(Green) Intermolecular Forces Flashcards
(Green) Intermolecular Forces
what is the key word called for the forces acting within a molecule
The bonding within a molecule is intramolecular,
what is the key word called for the forces acting outside a molecule
the forces between molecules are intermolecular
what are the three types of intremolcular forces
· London forces or dispersion forces also called Instantaneous dipole – induced dipole forces or Van der Waals forces
· Permanent dipole – permanent dipole forces
· Hydrogen bonds (which are intermolecular forces NOT bonds!)
Why would the melting points and boiling points of a substance give a measure of the strength of intermolecular forces?
· Forces of attraction between molecules need to be overcome for a substance to melt / boil
· Intermolecular forces are overcome by supplying heat energy
· Stronger intermolecular forces require more heat energy, hence M.Pt. higher
how do london forces work
This type of force is always present and varies in strength depending on the number of electrons and shape of the molecule. The electrons in a molecule or atom are always in constant motion. Although on average the electron charge is symmetrically distributed in a neutral molecule, at any given instant it may not be. At such an instant there may be more negative charge in one area of a molecule than another and more positive charge in another area. This creates an instantaneous dipole.
The instantaneous charge distribution in one such molecule influences the charge distribution in nearby molecules. An induced charge distribution then results. A negative area produces a positive area in the neighbouring section of the next molecule as it repels the electrons, whilst a positive area induces a negative charge as it attracts the electrons. An electrostatic attractive force results.
give three featurs of london forces
·The dipoles are not permanent, they arise and disappear all the time. The molecule remains neutral. The forces however are always attractive ones.
· The more electrons an atom/molecule possesses the stronger these forces will be. This is because the dipoles created can be greater.
· The shape of the molecule also affects the strength of attraction. Long thin molecules have larger instantaneous dipoles and can approach closer.
Explain why the boiling point increases with increasing chain length.
Increased chain length means more electrons and longer molecule so there will be a larger instantaneous dipole which induces the opposite charge distribution in a neighbouring molecule, these then attract
Do these molecules have the same number of carbon and hydrogen atoms?
So what can you say about the number of electrons of both?
Explain why their boiling points are so different.
pentane, 2-methylbutane , 2,2-dimethylpropane
Yes, they are isomers.
They have the same number of electrons.
Although they have the same number of electrons the shapes of the molecules influence the strength of these attractions.
Pentane has a higher boiling point because the London forces are greater.
The molecules are longer (and so set up bigger temporary dipoles) and they can lie closer together than the shorter, fatter 2,2-dimethylpropane molecules.
what are the states of the halogen gases at room tempreature
fluorine : gas
chlorine : gas
Bromine : luiqid
iodine : solid
Explain the different boiling points and change in state at room temp down the halogen group
number of electrons increases and radius of the molecule increases so the instantaneous dipole is larger.
This in turn induces larger dipoles in neighbouring molecules.
This results in stronger London forces which require more heat E to overcome.
So B.Pt increases down the group and halogens become solid.
Underline the molecule(s) which are polar:
CCl 4
CHCl 3
CH2Cl2
CH3Cl
non polar :
CCl 4
polar :
CHCl3
CH2Cl2
CH3Cl
draw out the displayed structural formula for CHCl3, giving the distribution of charge on each bond as δ+ and δ- where appropriate.
· Now draw a second molecule next to the first, oriented to show how a δ+ region in one molecule is attracted to a δ- region in the second.
· Show the resulting attractive force between the molecules as a dotted line
should have two molecules of CHCl3
with the hydrogen bonded with a dotted line from molecule 1 to a Chlorine on molecule 2
with negative dipoles on the chlorine molecules
and positive dipoles on the hydrogen molecule
what is the force called between perminent dipoles called
PERMANENT DIPOLE - PERMANENT DIPOLE FORCES
Account for the difference in boiling temperatures between the following pairs of molecules by considering both London forces and permanent dipole- permanent dipole interactions.
a) ethene which boils at -88oC and fluoromethane which boils at -78oC.
Ethene, CH2CH2 has 16 electrons and is a longer, non-polar molecule with weak London forces between molecules.
Fluoromethane, CH3F has 18 electrons but is a more spherical molecule which is polar so in addition to the London forces it also has permanent dipole / permanent dipole forces of attraction so more heat energy is required to overcome these resulting in a higher BPt.
