Science - General Chemistry Flashcards
What are commodity chemicals?
A group of chemicals made on very large scale to satisfy global markets.
Aka bulk chemicals.
What are fine chemicals and specialist chemicals?
Fine chemicals are usually of high purity and include bulk ingredients for pharmaceutical products, agrochemicals, fragrances, flavours, etc.
Speciality chemicals are also called performance chemicals and are sought for what they do (their functionality), rather than for what they are. Examples are paints, adhesives, stabilisers, thickening agents, etc.
What is E- factor and how is it determined?
A cumulative measure depending on many parameters, including how “good” the chemistry is.
E-factor = total mass of waste generated / total mass of product
What’s green chemistry?
An approach to addressing the environmental consequences of chemical products or processes at the design stage.
It follows the 12 principles of green chemistry
What are the 12 principles of green chemistry?
- It is better to prevent waste than to treat or clean up waste after it is formed
- Synthetic methods should be designed to maximise the incorporation of all materials used in the process into the final product
- Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment
- Chemical products should be designed to preserve efficacy of function while reducing toxicity
- The use of auxiliary substances (e.g. solvents, separation agents, etc.) should be made unnecessary wherever possible and, innocuous when used
- Energy requirements should be recognised for their environmental and economic impacts and should be minimised. Synthetic methods should be conducted at ambient temperature and pressure
- A raw material of feedstock should be renewable rather than depleting wherever technically and economically practicable
- Unnecessary derivatisation (blocking group, protection/ deprotection, temporary modification of physical/chemical processes) should be avoided whenever possible
- Catalytic reagents (as selective as possible) are superior to stoichiometric reagents
- Chemical products should be designed so that at the end of their function they do not persist in the environment and break down into innocuous degradation products
- Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances
- Substances and the form of a substance used in a chemical process should be chosen so as to minimise the potential for chemical accidents, including releases, explosions, and fires
What’s atom economy?
How is it determined?
How much of the molecular mass of reagents is included in
the molecular mass of the products.
Atom efficiency/economy is calculated on a theoretical basis,
assuming 100% chemical yield.
= Mr (product) / Mr (all reactants)
However it doesn’t account for the actual reaction yield (conversion & selectivity)
What is reaction mass efficiency?
It accounts for actual product yield, as well as excess reactants used.
RME = actual product mass / actual mass of all reactants
What is RME?
How is it found?
It accounts for actual product yield, as well as excess reactants used.
RME = actual product mass / actual mass of all reactants
What’s a Dalton?
Atomic mass unit (denoted u, or amu, or Dalton) is:
1 amu = 1.6605 x 10-27 kg
What are the 4 quantum numbers used to characterise electron orbitals?
(Since both the location and energy of electrons are linked to the size and shape of electronic density distribution.
These are charactirised by three quantum numbers.)
Principle quantum number, n
Orbital angular momentum, l
Magnetic quantum number, m
Magnetic spin, ms
What is the principle quantum number, n, (of electrons)?
A quantum number which labels energy levels. They describe the electron’s state.
An energy level corresponding to n = 1 is called “ground state” for hydrogen.
When electrons reach E = 0, i.e. n = infinity, it has left the atom.
This process is called ionization.
What is ionisation potential?
The energy required to remove electron from atom in the gas phase.
What is electron affinity?
The energy released when electron is added to a gas phase atom is called electron affinity - The property “opposite” to ionization
What’s orbital angular momentum, l, (for electrons)?
azimuthal number
Quantum number l specifies the number and energy of “sub-shells”.
It governs the size of the electron’s angular momentum and determines the orbital shape.
Thus,
n = 1 l = 0 subshell denoted “s”
n = 2 l = 0
l = 1 subshell denoted “p”
n= 3 l = 0
l = 1
l = 2 subshell denoted “d”
What’s magnetic quantum number, m?
Quantum number m specifies how many individual orbitals (wavefunctions) are there in a subshell.
