3.3-3.5 Flashcards
Important application of molecular shape research
Enzymes- large proteins that react with only specific molecules because of their shape. Catalyze specific reactions in living cells.
Linus Pauling’s main empirical work
X-ray analysis of crystals. The valence bond theory was created to explain what he observed in the laboratory. (extended the work of Gilbert Lewis)
What did Ronald Nyholm and Ron Gillespie create.
In 1957 they created VSEPR theory.
Stereochemistry
Study of the 3-D spatial configuration of molecules and how this affects their reactions.
VSEPR Theory
Valence-shell-electron-pair-repulsion theory. Pairs of electrons in valence shell of an atom stay as far apart as possible because negative charges repel each other. The type, number, and direction of bonds determine the shape of resulting molecule/ polyatomic ion.
In general formula, what do A, X, and E represent
A= central atom, X= bond pairs, E= lone pairs
Which electrons are important for molecular shape according to VSEPR theory
Valence electrons of the central atom(s)
How is molecular shape determined according to VSEPR theory
The positions of the electron pairs when they are a maximum distance apart.
Why does a trigonal pyramidal formula have an angle of 107.3 and not 109.5
There is a slightly stronger repulsion between the lone pair of electrons and the bonding pairs than between the bonding pairs. This causes bonding pairs to be pushed closer together.
What type of bond is strongest?
Multiple bonds (double, triple) are shorter and stronger than single bonds. They also react rapidly.
Polar molecule
A molecule in which the negative (electron) charge is not distributed symmetrically among the atoms making up the molecule.
Non polar molecule
Molecule with symmetrical electron distribution
What theories did Linus Pauling combine?
He combined valence bond theory and bond energy theory to create the concept of electronegativity. Pauling explained the polarity of a covalent bond as the difference in electronegativity of bonded atoms. Higher difference = higher polarity.
Nonpolar covalent bond
Bonded atoms have the same electronegativity and share electrons equally.
Polar covalent bond
Bonded atoms have different electronegativities. The higher the difference in electronegativity, the more polar the bond
What will happen when difference in electronegativity is very high?
The difference in attraction, combined with other factors, may transfer one or more electrons from one atom to the other, forming cations and anions. These ions will group together to form an ionic compound.
Bond dipole
The charge separation that occurs when electronegativity difference of two atoms shifts shared electrons, making one side of the bond positive and one negative. Arrow points from positive (low electronegativity) to negative (high).
Why can molecules with polar bonds be nonpolar?
If the bond dipoles balance each other producing a molecular dipole (vector sum) of zero, the molecule is non polar.
How does polarity of a molecule affect its solubility?
Polar compounds are soluble in polar solvents while nonpolar compounds are soluble in nonpolar solvents
Intermolecular forces
Forces of attraction and repulsion between molecules
Van der Waals forces
Attractive forces acting between molecules. In many substances, Van der Waals forces are a combination of many types of intermolecular forces.
Who suggested that there was a reason gases condense when cooled?
In 1873, Johannes Van der Waals suggested this. He assumed that molecules of gasses must have a small but definite volume, and the molecules must exert weak attractive forces on each other. (virgin)
What force is responsible for what can be physically observed about molecular substances , as well as physical behaviors?
Intermolecular forces. Ex. Crystalline structure, soluability
Dipole-dipole force
Attraction between any dipole and surrounding dipoles. Among the weakest intermolecular forces, strength determined by overall polarity of a molecule. Dipoles- oppositely charged sides of a molecule.
London force
London force (dispersion force): Although electron distribution may average to zero, momentary dipole can be created in an instant where electrons are not distributed perfectly evenly within a molecule. Strength of force related to the number of electrons in the molecule. Shape of and distance between molecules also affects force.
As number of electrons increases in a molecule, what will happen to the boiling point and why?
The boiling point will increase because the London force is higher causing the molecules to be difficult to separate.
Isoelectronic molecules
Molecules with the same number of electrons. They are predicted to have the same London force if the shapes of the molecules are similar.
Hydrogen bond
a hydrogen nucleus (proton) could be shared between pairs of electrons on adjacent molecules. It is a special type of polarity that results in a strong intermolecular force.
What two conditions must be true for a hydrogen bond to occur?
- Hydrogen atom must be bonded to another highly electronegative atom to pull the hydrogen’s electron strongly away from its proton, which is then unshielded.
- Must be at least one lone pair of electron on the atom bonded with hydrogen. This allows the lone pair of one atom to attract the unshielded proton on the next molecule.
What three structures show hydrogen bonding?
HF(g), Any molecule with an OH- bond or an NH- bond in any part of its structure will show hydrogen bonding.
Why is the reaction between water and glycerol exothermic?
New hydrogen bonds form, releasing energy and raising the temperature of the mixture.
Surface tension
Molecules on the surface of a liquid only have attractive forces acting downwards and sideways. This means the surface tends to stay intact. Strong intermolecular forces = high surface tension
Cohesion
Attractive forces between like molecules
Adhesion
Attractive force between unlike molecules
Capillary action
Movement of liquid up a narrow tube. Ex. water rises in a narrow tube, adhesion between water and glass thought to be higher than cohesion between water molecules
Volatility
How rapidly a liquid tends to evaporate. Weaker intermolecular attraction = quicker evaporation
Properties of Ionic compounds
hard solids, low reactivity, brittle, conductive liquids and solution, arranged in crystal lattice
What ideas does the model of metallic bonding operate on?
- low electronegativity of metal atoms to explain loosely held electrons
- empty valence orbitals to explain electron mobility
- electrostatic attraction of positive ions and negatively charged mobile valence electrons to explain the strong, nondirectional bonding.
What causes the shiny, silvery property of metals?
Valence electrons absorb and re-emit the energy from all wavelengths of visible and near visible light.
What causes the crystalline property of metals?
Electrons provide the electrostatic glue holding the metal ions together producing structures that are continuous and closely packed.
Properties of molecular crystals
low melting point, not very hard, non conductors
Covalent network
3-D arrangement of atoms continuously linked throughout the crystal by strong covalent bonds.
What gives a covalent network its strength?
The interlocking structure is responsible for the strength of the material. Ex. Carbon-carbon bonds in diamond are not different in strength from other carbon-carbon bonds. The final structure is stronger than individual components.
Semiconductors
The highest energy level of the atoms are filled with electrons that cannot move. A small amount of energy is required to jump to the next energy level. Here electrons can move freely.
Criteria for molecules only held together by London forces
No hydrogen bonds present, nonpolar (no dipole bonds present)
Hydrogen bonds require atoms with:
High electronegativity- pulls electron away from proton, lone pair to attract unshielded proton