Account for the difference in boiling temperatures between the following pairs of molecules by considering both London forces and permanent dipole- permanent dipole interactions.
b) butane which boils at -0.5oC and propanone, CH3COCH3, which boils at 56oC.
Butane with 34 electrons is a longer non-polar molecule with London forces of attraction between molecules.
Propanone has 32 electons and is a shorter molecule so one would expect the London forces to be weaker. However it contains an electronegative d-O atom resulting in a polar d+C = d-O bond which makes to molecule polar. This results in a permanent dipole / permanent dipole attraction so more heat energy is required to overcome these resulting in a higher BPt.
- Why do the boiling points of the hydrogen halides, H-Cl to H-I, increase even though the strength of the permanent dipole-permanent dipole forces decrease?
HCl (18 electrons) has fewer electrons than HI (54 electrons) and is smaller so the instantanous dipole is smaller. Stronger London forces between HI require more heat energy to be overcome and hence a higher BPt.
Note although the electronegativity difference between H and Cl would result in a more polar molecule than H and I the permanent dipole attraction is only minor and London forces are the major factor.
give the general rule for the increase in electro negativity
up and to the right of the periodic table
Nitrogen, oxygen and fluorine are the three most electronegative elements in the periodic table. Give the formula of the hydrides formed when they are bonded to hydrogen.
Nitrogen…NH3…
Oxygen…H2O…..
Fluorine……HF……
what kind of forces do you already know exist between the molecules :
NH3…
H2O…..
HF……
Permanent dipole-permanent dipole (as well as instantaneous dipole –induced dipole)
+ hydrogen bonds
info card (read and recite)
· The O, N and F atoms have lone pairs
· The O, N and F atoms attract electron(s) from the small hydrogen atom(s) so strongly that they leave exposed protons. These protons have a high charge density and strongly attract lone pairs from neighbouring O, N and F atoms.
what is the definition of hydrogen bonds
A hydrogen bond - is a particularly strong permanent dipole-permanent dipole attraction between the lone pair of electrons on a very electronegative atom (N, O or F) and a δ+ hydrogen atom directly covalently bonded to another very electronegative N, O or F atom.
look at chem folder and look at how HF , H20 and NH3 form hydrogen bonds , see how HF and NH3 can only form 1 hydrogen bond while H20 can form two therefore is much more stronger.
.
50 cm3 ethanol is pipetted into a 100 cm3 volumetric flask and the temperature recorded.
· 50 cm3 water is pipetted into the flask the meniscus level is recorded.Volume decreased to 95cm3
· The temperature is recorded. Temperature increased by 7o C
Explain the findings:
draw a diagram
More H-bonds are formed between ethanol and water allowing more efficient packing.
lone pair from the hydoxide group should be bonded with a hydrogen from the water molecule
info card (read and recite )
https://www.worldofmolecules.com/3D/why-does-ice-float.html
Ice is less dense than liquid water and therefore floats. This is unusual as most solids are more dense than the liquid from which they are formed. The low density of ice is due to the fact that there are more extensive and permanent hydrogen bonds in ice than in water . The hydrogen bonds in ice hold the molecules apart in the crystal structure. When it melts, some of the hydrogen bonds break and the structure collapses in on itself, increasing the density. As the water continues to heat up, more and more hydrogen bonds break until they are all broken and the water boils.
describe how a hydrogen halide looks like
The hydrogen halides are colourless gases at room temperature, producing steamy fumes in moist air
Why is the B.Pt. of HF so much higher than the other hydrogen halides?
F is very electronegative, it has less shielding and smaller radius, and so the H-F bond is polar. H atom will be an exposed proton and form relatively strong H bonds with lone pairs of electrons from an F atom in an adjacent molecule.H-F: IIII H-F: . Strong H-bonds require more heat energy to be overcome.
Bearing in mind that London forces are more significant than permanent dipole forces for the rest of the hydrogen halides, explain the trend in boiling points from HCl to HI
As HCl —–> HI, more electrons, so stronger London forces.
More heat energy is needed overcome them so the B.pt. increases
Why is there a general increase in B.Pts. of both alkanes and alcohols as the number of carbon atoms increases?