Thus,
n = 1 l = 0 m = 0 (only 1 orbital)
n= 2 l = 0 m = 0
l = 1 m = 1, 0, -1 (three orbitals)
n = 3 l = 0
l = 1
l = 2 m = 2, 1, 0, -1, -2 (5 orbitals)
What are quantum numbers, n, l and m responsible for?
n (principle quantum number) is related to the size of the orbital
l (orbital angular momentum) is responsible for its shape
m (magnetic quantum number) is related to its orientation in space
What is the order of orbital energies?
The order of energies of orbitals in a given shell is normally
s < p < d.
Note that in atoms with high atomic numbers the effects of electron penetration and shielding should be taken into account.
What is the Pauli exclusion principle?
No more than two electrons can occupy any given orbital.
What are valence electrons?
Electrons on the outermost shell are called valence electrons.
Why do most elements exist as compounds, and not atoms?
Two atoms form a chemical bond if the resulting arrangement of the system (two nuclei and their electrons) has the energy that is lower than the energy of the separate atoms.
Changes in energy occur due to the changes in location of electrons
Complete transfer of electrons from one atom to another leads to formation of ions, and the compound is held together by the attraction between these ions – by ionic bonds.
If the lowest energy can be achieved by sharing electrons, then the atoms form a covalent bond, which is characteristic for discrete molecules.
What are cations and anions?
Cations - positive
Anions - negative
An atom becomes a cation when it loses one or more electrons; it becomes an anion when it gains electrons.
What does the Lewis structure of a molecule show?
The atoms by their chemical symbols, the covalent bonds by lines, and the lone pairs by pairs of dots.
Lewis also postulated the octet rule:
In formation of a covalent bond, atoms go as far as possible toward completing their octets (s2p6) by sharing electron pairs.
Do electrons behave as particles or waves?
Both
They can produce diffraction patterns, as waves would.
What does ms represent? (for electrons)
Magnetic spin
ms = +/- 1/2
What does the VSEPR model of covalent bonds show?
The ‘valence-shell electron-pair repulsion model’ suggests different molecular shapes by considering the angles between bonds.
It considers coulombic interactions between electrons.
According to the model, bonding electrons and lone pairs position themselves as far apart as possible to minimise repulsion.
What are the bond angles in:
- Trigonal planar
- Trigonal pyramidal
- Tetrahedral
- Angular / non-linear
- Square planar
- Trigonal bipyramidal
- Square pyramidal
- Octahedral
Trigonal planar - 120
Trigonal pyramidal - 107
Tetrahedral - 109.5
Angular / non-linear - 104.5
Square planar - 90
Trigonal bipyramidal - 90 and 120
Square pyramidal - 90
Octahedral - 90
What is ‘valence-bond’ theory?
A bond is formed when 2 atomic orbitals overlap and an electron in the atomic orbital of one of the atoms pairs its spin with that of an electron in the atomic orbital of another atom.
! There must be a decrease in the system energy from the overlap of atomic orbitals.
How are sigma, σ, bonds formed?
By the ‘end-to-end’ (head on) overlap of two s orbitals or an s and p𝓏 orbitals.
How are pi bonds formed?
By the (side on) overlap of 2 p orbitals which are in the same plane.
What is sp3 hybridization?
When s and p orbitals combine to create identical orbitals of equal energies.
In hybridization, carbon’s 2s and three 2p orbitals combine into four identical orbitals, now called sp3 hybrids.
Now that carbon has four unpaired electrons it can have four equal energy bonds. The hybridization of orbitals is also greatly favored because hybridized orbitals are lower in energy compared to their separated, unhybridized counterparts.
In methane carbon forms four identical C-H bonds via sp3 hybridisation.
What is molecular orbital, MO, theory?
The theory that electrons don’t belong to the atoms in a molecule, but occupy molecular orbitals that spread throughout the entirety of the molecule.
When atomic orbitals overlap, molecular orbitals form.
2 types form: bonding and antibonding.
What are bonding and antibonding molecular orbitals?
Bonding molecular orbitals - occur when 2 atomic orbitals overlap and result in an overall energy decrease.
Antibonding molecular orbitals - occur when 2 atomic orbitals overlap and result in an overall energy increase (unfavourable).
These can be shown in energy-level diagrams.
How is bond order calculated?
b = (n - n*) * 1/2
Where:
b - bond order
n - number of e- on bonding orbitals
n* - number of e- on antibonding orbitals
What electronic configuration has the lowest energy state.
