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.
