsorbed gas molecules (θ) may be described in a simple way by using the Langmuir isotherm:
θ = K p / (1 + K p)
p: gas pressure, K: adsorption–desorption equilibrium constant
Write down a corresponding formula for the fraction θ(i) of surface sites that are occupied by the gas i, if two or more gases are adsorbed on the catalyst.
Model Answer
Knowing the derivation of the Langmuir isotherm from the law of mass action you can obtain:
θ(i) = (K_i * p_i) / (1 + Σ (K_j * p_j))