Triplet usually has a lower energy (unpaired e-) state than singlets (paired).
What are the 9 main reaction processes?
Acid-base and neutralisation
Precipitation
Crystallisation
Redox
Electrochemical
Radial
Synthesis
Polymerisation
Catalytic
What is the standard Gibbs energy of the reaction?
It determines the thermodynamic tendency of the reaction to go in forward or reverse directions.
The more negative ΔG is , the greater is the thermodynamic driving force for reaction to go in forward direction.
What’s the equation to find Gibbs energy of reaction?
ΔG = ΔH - TΔS
ΔG = -RT*ln(K)
Where K is the equilibrium constant.
What’s the equation to find Gibbs energy of reaction, also relating to reaction system composition?
ΔG = -RT*ln(K)
Where K is the equilibrium constant.
What does STM stand for?
Scanning tunnelling microscopy
A scanning tunneling microscope is an instrument for imaging surfaces at the atomic level.
Electrons tunnel between the metal tip and the surface and a current flows. Change in current as the tip passes a surface is used to produce an image.
(Resolution of nanometers)
What are typical defects present on solid surfaces?
Terraces
Steps
Adatoms
Kinks
Defects arise from non-uniform packing of atoms on solid surfaces.
What does TEM stand for?
Transition electron microscopy
Resolution of nanometers
What are zeolites?
Crystalline microporous aluminosilicate materials with three dimensional frameworks built up from SiO4 (neutral) and AlO4 (-ve) tetrahedra.
Some zeolites (as zeolite Y) have cages which are connected via channels formed by the rings of different diameter. These rings, in turn, are formed by the TO4 tetrahedra (T = Si or Al)
Their pores are similar to the size of molecules.
What are the structural fractures of zeolites?
They have pores that are similar sizes to molecules.
Some (zeolite Y) have cages connected by channels formed by the rings of different diameter.
Some (ZSM-5) have no cages. Their pore structure consists of the channels and channel intersections.
Zeolites can contain channels of different size, e.g. zeolite Ferrierite has 8- and 10-ring channels. It is considered as a medium-pore zeolite (largest channel gives the name).
What are the main zeolites used?
Zeolite Y (faujasite)
ZSM-5
How were the first zeolites formed?
By (alumina, alkali hydroxide and silica) undergoing hydrothermal crystallisation.
This occurs at 100-110°C.
What are mesoporous materials?
A material containing pores with diameters between 2 and 50nm.
They can form from surfactant molecules and micelles, forming rods which arrange as a hexagon.
What are the properties of mesoporous materials?
Well defined pore size which can be adjusted from 1.5 to 10 nm.
High thermal stability
Potential support for catalyst preparation
Disadvantage -
No active sites (just SiO2, no aluminium)
What are the essential features of zeolites?
Channels and cages of molecular size (unique feature)
Ion exchange properties
Potential catalysts: strong acid sites, introduced metal sites
Stability to heat and radiation
Reasonably good hydrothermal stability
Environmentally friendly materials
What are the main applications of zeolites?
Water treatment
Detergent builders
Radioactive waste clean up
Sorption (drying and separation using molecular sieve effect)
Catalysis
What are the 2 main zeolites used in industry/studied?
Zeolite Y (faujasite)
- 12 ring
- rings approx. 0.74nm in diameter
ZSM - 5
- 10 ring
- rings approx. 0.55nm in diameter
What is MCM-41?
A mesoporous material.
It consists of a regular arrangement of cylindrical mesopores that form a one-dimensional pore system. It is characterized by an adjustable pore diameter, sharp pore distribution, large surface and a large pore volume. The pores are larger than with zeolites and the pore distribution can easily be adjusted.
Contrary to zeolites, it has no Bronsted acid centres because there is no aluminium contained in the lattice.
What are hierarchical zeolites?
Zeolites that have meso or/and macro pores in addition to micro pores, which are characteristic to ‘normal’ (microporous) zeolites.
The presence of large pores in hierarchical zeolites results in higher rates of diffusion of molecules.
This, in turn, affects catalytic performance of zeolites
What’s a Lewis-acid?
