Chapter 4 Intermolecular Forces Flashcards
List all the molecular shapes.
- Linear Shape - 180*
- Triangular Planar - 120*
- Tetrahedral - 109.5*
- Pyramidal - 109.5*
- Bent or V-Shape - 109.5*
Define VSEPR.
Valence Shell Electron Pair Repulsion, results from repulsion between groups of electrons in the valence shell of the central atom in a molecule.
Define electronegativity.
Electronegativity measures the tendency of an atom to attract electrons.
How is a Dipole bond form?
- When 2 atoms of different electronegativity form a covalent bond then an uneven sharing of the bonding electrons will occur.
- This happens as the shared electrons of the covalent bond spend more time close to the more highly electronegative element.
- As a result of this uneven sharing of electrons, the more electronegative atom gains a small negative charge and the less electronegative atoms has a small positive charge.
What are polar molecules?
Polar molecules are those with slight positive and slight negative charged ends to the molecule. Molecules like this are said to have a net dipole.
What is a non polar molecule?
It is one in which bond dipoles act to cancel one another out resulting in a net dipole of zero.
What is the difference between intramolecular forces and intermolecular forces?
- Intramolecular forces are the covalent Bonds that keep atoms clustered together within the molecule. The covalent bonds between atoms within the molecule are not affected during melting, boiling or evaporating.
- Intermolecular forces aka Van Der Waals forces are much weaker as they keep molecules bonded to one another. These are the forces that are easily overcome when a molecular substance melts, boils or evaporates.
When does a dipole-dipole force occur?
Polar molecules are able to attract one another due to the weak EA between their dipoles. Such attractions are known as dipole-dipole forces.
What does dipole-dipole forces do?
Increase the melting points and building points of a compound.
How do dipole-dipole forces and molecular polarity affect the solubility of polar substance on other polar substances?
This can be understood as the molecular dipoles of different polar molecules will be attracted to one another resulting in a tendency for them to form a homogenous mixture of the different polar molecules. Whereas non-polar substances won’t be soluble in polar substances as there is no opportunity for dipole-dipole attraction between such combinations of molecules.
-> like dissolves like
What are dispersion forces?
They occur in all molecular materials these forces are typically weak for molecules with few electrons and show increasing strength for molecules with a greater number of electrons.
When are dispersion forces stronger?
• In molecules whose shape allows maximum surface contact between molecules. (Linear)
How are dispersion forces formed from temporary dipoles?
Dispersion forces arise when the random motion of electrons within a molecule produce short lives temporary dipoles. This can make otherwise non-polar molecules temporarily polar. Temporarily polar molecules then cause nearby molecules to form similar temporary dipoles called induced dipoles.
-> The molecule with the temporary dipole and the one with the induced dipole then experience weak EA known as dispersion forces. These attractive forces are in a state of rapid change as the temporary and induced dipoles constantly form, rapidly disappear and then reform once again.
What is a hydrogen bond?
Hydrogens bonds are the results of EA between a lobe pair of electrons of an F, O or N atom and an H atom already covalently bonded to another F, O or N atom.
Properties of substances that have a hydrogen bond:
- MP and BP are much higher
- Solubility; substances which experience hydrogen bonding between their molecules are extremely soluble in other substance which can also hydrogen bond.