An increase in C atoms is accompanied by an increase in the number of electrons and length of the molecule resulting in stronger London forces of attraction which require more heat energy to be overcome
Why is the B. Pt. of an alcohol higher than the corresponding alkane?
Due to the electronegative O atom causing the attached H atom to become d+. This d+H is attracted to the lone pair of electrons on O of an adjacent molecule resulting in relatively strong H-bonding. This requires more heat energy to be overcome then the weak London forces in alkanes.
What is meant by the specific heat capacity of a liquid?
The amount of energy to raise the temperature of 1g/1Kg by 1oC
Water’s specific heat capacity is much greater than that of most other liquids. Explain why:
Relatively strong H-bonding in 3D which requires more heat energy to be overcome..
the force is : metalic
it is between which species ?
typical strength ?
Factors that affect the force ?
between :
Metal cations and delocalised ‘sea’ of electrons
Typical strength kJ mol-1:
variable
Factors affecting the force.:
Number of delocalised electrons and radius of cation
the force is : ionic
it is between which species ?
typical strength ?
Factors that affect the force ?
between :
Cations and anions
Typical strength kJ mol-1:
250
Factors affecting the force.:
Charge on ion Radius of ion (any covalent character)
the force is : covalent (intramolecular)
it is between which species ?
typical strength ?
Factors that affect the force ?
between :
Atoms in a molecule or giant structure
Typical strength kJ mol-1:
200 - 500
Factors affecting the force.:
Bond enthalpy (affected by atomic radius and whether single, double or triple)
the force is : hydration
it is between which species ?
typical strength ?
Factors that affect the force ?
between :
Ions and water molecules
Typical strength kJ mol-1:
100
Factors affecting the force.:
Charge on ion and ionic radius
the force is : H-bonds
it is between which species ?
typical strength ?
Factors that affect the force ?
between :
d+H attached to N,O or F and lone pair on N O F
Typical strength kJ mol-1:
5 - 40
Factors affecting the force.:
Electronegativity difference between d+H and N O or F. Strongest if in a straight line with bond
the force is : london forces
it is between which species ?
typical strength ?
Factors that affect the force ?
between :
All molecules and atoms
Typical strength kJ mol-1:
10
Factors affecting the force.:
Number of electrons. Shape of molecule/ radius of atom Distance apart
the force is : Permanent.dipole
– permanent dipole
it is between which species ?
typical strength ?
Factors that affect the force ?
between :
Polar molecules
Typical strength kJ mol-1:
1.5
Factors affecting the force.:
Electronegativity difference of atoms in molecule. Geometry of molecule and distance apart.
Using your understanding of intermolecular forces, underline the compound with the highest boiling point and justify you answer.
1) CH3F (-78oC) and CH3I (42- 43oC)
Both have London, stronger for CHI3 because more electrons and larger.
C-F electronegativity difference —> bond polar molecule not symmetrical —> molecule polar —> pd/pd forces stronger BUT proportionally a lesser effect.
Using your understanding of intermolecular forces, underline the compound with the highest boiling point and justify you answer.
2) CH3Cl (a polar molecule) (-24.2oC)and CCl4 (a non polar molecule) (76.7oC)
Both have London, much stronger for CCl4 because more electrons.
C-Cl electronegativity difference —-> bond polar. CCl4 molecule is symmetrical so molecule is non-polar. CH3Cl molecule not symmetrical —> molecule polar —> pd/pd forces; they do not outweigh the stronger London in CCl4
Using your understanding of intermolecular forces, underline the compound with the highest boiling point and justify you answer.
CH3CH2F (-37.7 oC) and CH3CH2OH (78 oC)
Both have London, same number of electrons.
C-F electronegativity difference —> bond polar molecule not symmetrical —-> molecule polar —> pd/pd forces stronger.
O-H electronegativity difference —> Hd+ and lone pair on O so forms H-bonds —> forces stronger so higher B.Pt.
Using your understanding of intermolecular forces, underline the compound with the highest boiling point and justify you answer.
CH3CH2OH (78 oC) and CH3CH2NH2 (16.6 oC)
Both have similar London forces
Both make H-bonds but O-H electronegativity difference à Hd+ and lone pair on O so forms H-bonds. N-H electronegativity difference not as great so weaker H-bonds.