Any compound ready to accept a pair of electrons
What’s zeolite dehydroxylation?
The formation of Lewis acid sites, in zeolites.
What’s zeolite dealumination?
The chemical removal of alumina from a material. This occurs under high temperature and the presence of water vapour.
What’s infrared spectroscopy?
Infrared (IR) spectroscopy is based on the ability of molecules to absorb electromagnetic radiation with the energies in the infrared region of the electromagnetic spectrum.
These energies correspond to the difference in the energies between different vibrating states of the molecules, where atoms are vibrating around their equilibrium positions.
The frequencies (and, hence, the energies) of these vibrations depend on the mass of atoms and the length and strength of the bonds between these atoms (the latter depend on other atoms present in the molecule).
The IR spectrum of a sample is obtained by passing a beam of infrared radiation through the sample and recording the absorbance of the energy, which occur at different frequencies.
How can the effective pore/channel diameter in a zeolite be changed?
By using a different cation.
Different cations have different sizes.
Effective channel diameter can be reduced by exchanging the protons with larger cations.
Is it possible to achieve different degree in the changes of the effective pore/channel diameter by changing the cations used?
Yes - degree can be changed by using different cations or by the degree of ion exchange.
What are the expected Bronsted acid site peak wave numbers after infrared spectroscopy?
3603 /cm (indicates Bronsted acid site)
[3745 /cm indicates SiOH groups, which are NOT acidic]
What are the expected Bronsted and Lewis acid site peak wave numbers after infrared spectroscopy with pyridine?
1545 /cm - pyridine adsorbed on BAS
1455 /cm - pyridine adsorbed Ln zeolite LAS
1490 /cm - pyridine adsorbed on BAS and LAS (not normally used)
Consider two solid oxides Al2O3 and Ga2O3. Both oxides have unsaturated surface Al3+ or Ga3+ cations, which act as Lewis acids (or Lewis acid sites).
(a) Suggest an experiment, which should demonstrate the presence of these Lewis acid sites on the oxide surface.
(b) Suggest an experiment, which should allow you to conclude what cations (Al3+ or Ga3+) form the stronger Lewis acid sites on the oxide surface.
a) Carry out adsorption of pyridine with the compounds then analyse by IR spec.
Lewis acids will produce a specific IR spec pattern.
b) React with pyridine again. This time, heat the sample - the pyridine will start to dissolve.
The weaker the Lewis acid, the weaker the interactions between molecules in the structure, and so the lower the temperature at which the solid melts/dissolves.
What’s the activated complex and activation energy?
An activated complex is an intermediate state that is formed during the conversion of reactants into products. An activated complex is the structure that results in the maximum energy point along the reaction path.
As reactants approach each other, a transition state is formed, called activated complex, and the energy difference between the activated complex and the reactants is known as activation energy (Ea ).
What’s the Arrhenius equation?
k(T) = k(o)e^-Ea/RT
What are the essential features of catalysts?
A catalyst decreases activation energy of the reaction, thus accelerating the reaction rate.
Catalysts can accelerate (in principle) all thermodynamically possible reactions. However, catalysts do not change equilibrium constants of these reactions.
A catalyst can provide a new reaction pathway, which leads to new products and does not exist without this catalyst.
What are the steps of catalytic reactions on solid surfaces?
Physical adsorption of at least one reactant (A)
Chemical adsorption of that reactant
Chemical transformation (Achem,sorb Pchem,sorb)
Change in the adsorption state of the product
Desorption of the product
What are the (4) classifications of catalysts?
Massive - consists only of active component
Supported - active component is distributed on support
Structured - active sites located inside pores of catalyst
Anchored - active sites are anchored to a catalytically inert support
What’s involved in a heterogeneous catalytic process?
It consists of a packed-bed reactor.
Mixing and dispersion of the fluid occur to transport substances to the catalyst surface.
Diffusion in catalyst pores occurs, as well as adsorption on active sites, followed by a reaction and then desorption.
Why do molecules attach to catalyst surfaces?
Surface atoms possess certain ‘unsaturation’, which can be balanced by adsorption of molecules, which are in the vicinity of the surface.
