Core 3: Molecular Interactions Flashcards
How are real gases modelled from a perfect gas?
A compressability factor, z, is required which is the Vm of gas/Vm of a perfect gas at the same temperature and pressure.
What factors does the state of matter depend on?
The attraction between molecules and the kinetic energy of the system.
How can dipole strength be calculated? What are the units? How is this related to electronegativity?
μ=qR where q is the charge on one atom and R is the distance between atoms/charges. Cm or D where D is a Debye and represents 3.335x10-30Cm (databook)
The dipole in a molecule is roughly equal to the difference in electronegativity
μ≈ΔΧ
How is percentage ion character calculated?
(actual dipole/+,- approximation of bond dipole) x100
What must be considered when comparing dipoles?
They have a direction as well as a value.
How can the overall dipole in more complex molecules be considered for strength?
The dipoles can be considered to be vectors. Opposite vectors of the same value cancel out, parallel vectors add.
What is polarisability? How are induced dipoles represented?
The ability to have an electron cloud distorted by a weak electric field. Induced dipole, μ*=αE where α is the polarisability and E is the electric field.
Strong fields require require more complex equations.
What are the units and alternate forms of polarisability?
Units of α is C2m2J-1
Polarisability volume is a useful measure, α’=α/4πε0 units=m3
What is the Coulombic potential energy?
V=q1q2/4πε0r
What four key steps are there for deriving the energy between charges?
- multiply by r/r to remove a factor of r
- substitute l/nr for x where n is a number and l is the bond lenth
- expand 1/1±x assuming r>>l
- μ=ql
Why are Van der Waals forces always attractive?
Molecules are nearly freely rotating, if they were freely rotating the interactions would cancel out. They will adopt lower energy configurations more favourably meaning all interactions overall are attractive.
What is the name for dipole-dipole interactions and what are its important features?
Keesom interaction.
Negative sign, 1/r6 distance dependance, inverse temperature dependance.
What is the name and key features for dipole-induced dipole interactions?
Debye interactions.
Negative sign, 1/r6 distance dependance, no temperature dependance(cannot be unalligned).
What is the name and key features for induced dipole-induced dipole interactions?
London (dispersion) interaction.
Negative sign, 1/r6 distance dependance, no temperature dependance.
What is the Van der Waals interaction?
The sum of the 3 types of intermolecular interactions. V=-C/r6 where C depends on the molecule.
What additionally need to be considered for molecules without dipoles?
Electrostatic interactions which decay faster than dipole interactions but are significant in non-polar systems. They are called multipoles or n-poles.
Vn∝1/rn+m-1
n and m depend on the molecule being considered, monopoles are only single atoms, dipoles are two atoms, quadrupoles are CO2 equvalents or 4 atom squares, octupoles are cube shape or tetrahedral with a central heteroatom.
How are three body interactions accounted for? How significant are these interactions?
Using the Axilrod-Teller formula. Not large but not insignificant, they account for 10% of the total interactions of argon.
How does shape affect polarisability and boiling point? Describe using n-pentane and neopentane.
n-pentane has a higher boiling point as the chain can stack better than the neopentane molecules and its polarisability is more spread out meaing it can shift around easier.
Give an approximate strength of attaction using the boiling point of a substance.
Strength of attraction, ε≈kBTBP
How does strength of attraction relate to troutons rule?
ΔHvap∝ε as the energy required to vapourise a molecule depends on its attractions to surrounding molecules.
Therefore ΔHvap≈constant x TBP which is troutons rule.
How do the strengths of hydrogen bonds vary for NH3, H2O and HF?
HF and NH3 both can form two hydrogen bonds per molecule whereas H2O can form four per molecule, meaning it has stronger intermolecular forces.
HF has stronger hydrogen bonds than NH3 as it has a greater dipole over the X-H bonds.
How can hydrogen bond strengths be estimated?
By using the trend of ΔHvap for the other hydrides in the group, the value for the first vaule can be predicted. The difference should be large and can be estimated to be the influence of the hydrogen bonds.
For a plot of ΔHvap vs T, what do liquids that lie above and below the line mean for their liquid and gas structures?
Above the line, such as alcohols and water, means there is additional structure in the liquid such as hydrogen bonds.
Below the line, such as methan/ethanoic acid, form additional structure in the gas e.g methanoic and ethanoic acid form dimers in the gas phase.
What is special about sodium polyacrylate?
In dry conditions it packs tightly together as each O- has a Na+ and the groups are happy to be close. In water the Na+ ions dissolve into solution and the chains straighten as the O- ions repel each other. They also bind up water as the gel like structure holds water molecules.
What is the strengths of typical bonds and interactions?
Covalent bond=ion ion interaction= 200-800kJmol-1
Ion-dipole= 10-100kJmol-1
H-bond= 10-40kJmol-1
Dipole-dipole= 2-10kJmol-1
London= 0.05-10kJmol-1
Explain the origins of surface tension.
At the surface of the liquid there is fewer neighbours so the molecules are at a higher potential energy (fewer intermolecular bonds). This means the liquid wants to minimise the molecules on the edge. When an object is introduced to the top of the liquid, it creates a greater area of higher potential molecules. They provide a restoring force to get to their ideal position. If the restoring force is large enough it will counteract the objects weight and allow it to float on the surface.
What are the three ways to represent the total interaction of two atoms? When do each apply?
Hard spheres - no attraction, inifinite repulsion when atoms collide. This applies when the atoms are hot so attractions are negliable or for ideal gases.
Square well - set attraction between two values, infinite repulsions when atoms collide. Models simple attraction.
Mie potential - curve of increasing attraction until a minimum is reached and fast increase in repulsion as atoms collide to a practially infinite value.
What is the Mie potential and how has it been adapted?
V=(Cn/rn)-(Cm/rm)
Lennard-Jones V=4ε{(σ/r)12-(σ/r)6} where ε is the well depth and σ is the seperation where V=0
The repulsion, (σ/r)12, term can be improved with an exponential term but this not significantly different.
How can interactions between two nobel gases be estimated?
εHe-Ar≈(εHe-HeεAr-Ar)½
What has to be considered when describing all interactions between large molecules? Why doesn’t this apply to small molecules?
Sum of bond vibrations, angle vibrations, dihedrals(bonds rotating) and interactions between molecules.
For small molecules, they rotate so fast that they can be estimated to be spherical.
Why do non-polar molecules not mix with water despite the entropy increase?
They disrupt the hydrogen bonding and produce a cage structre around the non-polar molecules which decreases the entropy of the water molecules overall.
Explain the benefits and drawbacks of forming micelles.
In micelles - Pros: Chains are free to move, H-bond structure retained
Cons: Chains are confined, head groups close together
In solution - Pros: Chains dispersed, headgroups dispersed
Cons: Chains immobile, H-bond structure broken
How does the shape of the micelle formed realate to the size of the surfactant?
Ratio used is V/al where V is the volume of the tail, a is the width of the head group and l is the length of the tail.
If: V/al≈1 then a bilayer is formed
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