Unit 1 - Intermolecular forces Flashcards
What are intermolecular forces?
Forces of attraction between molecules
What is intramolecular bonding?
Bonding within molecules
Eg Covalent / Ionic / Metallic
What is intermolecular bonding?
Forces of attraction between molecules
Eg Van Der Waals forces
What are London dispersion forces?
London dispersion forces are forces of attraction that can operate between all atoms and molecules.
These forces are much weaker than all other types of bonding.
How do London Dispersion Forces arise?
London Dispersion Forces are formed as a result of electrostatic attraction between temporary dipoles and induced dipoles caused by movement of electrons in atoms and molecules.
What is overcome when liquids become gases?
Intermolecular forces of attraction
Van Der Waals forces
What type of bonding is found in the noble gases?
Intermolecular forces of attraction
Weak London Dispersion Forces
Why does sulfur have a higher boiling point than chlorine?
Sulfur is a larger molecule (S₈ vs Cl₂)
More electrons are present in a sulfur molecule than a chlorine molecule. London Dispersion Forces are stronger therefore there is greater attraction between molecules. Higher boiling point.
Why does iodine have a higher boiling point than fluorine?
As you go down a group on the periodic table, atoms get larger so have more electrons.
This leads to larger temporary and induced dipoles being formed. Greater temporary dipoles mean stronger London Dispersion Forces. Higher boiling point.
Why do the boiling points of the noble gases increase as you go down the group?
As you go down a group on the periodic table, atoms get larger so have more electron shells.
More shells mean more electrons. This leads to larger temporary and induced dipoles being formed. Greater temporary dipoles mean stronger London Dispersion Forces. Higher boiling point.
Name the three different types of Van Der Waals forces?
London Dispersion Forces
Permanent dipole-permanent dipole interactions
Hydrogen bonding
How do polar molecules arise?
A molecule is described as polar if it has a permanent dipole.
The spatial arrangement of polar covalent bonds can result in a molecule being polar.
What evidence is there for intermolecular forces of attraction?
All molecular elements and compounds and monatomic elements condense and freeze at sufficiently low temperatures.
For this to occur, some attractive forces must exist between the molecules or discrete atoms.
What are Permanent dipole-permanent dipole interactions?
Permanent dipole-permanent dipole interactions are additional electrostatic forces of attraction between polar molecules.
Permanent dipole-permanent dipole interactions are stronger than London dispersion forces for molecules with similar numbers of electrons.
Is phosphine, PH₃, a polar or a non polar molecule?
Non polar
∆E between P and H is less than 0.5
Is CH₃F a polar or a non polar molecule?
Polar
The molecule is tetrahedral. The molecule contains 3 non polar C-H bonds. The molecule contains one polar C-F bond. There is a temporary dipole ς⁻ on the F atom in the C-F bond.
Is carbon dioxide (O=C=O) a polar or a non polar molecule?
Non Polar
The molecule is linear. The molecule contains 2 polar C=O bonds. There is a temporary dipole ς⁻ on each of the O atoms in the C=O bonds. Symmetry cancels out the opposing ς⁻ dipoles on the O=C=O molecule.
Is H-H a polar or a non polar molecule?
Non polar
∆E between H atoms is 0
Is H-Cl a polar or a non polar molecule?
Polar
Linear molecule. ∆E between more than 0.5. ς⁻ dipole on the Cl atom. ς⁺ dipole on the H atom.
Is CH₂Cl₂ a polar or a non polar molecule?
Polar
The molecule is tetrahedral. The molecule contains 2 non polar C-H bonds. The molecule contains 2 polar C-Cl bonds. There is a temporary dipole ς⁻ on each of the Cl atoms in the C-Cl bonds.
Is CH₄ a polar or a non polar molecule?
Non polar
∆E between C and H is less than 0.5
Is NH₃ a polar or a non polar molecule?
Polar
The molecule is trigonal pyramidal. The molecule contains 3 polar N-H bonds. There is a temporary dipole ς⁺ on each of the H atoms in the N-H bonds. There is a temporary dipole ς- on the N atom in the N-H bonds.
Is H₂O a polar or a non polar molecule?
Polar
The molecule is angular. The molecule contains 2 polar O-H bonds. There is a temporary dipole ς⁺ on each of the H atoms in the O-H bonds. There is a temporary dipole ς- on the O atom in the O-H bonds.
What type of molecules have London Dispersion Forces as significant interactions?
Non polar molecules
Noble gases
When comparing Van Der Waals forces, why should you compare molecules of a similar size?
As molecules get larger, the London Dispersion Forces become stronger.
What is a polar molecule?
A molecule is described as polar if it has a permanent dipole.
The spatial arrangement of polar covalent bonds can result in a molecule being polar.
What effect does a permanent dipole have on boiling point?
A permanent dipole will dramatically increase boiling point
A permanent dipole is always there and the molecules arrange themselves around the dipoles.
What is hydrogen bonding?
Bonds consisting of a hydrogen atom bonded to an atom of a strongly electronegative element such as fluorine, oxygen or nitrogen are highly polar.
Hydrogen bonds are electrostatic forces of attraction between molecules that contain these highly polar bonds. A hydrogen bond is stronger than other forms of permanent dipole-permanent dipole interaction but weaker than a covalent bond.
Name three molecules which exhibit hydrogen bonding
Ammonia
Water
Hydrogen Fluoride
Methanol
Ethanol
Give an example which proves the strength of hydrogen bonding
The anomalously high boiling points of ammonia, water and hydrogen fluoride
(Graph in class notes)
Clearly explain the differences in boiling point between methylpropane (-12°C), propanone (57°C) and propan-2-ol (82°C)
Different Van Der Waals forces between molecules
The molecules can be compared as they have similar molecular masses. Methylpropane has the lowest boiling point as only London Dispersion Forces between molecules. Propanone has the next highest boiling point as it has permanent dipole - permanent dipole interactions between molecules. Propan-2-ol has the highest boiling point as it exhibits hydrogen bonding between its molecules.
What types of solutes are soluble in water?
Ionic compounds
Polar solutes
What types of solutes dissolve in non polar solvents?
Non polar solutes
Explain the process of hydration when an ionic compound dissolves
Water molecules are highly polar so are attracted to and surround the ions.
Explain how Van Der Waals forces affect viscosity
The greater the Van Der Waals force, the more viscous the liquid will be.
Why is ice less dense than water?
When water freezes, hydrogen bonding causes the molecules to arrange themselves in a lattice structure with hexagonal symmetry.
The molecules are further apart in the solid than they are when moving randomly in the liquid so less dense.
Why is hydrogen bonding essential to life on Earth?
DNA has a double helix structure due to hydrogen bonding between base pairs.
The boiling point of water is estimated to be -60°C without hydrogen bonding so all water on the planet would be water vapour.