Physical adsorption: the forces of attraction between the adsorbed molecules and the solid surface are weak.
During chemisorption the adsorbed molecules are held to the surface by valence forces. As a result, the electronic structure of the chemisorbed molecules (species) is perturbed significantly, and this makes these species highly reactive.
What are the 2 types of adsorption?
Chemical and physical, both of which are exothermic.
Physical adsorption is from weak, intermolecular forces.
Chemical adsorption results from stronger, covalent or ionic interactions.
What’s a common feature of all catalysts?
They all have many active sites.
What causes catalyst deactivation?
Poisoning - strong chemisorption of impurity in feed (competitive adsorption, reversible)
Fouling - secondary reactions of reactants or products, ‘coke’ formation
Thermal degradation - sintering (loss of surface area or active sites), evaporation
Hydrothermal degradation - sintering, loss of structure
Mechanical damage - loss of catalyst structure
Corrosion / leaching - loss of active sites, loss of structure
What’s the relationship between time scale of deactivation and reactor time?
Years - fixed-bed reactor, no regeneration
Months - fixed-bed reactor, regeneration while reactor is off-line
Weeks - moving-bed reactor, continuous regeneration
Hours/days - fluidised bed reactor, slurry reactor, continuous regeneration
Seconds - ‘riser reactor’ with continuous regeneration
(Time catalyst spends in reactor)
What happens in a methane steam reforming reaction?
Methane steam is converted into synthesis gas (CO and H2)
The synthesis gas is then used for methanol synthesis and FT synthesis of liquid fuel cells.
The hydrogen from the synthesis gas is used for fuel cell power and ammonia synthesis.
It’s a highly endothermic catalytic process.
Radiant energy is supplied from burners which are normally located in the ceiling (some burners could be on the floor or on the sides of the firebox).
Each reformer furnace contains hundreds of vertical catalyst containing tubes.
Typical catalyst:
32 wt% NiO + 14 wt% CaO
supported on Al2O3 (metal on alumina)
What are the preferred Industrial catalysts (for reforming reactions)
Ni, nickel based catalysts are preferred.
Precious metal (PM) based catalysts e.g. Ru, Rh and Pt can also be used, and are potentially better, but are too expensive.
What happens / what is the equation for the (methane steam) reforming reaction?
What’s the accompanying enthalpy change?
CH4 + H2O -> CO + 3H2
ΔHr = +206 kJ/mol
What are the conditions for the methane steam reforming reaction?
The reforming reaction is favoured by high temperature and low pressure.
However, the water-gas shift reaction (WGS) reaction is favoured by low temperature and is largely not affected by pressure.
Temperature range of steam reforming: 800-950 °C.
Operating pressures: 20 – 35 bar (elevated pressure is used to increase the reaction rate)
CH4 + H2O -> CO + 3H2
Why are high temperatures and pressures used for the methane steam reforming reaction?
High temperature used as the forward reaction is endothermic so favours high temp.
High pressure used to increase reaction rate.
Additional water (threefold excess of water) is also needed for the water-gas shift reaction.
Why could it be considered that the MSF reaction (methane steam reforming) contradicts (some of) the 12 principles of Green Chemistry.
1) It uses methane, CH4, which is not a renewable resource.
2) The MSF process would have high energy requirements due to using high temperature and pressure (high p used for rate)
3) Fuels are used which result in formation and release of CO2.
What is the equation for the water gas shift reaction?
What’s the accompanying enthalpy change?
CO + H2O -> CO2 + H2
ΔHr = -41 kJ/mol
What are the equations for the formation of carbon, the side reaction during the methane steam reforming reaction?
2CO -> C + CO2
CH4 -> C + 2H2
What are the equations for carbon gasification, the side reaction during the methane steam reforming reaction?
What’s the accompanying enthalpy change?
C + H2O -> CO + H2
ΔHr = +131.2 kJ/mol
C + 2H2O -> CO2 + 2H2
ΔHr = +90 kJ/mol
What are the (4) classifications of catalysts?
Massive - consists of an active component only
Supported - active components distributed on a support
Structured - active sites are located inside pores of the catalyst (e.g. zeolites)
Anchored - active sites are anchored to catalytically inert support
