Exam Tut 2017 Flashcards
Mr Shabalala is a PhD student at a local university, his research is on the synthesis of graphene-based polymer nanocomposites after he synthesised his graphene from graphite following modified Hammer’s method he used Raman Spectroscopy and Atomic force microscopy. Briefly, explain what information Mr Shabalala will get from the two characterization techniques
Raman Spectroscopy:
He will get the vibrational, rotational, and other low frequency modes of the synthesized material. Since vibrational information is very specific for the chemical bonds in molecules, it therefore provides a fingerprint by which the molecule can be identified in the range 500 - 2000cm^-1
Atomic Force Microscopy:
the atomic force microscope is ideal for measuring nanometre-scale surface roughness and visualizing surface nanotexture. Therefore a single graphene sheet is a one atom thick material and can be used to measure the thickness and the number of layers of graphene.
Vapor phase synthesis of nanomaterials broadly classified into two methods, Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). Explain the difference between the two and if you are asked to make multiwall carbon nanotube using acetylene gas as a carbon source, which method do you think is appropriate
a) Physical Vapor Deposition
Involves conversion of solid material into gaseous phase through physical processes, the material is then cooled and re-deposited onto the substrate with perhaps some modification, like reaction with a gas.
b) Chemical Vapor Deposition
Its nanomaterials are formed in a reaction chamber by pyrolysis reduction, oxidation or nitridation and then are deposited onto the surface and are well known in a semiconductors industry vapor. e.g. synthesis of carbon nanotubes.
A well-known theoretical physicist, Dr Sheldon Cooper, and Penny, a waitress at cheesecake factory were arguing about taking Penny’s cars to a local Motor Workshop and replacing the petroleum-based engine with a fuel cell. Dr Sheldon Cooper suggested that she should consider a Polymer Electrolyte Membrane (PEM) fuel cell, however, Penny, wanted to a Solid oxide fuel cells (SOFCs); assume that you are in a position to advise them, what would be your advice on the advantage and disadvantages of the two types of Fuel cells and finally what would you recommend. [10 Marks]
Note: The story and persons mentioned in this question are fictitious.
ADVANTAGES
Polymer Electrolyte Membrane(PEM):
- Delivers high-density power and the advantages of low weight and volume.
- Only needs hydrogen, oxygen from air and water to operates. Doesn not require corrosive fluids
- Operates at relatively low temperatures, about 80degrees celcius
- Low temp. operation allows it to start quickly(less warm up time), this results in less wear on the system components and thus resulting in better durability.
- Use solid polymer electrolytes and porous carbon electrodes containing platinum catalyst
- The platinum catalyst is extremely sensitive to CO poisoning
- Making it necessary to employ an additional reactor to reduce the CO in the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel.
Solid Oxide Fuel Cells(SOFC):
- Uses a hard, non-porous, ceramic compound as the electrolyte. Since the electrolyte is solid the cells dont have to be constructed in a plate like configuration
- High temperature operation removes the need for precious metal catalysts therefore reducing costs.
- Do not get poisoned by CO, which can actually be used as fuel.
#DISADVANTAGES *Operates at high temperature, around 1000degrees celcious.
#CONCLUSION Polymer Electrolyte Membrane is a good candidate since it operates at low temperature which is ideal for running vehicles. it also has a high power density and offers the advantages of low weight and volume compared to other fuel cells, although it is expensive to operate due to the Pt catalyst which is very sensitive to CO poisoning.
Read carefully the following passage, it is extracted from Climent et al study
published on Green chemistry, 2010,12, 99-107.
4-(4-Methoxyphenyl-) butan-2-one (2) is an odorous molecule with a raspberry scent
which is extracted from aloe wood. The first Route, for the preparation of 2 in one step
(Reaction route 1) is the Friedel-Crafts alkylation of anisole with 4-hydroxybutane-2-
one or methyl vinyl ketone using acid catalysts, HCl. There is another alternative route
to synthesise 2 (reaction route 2), a one-pot catalytic process that consist of reacting
4-methoxybenzaldhyde with acetone under hydrogen atmosphere (5 bar) at 75 oC in the
presence of PdMgO catalyst to give an intermediate ,1 , which is hydrogenated to give
2 (Reaction route 2).
Based on the information given above answer the following questions.
a) Compare the two reaction routes from the green chemistry point of view
b) Calculate the atom economy of the two reactions
a)
Rxn 1:
*Side reactions(unwanted products)
*Low atom economy
Rxn 2:
- High atom economy
- No side reactions
- Under pressure(low energy consumption)
- One pot catalytic reaction
b) Rxn-1 atom economy total molecular mass of the products is 178+178+166= 522 Target molecule = 178 g/mol Therefore AE %= 187/522= 34.1%
Rxn-2: there is no side reactions and only the target molecule is produced
therefore the AE= 100%
What is Life-cycle assessment (LCA) in responsible product design?
Is a tool for assessing the environmental performance of a product, process or activity from cradle to the grave.
State four objectives of Life-cycle assessment (LCA)
- To provide as complete picture as possible of the interactions of an activity with the environment.
- To compare environmental impacts of alternative products, processes or activities
- To identify major environmental impacts and life cycle stages contributing to these impacts.
- To provide decision makers with information on the environmental effects of these activities and identify opportunities for environmental improvements.
What is solvent-less reaction?
Is a chemical reaction system in the absence of a solvent
State advantages and disadvantages of solvent-less reactions
ADVANTAGES:
- Economics
- Environmentally friendly
- Ease of purification
- High reaction rate
DISADVANTAGES:
- High viscosity in reactant system
- Side reaction accelerated
- Reactants should mix to a homogeneous system
- Unsuitable for solvent assisted chemical reactions
What are Green Solvents. Why we are interested in green solvents?
- Low solvents
- Easy recyclability
- Easy removal from products
- Low reactivity
Explain why water is a good candidate as a reaction medium?
- Economically & Environmentally attractive
- Inexpensive and abundantly available
- Non-inflammable and non-toxic
- Odourless and colourless
- Highly polar reaction medium
- Novel reactivity of organometallic complexes of organometallic complexes
- Facile product separation/catalyst recycling
- Reduced product contamination
What is a supercritical fluid ?
A substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist.
Explain why supercritical CO2 liquid is preferred over chlorinated organic solvent in a chemical reaction
- Simple
- Safe
- Selective
- Efficient
- Clean
- Versatile
Explain the advantages of biocatalysts in a chemical reaction
- Mild condition
- Fewer conditions
- Largely avoids toxix reagents and solvents
- High chemo-, regio-, and strereoselectivities
What is recycling?
Is a process to change waste materials into new products
Why recycling of waste materials is important?
- Prevent waste of potentially useful materials
- Reduce consumption of fresh raw materials
- Reduce energy usage
- Reduce air pollution
