GGRA03H3 Lecture : Milestone6
OneClassC-18,OneClassE-18.5.,OneClassP-807,OneClassZ-60
OneClassQuestion:- The hydrogenation reaction shown here was performed with an iridium
catalyst, both in supercritical CO2 (scCO2) and in the chlorinated solvent CH2Cl2. The kinetic
data for the reaction in both solvents are plotted in the graph. In what respects is the use of
scCO2 a good example of a green chemical reaction?
OneClassSolution:-This is a good example of a green chemical reaction as the reaction rate in
green solvent is greater than the reaction rate in conventional solvent CH2Cl2. The rate of
reaction can be estimated as the slope of this line, i.e. the ratio of conversion % and the time.
The usage of CO2 replaces the volatile, chlorinated solvent. Besides the toxicity to humans,
chlorinated solvent like CH2Cl2 may lead to ozone depletion once they reach the stratosphere.
As we know that CO2 is a greenhouse gas it may seem strange to call this kind of solvent a
green solvent. The reason why this is a green technology is in the fact that we need to take
carbon dioxide from the environment and turn it into supercritical fluid. After the reaction, the
amount of gas taken from the atmosphere is released back so the net CO2 is the same. Also,
there is no waste production, the catalyst is regenerated easily,
OneClassC-18,OneClassE-18.5.,OneClassP-808,OneClassZ-61
OneClassQuestion:- In the following three instances which choice is greener in each situation?
Explain. (a) Benzene as a solvent or water as a solvent. (b) The reaction temperature is 500 K,
or 1000 K. (c) Sodium chloride as a by-product or chloroform (CHCl3) as a by-product.
OneClassSolution:-a) Water is greener choice. Benzene is carcinogen and
b) The 500 K reaction temperature is greener process, as it consumes less energy for the same
process – the process at 500 K is more energy efficient.
c) NaCl as a by-product is greener choice, as the salt is normally present in the humans and the
environment, and CHCl3 is an environmental hazard due to the potential for ozone depletion.
OneClassC-18,OneClassE-18.5.,OneClassP-808,OneClassZ-62
OneClassQuestion:- In the following three instances which choice is greener in a chemical
process? Explain. (a) A reaction that can be run at 350 K for 12 h without a catalyst or one that
can be run at 300 K for 1 h with a reusable catalyst. (b) A reagent for the reaction that can be
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obtained from corn husks or one that is obtained from petroleum. (c) A process that produces
no by-products or one in which the by-products are recycled for another process.
OneClassSolution:-a) The 1h reaction time at 300 K and with a reusable catalyst is a greener
choice. It is more energy efficient due to shorter reaction time and lower temperature.
Renewable catalyst makes it a green process, as it does not generate a waste.
b) A reagent obtained from corn husks in a greener choice as it comes from the renewable
source.
c) It is better to have a process that produces no waste, so the first one is a greener choice.
OneClassC-18,OneClassE-A,OneClassP-808,OneClassZ-63
OneClassQuestion:- A friend of yours has seen each of the following items in newspaper
articles and would like an explanation: (a) acid rain, (b) greenhouse gas, (c) photochemical
smog, (d) ozone depletion. Give a brief explanation of each term and identify one or two of the
chemicals associated with each.
OneClassSolution:-a) Acid rain is the occurrence of rain with a low pH value. It happens when
water dissolves pollutants such as SO2 and nitrogen oxides (NOx). SO2 is oxidized to SO3,
which forms sulfuric acid (H2SO4) with water.
b) A greenhouse gas in the gas that reflects the heat (IR radiation) back to the Earth’s surface,
causing the temperature increase. Two most common greenhouse gases are water vapor and
carbon dioxide. Some other gases such as methane are even more potent greenhouse gases.
c) A photochemical smog is a type of smog caused by the photochemical processes. Smog is a
type of visible air pollutant. The primary cause of photochemical smog is the reaction
betweeen nitrogen and oxygen in the internal combustion engines.
N2(g) + O2(g) 2 NO(g) ΔH > 0.
d) Ozone depletion happens in the stratosphere. O3 has a protective role against high energy
UV radiation, and thinning of the ozone layer (ozone holes) leads to higher penetraton of such
radiation to the Earth’s surface. It is primarily caused by chlorofluorocarbons, chemicals
nontoxic to humans but devastating for ozone. C–Cl bond in CFCs dissociates when irradiated,
making the atomic Cl that starts the chain reaction in which O3 is destroyed. One example of
CFCs is CFCl3.
OneClassC-18,OneClassE-A,OneClassP-808,OneClassZ-64
OneClassQuestion:- Suppose that on another planet the atmosphere consists of 17% Kr, 38%
CH4, and 45% O2. What is the average molar mass at the surface? What is the average molar
mass at an altitude at which all the O2 is photodissociated?
OneClassSolution:-The average molar mass at the surface is:
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=.×�K+.×CH+.×O
=.×.+.×.+.×.=. g/mol
If we assume that photodissociation of O2 leads to the formation of two O atoms, without
recombination or O3 formation, the molar mass of photodissociated gas is:
=×�K+×CH+×O
++=. g/mol
Calculation is based on 100 gas molecules. When 45 molecules of gas dissociates, 90 O atoms
are formed.
OneClassC-18,OneClassE-A,OneClassP-808,OneClassZ-65
OneClassQuestion:- If an average O3 molecule “lives” only 100–200 seconds in the
stratosphere before undergoing dissociation, how can O3 offer any protection from ultraviolet
radiation?
OneClassSolution:-After ozone dissociates to molecular and atomic oxygen (O2 and O) it is not
lost - it can recombine and form ozone, which is able to absorb new photons of ultraviolet
radiation.
OneClassC-18,OneClassE-A,OneClassP-808,OneClassZ-66
OneClassQuestion:- Show how Equations 18.7 and 18.9 can be added to give Equation 18.10.
OneClassSolution:-The equations 18.7 and 18.9 are:
Cl + O3 → ClO +O2
ClO + hv → Cl + O
Multiplying both equations with 2 gives:
2 Cl + 2 O3 → 2 ClO + 2 O2
2 ClO + hv → 2 Cl + 2 O
Canceling molecules at both sides gives:
2 Cl + 2 O3 → 2 ClO + 2 O2
2 ClO + hv → 2 Cl + 2 O
As 2 O atoms recombine into O2 molecule, giving Eq. 18.10:
2 O3 → 3 O2
OneClassC-18,OneClassE-A,OneClassP-808,OneClassZ-67
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