Using your understanding of intermolecular forces, underline the compound with the highest boiling point and justify you answer.
Br2 (58.8 oC) and HBr (-66.8 oC)
Both have London, much stronger for Br2 because more electrons and larger.
HBr is polar so will have pd/pd as well.
info card (read and recite)
When an ionic solid dissolves the ions become hydrated (electrostatic attractions form between the ions and polar water molecules) this is an exothermic process. The energy released is enough to overcome the lattice energy binding the ions together in the solid.
Sodium ions and chloride ions leaving a crystal lattice and becoming hydrated as they dissolve in water. Here there are electrostatic attractions between the ions and the polar water molecules which compensate for the attractions between oppositely charged ions in the solid.
Let us consider the old adage ‘Like dissolves like’. This means that polar molecules generally dissolve in polar solvents and non-polar molecules usually dissolve in non-polar solvents.
For a substance to dissolve the forces of attraction between the solute molecules and those between the solvent molecules must be overcome.
Name the 3 types of force between covalent molecules that may have to be overcome:-
and rank them in order or strength
H-bonds. > London forces > permanent dipole.
infocard (Read and recite )
If the total energy required to separate the solute particles and to separate the solvent molecules is less than the energy released when new forces between the solute and solvent are formed, then the substance will dissolve
infocard (Read and recite )
Solubility can be described as: Very soluble or Soluble or Sparingly soluble or Insoluble.
The degree of solubility is largely determined by the overall energy of these processes
Polar substances contain permanent dipole / permanent dipole or H-bonds as well as
London intermolecular forces. When a polar solvent dissolves a polar solute the new intermolecular forces between solute and solvent molecules that could be formed would be
hydration enthalpy, permanent dipole/permanent dipole or H-bonds as well as
London forces. Therefore, the solute would probably dissolve because
energy required to overcome forces < energy released when new bonds are made
Non-polar substances contain London. intermolecular forces only. When a
non-polar solvent dissolves a non polar solute the new intermolecular forces between solute and solvent molecules that could be formed would be London. Therefore, the solute
would probably dissolve because energy required to overcome London forces = energy released when new London force form
Why are polar solutes less soluble in non-polar solvents and vice versa?
Energy required to overcome pd/pd or H-bonds and London forces > energy released when weak London forces are made.
Explain the relative solubility of each of the following solutes in each of the three solvents.
Iodine is most soluble in cylclohexane
Iodine is most soluble in cylclohexane because overcoming London between iodine molecules and cyclohexane molecules is compensated by making new London forces between the iodine and cyclohexane molecules.
In water and ethanol the new London forces do not compensate for overcoming the H-bonds in the solvent.
Explain the relative solubility of each of the following solutes in each of the three solvents.
Sucrose is most soluble in water because
Sucrose is most soluble in water because overcoming the H bonds between sucrose molecules and water molecules are compensated by formation of new H-bonds between the sucrose and water molecules.
Partially soluble in ethanol because fewer H-bonds are made.
Insoluble in cyclohexane because the London forces made do not compensate….
Explain the relative solubility of each of the following solutes in each of the three solvents.
Calcium chloride is most soluble in waterthan ethanol because
Calcium chloride is most soluble in water because in water the energy released as the ions are hydrated compensates for the energy needed to overcome the H-bonds in water and the lattice enthalpy between ions in the solid.
In ethanol only weak p.d. can form which partially compensates for the lattice enthalpy and in cyclohexane London forces do not compensate for overcoming the lattice.
describe the solubility and observations of adding :
iodine and water
V slight colour change Insoluble
describe the solubility and observations of adding :
iodine and ethanol
Deep brown colour + solid Sparingly soluble
describe the solubility and observations of addingto I2 :
cyclohexane
Purple solution Very soluble
describe the solubility and observations of adding :
sucrose and water
Very soluble
describe the solubility and observations of adding :
ethanol
Sparingly soluble
describe the solubility and observations of adding :
cyclohexane
Insoluble
describe the solubility and observations of adding :
Calcium chloride and water
Thick ppt. of AgCl Solubl
describe the solubility and observations of adding :
Calcium chloride and ethanol
Slight ppt. Sparingly soluble
describe the solubility and observations of adding :
Calcium chloride and cyclohexane
No ppt. Insoluble
