polarities and intermolecular forces Flashcards
Octet rule exceptions
Beryllium is stable with 4 electrons, boron is stable with 6 electrons, phosphorous has 10 electrons in its shell in PCl 5, sulfur has 12 electrons in its shell in SF6
Valence shell electron pair repulsion theory
way of predicting the shape of a molecule based on the number of bonding and lone pairs of electrons in polyatomic species
What is valence shell electron pair repulsion theory based on
Based on the fact that the electron pairs interact and repel each other due to electrostatic repulsion meaning they adopt a spatial arrangement where the electron pairs are as fair apart as possible meaning electrostatic attraction is minimised
What is polarity
Results form uneven partial charge distrubution
What does polarity result from
Results from uneven partial charge distribution between various atoms in a molecule
What atoms have partial negative charges
Atoms with more electronegativity
What atoms have a partial positive charges
Atoms that are more neutral
Electrons in a polar covalent bond
unequally shared between the 2 bonded atoms resulting in positive and negative charges
Dipole
Separation of partial charges
Polar bonds
covalent bonds which have a separation of charge, larger the difference in electronegativity between the 2 atoms the more polar the bond
Non-polar covalent bonds
forms between 2 identical atoms when the electronegativities of the 2 atoms are equal and the electrons are shared equally (electronegativity difference is 0)
Polar covalent bond
difference in electronegativities of the 2 atoms is greater than 0 but less than 2.0
When is a bond ionic
If the difference in electronegativities is 2.0+
What must occur for a molecule to be a polar
Contain one or more polar bonds, not be symmetrical
3 types of intermolecular forces
Dispersion, dipole-dipole, hydrogen bonding
Dispersion forces (4)
Exists between all molecules, Weakest intermolecular force, Only force of attraction between non-polar molecules, More significant as the molecules become larger
What makes dispersion forces
Due to the creation of temporary dipoles when the molecules come close together, Dipoles form from uneven spread or dispersion of the constantly moving electrons, induced on one molecule by the other by forcing the electrons to the other side of the molecule, Dispersion forces result from the opposite charges attracting each other
Dipoles
in polar molecules where the electron arrangement which causes one end of the molecules to be positively charged whilst the other end is negatively charged
Dipole-Dipole interactions (forces)
Attraction between oppositely charged regions of different polar molecules, Stronger than dispersion forces, Significant intermolecular forces for small molecules
Hydrogen bond
Extreme form of dipole-dipole attraction occurring in polar molecules, Strongest intermolecular force as the hydrogen is attracted to the most electronegative atoms (N,O,F)
Strength of intermolecular forces
Hydrogen bonding > dipole-dipole > dispersion forces
Comparing substances with the same intermolecular forces
Substance with the largest molecular mass/longest chain has higher melting/boiling point as it has a greater No of electrons
Solubility
Measure of the ability for a solute to dissolve in a solvent at a given temperature, determined by knowing the type of binding that exists in both the solvent and solute, Depends on the strength of the attraction between the particles
Solubility when substances have a similar type of bonding
solute will dissolve, polar solvents dissolve in a polar solute (like dissolves like)
Solubility when substances have a different type of bonding
Won’t dissolve, non-polar solvents dissolve in non-polar solute
Evaporation
when there is space above the liquid the particles at the surface have enough genetic kinetic energy to break the forces holding them and escape
Vapour pressure
particles that escape a liquid in a closed container that collide with the walls of the container exerting pressure on the wall, tendency of molecules to escape the surface of a liquid
When do particles collide with the liquid
If they have small enough kinetic energy meaning the particles can go back and forth between the liquid and the gas, As the amount of gas increases, it is more likely to bump the surface and get stuck meaning the rates of become gas and becoming liquid become the same establishing an equilibrium
What does vapour pressure depend on
Intermolecular forces and temperature
How do temperature and vapour pressure work together
if the temperature increases more molecules will have enough kinetic energy to escape the liquid and be gas meaning as the temperature increases the vapour pressure of a substance increases
How do intermolecular forces and vapour pressure work together
if the molecule has strong intermolecular forces it will take more kinetic energy to escape the liquid meaning the vapour pressure is low, if the molecule has weak intermolecular forces they are easier to break meaning the particles in the liquid can escape easily meaning the vapour pressure will be high
Volatile liquids
Have high vapour pressure as they evaporate quickly
How does vapour pressure decrase/lower
addition of non-volatile solute which decreases the number of solvent molecules available on the surface of a liquid, greater the concentration of solute particles in the solution = lower vp
Vapour pressure at boiling point
Equal to the external pressure applied to the liquid (most cases- atmospheric pressure)
Bubble formation when boiling
form when the vapour pressure is equal to the external pressure, bubbles rise to the surface where they burst releasing the gas formed throughout the liquid, vapour inside the bubble has enough pressure to prevent bubble from collapsing
Colligative properties of solutions
When a solute is dissolved in a solvent the resulting solution has different chemical and physical properties from the pure solvent, Change that occurs dependent on the concentration of the solute particles rather than the type of solute
Colligative properties
properties of a solution that differ from the pure solvent
Freezing point
temperature at which the vapour pressure of the solid and liquid phases is the same
How is freezing point lowered
addition of a solute as the vapour pressure of the solvent is lowered, Freezing point is depressed as the average kinetic energy decreases when a liquid starts to freeze turning into a crystalline solid by losing energy that needs to be cooled further so the solute particles fit into the crystal pattern
Water
Covalent compound that consists of a highly electronegative central oxygen atom covalently bonded to 2 hydrogen atoms, has hydrogen bonding, bent shape due to asymmetrical polar bonds
How is density calculated
Dividing the mass of a substance by its volume
What is the density of water
Solid state is less dense than liquid state (ice floats on water) due to the changes in the hydrogen bonds between water molecules as when cooled, molecules slow down losing their kinetic energy allowing the hydrogen bonds to bring them closer together
Surface tension of water
Forces acting on the surface of liquids, Measure of the resistance of a liquid to increase in its surface area, Due to the strong forces of attraction (due to the hydrogen bonds) between the water molecules at the surface , Molecules on the surface feel attractive forces on one side (drawn to the liquid), Molecules in the middle are attracted in all directions
Water melting and boiling point
High compared to other covalent molecular compounds due to strong hydrogen bonding that has to be overcome for it to change state
