Chapter 12.2 Properties Of Covalent Molecular Substances Flashcards
What is electronegativity?
The tendency of an atom to attract a shared pair of electrons towards itself.
- affected by core charge and distance from nucleus
- francium (least) diagonally to Fluorine (most)
What are intermolecular forces?
They Forces of attraction (electrostatic forces) that exist between molecules.
*10-100x weaker than INTRAMOLECULAR forces such as ionic, metallic and covalent bonds.
Why do COVALENT MOLECULAR SUBSTANCES tend to have much LOWER MELTING AND BOILING POINTS than ionic, metallic and covalent network substances?
This is because the forces BETWEEN the molecules are much weaker
It is the WEAK INTERMOLECULAR FORCES that are broken when a covalent molecular substance is converted from a solid - liquid - gas.
What is vapour pressure?
The pressure excerted by molecules that evaporate from a liquid in an enclosed vessel at the point where the rates of evaporation and condensation are in the equilibrium
How does intermolecular forces affect vapour pressure?
In a liquid with strong intermolecular forces, the molecules are held together tightly, making it harder for them to escape from the surface of the liquid.
Therefore Vapour pressure is LOWER
When a liquid is heated, it shows a greater tendency to evaporate as the molecules have a higher average kinetic energy that enables them to overcome the intermolecular forces holding the molecules together.
As the temperature increases a greater proportion of the particles will have sufficient kinetic energy to escape from the liquid to produce the gaseous phase - vapour pressure of liquid increases
What is the Boiling Point?
The boiling point of a liquid is defined as the Temperature at which the liquid’s vapour pressure reaches atmospheric pressure of Yeh surroundings
Property; Melting and Boiling Point
What is its effect on Non Polar and Polar molecules?
Polar molecules;
- has dispersion forces
- dipole-dipole forces
Therefore the melting and boiling points are relatively high
Non -Polar
- dispersion forces
- therefore low melting and boiling points
Property; Surface Tension
What is its effect on Non Polar and Polar molecules?
Polar;
- More attractive forces making molecules have greater attraction
High surface tension
Non-polar
- low surface tension because there is less attraction between molecules
Property; Vapour Pressure
What is its effect on Non Polar and Polar molecules?
Polar
- has strong intermolecular forces
- therefore Lower Vapour Pressure
Non polar
- has weak intermolecular forces
- high vapour pressure
How to determine the POLARITY OF polyatomic particles?
Polyatmoic molculed polarity depends on the shape of the molecule as well as the Polarity of covalent bonds.
GENERAL RULE =
Symmetrical molecules = Non polar
Asymmetrical molecules = polar
Explain Polar molecules
In asymmetrical molecules the individual dipoles do not cancel each other out, resulting in a net dipole making overall molecule POLAR
Explain non- polar molecules
Is symmetrical therefore no net dipole making one end more -/+ than the other.
Explain the polarity of diatomic particles.
When 2 atoms from a covalent bond - the atoms are competing for the electrons being shared between them.
If the 2 atoms in a covalent bond are the same (ie identical electronegativity) then the electrons shared are equally between the two atoms.
Diatomic molecules; Cl2
Bonds with equal distribution of valence outer shell electrons = NON POLAR
*because there is no charge on either end of molecule .. neutral
If a covalent bond is between atoms of two different elements, then the electrons will stay closer to the most electronegative atom as it has a stronger pull on the electrons in the Bond.
Molecules with an imbalanced electron configuration = POLAR
What is a dipole
The separation of the positive and negative charges is known as an electric dipole or dipole, as they have two opposite charged poles at each end of the molecule
How does the electronegativity impact the sharing of electrons?
The level of polarity will depend on the difference between electronegativities of the 2 atoms.
As the difference in electronegativities of two atoms increases, the covalent bond formed between them increases in polarity.
In other words, it becomes more like an ionic bond.
